The large, partially double-walled tube consists of a ground jacket joint, a cooler, a cooling vessel and an inlet. The temperature is measured with a suitable temperature sensor e. The necessary connections are made to the pressure regulation and measuring equipment. The bulb, which acts as a buffer volume, is connected with the measuring apparatus by means of a capillary tube. The boiling vessel is heated by a heating element e. The heating current required is set and regulated via a thermocouple.
The necessary vacuum of between 10 2 Pa and approximately 10 5 Pa is produced with a vacuum pump. A suitable valve is used to meter air or nitrogen for pressure regulation measuring range approximately 10 2 to 10 5 Pa and ventilation. The vapour pressure is measured by determining the boiling temperature of the sample at various specified pressures between roughly 10 3 and 10 5 Pa. A steady temperature under constant pressure indicates that the boiling temperature has been reached.
Frothing substances cannot be measured using this method. The substance is placed in the clean, dry sample vessel. Problems may be encountered with non-powder solids but these can sometimes be solved by heating the cooling jacket. Once the vessel has been filled the apparatus is sealed at the flange and the substance degassed.
The lowest desired pressure is then set and the heating is switched on. At the same time, the temperature sensor is connected to a recorder. Equilibrium is reached when a constant boiling temperature is recorded at constant pressure. Particular care must be taken to avoid bumping during boiling. In addition, complete condensation must occur on the cooler. When determining the vapour pressure of low melting solids, care should be taken to avoid the condenser blocking.
After recording this equilibrium point, a higher pressure is set. The process is continued in this manner until 10 5 Pa has been reached approximately 5 to 10 measuring points in all. As a check, equilibrium points must be repeated at decreasing pressures. The apparatus comprises a container for the sample, a heating and cooling system to regulate the temperature of the sample and measure the temperature.
The apparatus also includes instruments to set and measure the pressure. Figures 2a and 2b illustrate the basic principles involved. The sample chamber figure 2a is bounded on one side by a suitable high-vacuum valve. A U-tube containing a suitable manometer fluid is attached to the other side. One end of the U-tube branches off to the vacuum pump, the nitrogen cylinder or ventilation valve, and a manometer. A pressure gauge with a pressure indicator can be used instead of a U-tube figure 2b.
The temperature measurements are taken on the outside wall of the vessel containing the sample or in the vessel itself. In method 2a the vapour pressure of the substance is measured indirectly using a zero indicator. This takes into account the fact that the density of the fluid in the U-tube alters if the temperature changes greatly. The following fluids are suitable for use as zero indicators for the U-tube, depending on the pressure range and the chemical behaviour of the test substance: silicone fluids, phthalates.
The test substance must not dissolve noticeably in or react with the U-tube fluid. For the manometer, mercury can be used in the range of normal air pressure to 10 2 Pa, while silicone fluids and phthalates are suitable for use below 10 2 Pa down to 10 Pa. Heatable membrane capacity manometers can even be used at below 10 -1 Pa. There are also other pressure gauges which can be used below 10 2 Pa. Before measuring, all components of the apparatus shown in figure 2 must be cleaned and dried thoroughly. For method 2a, fill the U-tube with the chosen liquid, which must be degassed at an elevated temperature before readings are taken.
The test substance is placed in the apparatus, which is then closed and the temperature is reduced sufficiently for degassing. The temperature must be low enough to ensure that the air is sucked out, but — in the case of multiple component system — it must not alter the composition of the material. If required, equilibrium can be established more quickly by stirring. The sample can be supercooled with e. For low-temperature measurements use a temperature-regulated bath connected to an ultra-cryomat.
With the valve over the sample vessel open, suction is applied for several minutes to remove the air. The valve is then closed and the temperature of the sample reduced to the lowest level desired. If necessary, the degassing operation must be repeated several times. When the sample is heated the vapour pressure increases.
This alters the equilibrium of the fluid in the U-tube. To compensate for this, nitrogen or air is admitted to the apparatus via a valve until the pressure indicator fluid is at zero again. The pressure required for this can be read off a precision manometer at room temperature.
This pressure corresponds to the vapour pressure of the substance at that particular measuring temperature. The temperature-dependence of vapour pressure is determined at suitably small intervals approximately 5 to 10 measuring points in all up to the desired maximum. Low-temperature readings must be repeated as a check. If the values obtained from the repeated readings do not coincide with the curve obtained for increasing temperature, this may be due to one of the following:. In this case liquid nitrogen is used as the cooling agent.
A complete description of this method can be found in reference 7. The principle of the measuring device is shown in figure 3. Similarly to the static method described in 1. In the case of liquids, the substance itself serves as the fluid in the auxiliary manometer. A quantity of the liquid, sufficient to fill the bulb and the short leg of the manometer section, is put in the isoteniscope. The isoteniscope is attached to a vacuum system and evacuated, then filled by nitrogen. The evacuation and purge of the system is repeated twice to remove residual oxygen. The filled isoteniscope is placed in an horizontal position so that the sample spreads out into a thin layer in the sample bulb and manometer section U-part.
The pressure of the system is reduced to Pa and the sample gently warmed until it just boils removal of dissolved fixed gases. The isoteniscope is then placed so that the sample returns to the bulb and short leg of the manometer, so that both are entirely filled with liquid. The pressure is maintained as for degassing; the drawn-out tip of the sample bulb is heated with a small flame until sample vapour released expands sufficiently to displace part of the sample from the upper part of the bulb and manometer arm into the manometer section of the isoteniscope, creating a vapour-filled, nitrogen-free space.
The isoteniscope is then placed in a constant temperature bath, and the pressure of nitrogen is adjusted until its pressure equals that of the sample. Pressure balance is indicated by the manometer section of the isoteniscope. At the equilibrium, the vapour pressure of nitrogen equals the vapour pressure of the substance. In the case of solids, depending on the pressure and temperature range, the manometer liquids listed in 1. The degassed manometer liquid is filled into a bulge on the long arm of the isoteniscope.
Then the solid to be investigated is placed in the bulb and is degassed at elevated temperature. After that the isoteniscope is inclined so that the manometer liquid can flow into the U-tube. The measurement of vapour pressure as a function of temperature is done according to 1. Various versions of the apparatus are described in the literature 1. The apparatus described here illustrates the general principle involved figure 4. Figure 4 shows the main components of the apparatus, comprising a high-vacuum stainless steel or glass container, equipment to produce and measure a vacuum and built-in components to measure the vapour pressure on a balance.
The following built-in components are included in the apparatus:. The evaporator furnace is a cylindrical vessel, made of e. A glass vessel with a copper wall can also be used. There are between one and three openings of different sizes for the vapour stream. The furnace is heated either by a heating spiral around the outside. To prevent heat being dissipated to the base plate, the heater is attached to the base plate by a metal with low thermal conductivity nickel-silver or chromium-nickel steel , e. This arrangement has the advantage of allowing the introduction of a copper bar.
This allows cooling from the outside using a cooling bath,. Large openings are used for low vapour pressure and vice versa. By rotating the furnace the desired opening or an intermediate position in the vapour stream furnace opening — shield — balance pan can be set and the stream of molecules is released or deflected through the furnace opening onto the scale pan. In order to measure the temperature of the substance, a thermocouple or resistance thermometer is placed at a suitable point,.
Gold-plated aluminium is a suitable material,. Depending on the type of balance, it has openings for the balance beam and a shield opening for the stream of molecules and should guarantee complete condensation of the vapour on the balance pan. Heat dissipation to the outside is ensured e. The bar is routed through the base plate and thermally insulated from it, e.
The bar is immersed in a Dewar flask containing liquid nitrogen under the base plate or liquid nitrogen is circulated through the bar. The balance pan is cooled exclusively by radiation and is satisfactory for the pressure range under investigation cooling approximately 1 hour before the start of measurement ,. Suitable balances are e. The pressure is regulated with a suitable ionisation manometer.
The vessel is filled with the test substance and the lid is closed. The shield and refrigeration box are slid across the furnace. The apparatus is closed and the vacuum pumps are switched on. The final pressure before starting to take measurements should be approximately 10 -4 Pa. Cooling of the refrigeration box starts at 10 -2 Pa. Once the required vacuum has been obtained, start the calibration series at the lowest temperature required. The corresponding opening in the lid is set, the vapour stream passes through the shield directly above the opening and strikes the cooled balance pan.
The balance pan must be big enough to ensure that the entire stream guided through the shield strikes it. The momentum of the vapour stream acts as a force against the balance pan and the molecules condense on its cool surface. The momentum and simultaneous condensation produce a signal on the recorder. Evaluation of the signals provides two pieces of information:.
Geometrical factors such as the furnace opening and the angle of the molecular stream must be taken into account when evaluating the readings;. The vapour pressure can also be calculated from the rate of evaporation and molecular weight using the Hertz equation 2. After the necessary vacuum is reached, the series of measurements is commenced at the lowest desired measuring temperature. For further measurements, the temperature is increased by small intervals until the maximum desired temperature value is reached. The sample is then cooled again and a second curve of the vapour pressure may be recorded.
If the second run fails to confirm the results of the first run, then it is possible that the substance may be decomposing in the temperature range being measured. An electrically heated, aluminium vacuum tank with four stainless steel effusion cells is shown in figure 5 for example. The reference and test substances are filled into each effusion cell, the metal diaphragm with the orifice is secured by the threaded lid, and each cell is weighed to within an accuracy of 0,1 mg.
The cell is placed in the thermostated apparatus, which is then evacuated to below one tenth of the expected pressure. At defined intervals of time ranging from 5 to 30 hours, air is admitted into the apparatus, and the loss in mass of the effusion cell is determined by reweighing. In order to ensure that the results are not influenced by volatile impurities, the cell is reweighed at defined time intervals to check that the evaporation rate is constant over at least two such intervals of time. The correction factor K depends on the ratio of length to radius of the cylindrical orifice:.
This effusion cell constant E may be determined with reference substances 2,9 , using the following equation:. A typical apparatus used to perform this test comprises a number of components given in figure 6a and described below 1. The carrier gas must not react chemically with the test substance. Nitrogen is usually sufficient for this purpose but occasionally other gases may be required The gas employed must be dry see figure 6a, key 4: relative humidity sensor.
A suitable gas-control system is required to ensure a constant and selected flow through the saturator column. These are dependent on the particular sample characteristics and the chosen method of analysis. The vapour should be trapped quantitatively and in a form which permits subsequent analysis. For some test substances, traps containing liquids such as hexane or ethylene glycol will be suitable. For others, solid absorbents may be applicable. As an alternative to vapour trapping and subsequent analysis, in-train analytical techniques, like chromatography, may be used to determine quantitatively the amount of material transported by a known amount of carrier gas.
Furthermore, the loss of mass of the sample can be measured. For measurements at different temperatures it may be necessary to include a heat-exchanger in the assembly. The test substance is deposited from a solution onto a suitable inert support. The coated support is packed into the saturator column, the dimensions of which and the flow rate should be such that complete saturation of the carrier gas is ensured. The saturator column must be thermostated.
For measurements above room temperature, the region between the saturator column and the traps should be heated to prevent condensation of the test substance. In order to lower the mass transport occurring by diffusion, a capillary may be placed after the saturator column figure 6b. A solution of the test substance in a highly volatile solvent is added to a suitable amount of support. Sufficient test substance should be added to maintain saturation for the duration of the test.
The solvent is totally evaporated in air or in a rotary evaporator, and the thoroughly mixed material is added to the saturator column. After thermostating the sample, dry nitrogen is passed through the apparatus. The traps or in-train detector are connected to the column effluent line and the time recorded. The flow rate is checked at the beginning and at regular intervals during the experiment, using a bubble meter or continuously with a mass flow-meter.
The pressure at the outlet to the saturator must be measured. This may be done either:. The time required for collecting the quantity of test substance that is necessary for the different methods of analysis is determined in preliminary runs or by estimates. As an alternative to collecting the substance for further analysis, in-train quantitative analytical technique may be used e. Before calculating the vapour pressure at a given temperature, preliminary runs are to be carried out to determine the maximum flow rate that will completely saturate the carrier gas with substance vapour.
This is guaranteed if the carrier gas is passed through the saturator sufficiently slowly so that a lower rate gives no greater calculated vapour pressure. The specific analytical method will be determined by the nature of the substance being tested e. The quantity of substance transported by a known volume of carrier gas is determined. Measured volumes must be corrected for pressure and temperature differences between the flow meter and the thermostated saturator. If the flow meter is located downstream from the vapour trap, corrections may be necessary to account for any vaporised trap ingredients 1.
The spinning rotor technique can be carried out using a spinning rotor viscosity gauge as shown in figure 8. A schematic drawing of the experimental set-up is shown in figure 7. A high-vacuum pump device is connected to the system by means of high-vacuum valves. The ball is suspended and stabilised in a magnetic field, generally using a combination of permanent magnets and control coils. The ball is made to spin by rotating fields produced by coils. Pick-up coils, measuring the always present low lateral magnetisation of the ball, allow its spinning rate to be measured.
When the ball has reached a given rotational speed v o usually about revolutions per second , further energising is stopped and deceleration takes place, due to gas friction. The drop of rotational speed is measured as a function of time. As the friction caused by the magnetic suspension is negligible as compared with the gas friction, the gas pressure p is given by:. The average speed of the gas molecule is given by:. The vapour pressure from any of the preceding methods should be determined for at least two temperatures.
If a transition change of state, decomposition is observed, the following information should be noted:. Ambrose, D. Vodar, Eds. ASTM D , Standard test method for vapour pressure-temperature relationship and initial decomposition temperature of liquids by isoteniscope. Messer, P. Rohl, G. Grosse and W. A , , 'Vol. Thermodynamics , vol. Comsa, J. Fremerey and B. The vapour pressure of liquids and solids can be estimated by use of the modified Watson Correlation a.
The only experimental data required is the normal boiling point. The method is applicable over the pressure range from 10 5 Pa to 10 -5 Pa. For several compound types, K F factors are listed in reference b. Quite often, data are available in which a boiling point at reduced pressure is given. In such a case, according to b , the vapour pressure is calculated as follows:. When using the estimation method, the report shall include a comprehensive documentation of the calculation.
Lyman, W. Reehl, D. Apparatus for determining the vapour pressure curve according to the dynamic method. Apparatus for determining the vapour pressure curve according to the static method using a U-tube manometer. Apparatus for determining the vapour pressure curve according to the static method using a pressure indicator.
Apparatus for determining the vapour pressure curve according to the vapour pressure balance method. An example of a flow system for the determination of vapour pressure by the gas saturation method. An example of system for the determination of vapour pressure by the gas saturation method, with a capillary placed after the saturation chamber.
Example of the experimental set-up for spinning rotor method. The described methods are to be applied to the measurement of the surface tension of aqueous solutions. It is useful to have preliminary information on the water solubility, the structure, the hydrolysis properties and the critical concentration for micelles formation of the substance before performing these tests. The following methods are applicable to most chemical substances, without any restriction in respect to their degree of purity. The measurement of the surface tension by the ring tensiometer method is restricted to aqueous solutions with a dynamic viscosity of less than approximately mPa s.
The free surface enthalpy per unit of surface area is referred to as surface tension. Reference substances which cover a wide range of surface tensions are given in references 1 and 3. The methods are based on the measurement of the maximum force which is necessary to exert vertically, on a stirrup or a ring in contact with the surface of the liquid being examined placed in a measuring cup, in order to separate it from this surface, or on a plate, with an edge in contact with the surface, in order to draw up the film that has formed.
These methods are capable of greater precision than is likely to be required for environmental assessment. A solution of the substance is prepared in distilled water. Commercially available tensiometers are adequate for this measurement. They consist of the following elements:. Mobile sample table. The mobile sample table is used as a support for the temperature-controlled measurement vessel holding the liquid to be tested. Together with the force measuring system, it is mounted on a stand. Force measuring system.
The force measuring system see figure is located above the sample table. Measuring body ring. The wire ring is suspended horizontally from a metal pin and a wire mounting bracket to establish the connection to the force measuring system see figure. Measurement vessel. The measurement vessel holding the test solution to be measured shall be a temperature-controlled glass vessel.
It shall be designed so that during the measurement the temperature of the test solution liquid and the gas phase above its surface remains constant and that the sample cannot evaporate. Glass vessels shall be cleaned carefully. The ring shall first be rinsed thoroughly in water to remove any substances which are soluble in water, briefly immersed in chromo-sulphuric acid, washed in double-distilled water until a neutral reaction is obtained and finally heated briefly above a methanol flame.
Contamination by substances which are not dissolved or destroyed by chromo-sulphuric acid or phosphoric acid, such as silicones, shall be removed by means of a suitable organic solvent. Calibration of the apparatus. The apparatus shall be levelled, for instance by means of a spirit level on the tensiometer base, by adjusting the levelling screws in the base.
After mounting the ring on the apparatus and prior to immersion in the liquid, the tensiometer indication shall be adjusted to zero and the ring checked for parallelism to the liquid surface. For this purpose, the liquid surface can be used as a mirror. The actual test calibration can be accomplished by means of either of two procedures:. This procedure is accomplished faster than the weight calibration but there is always the danger that the surface tension of the water is falsified by traces of contamination by surfactants.
Aqueous solutions shall be prepared of the substances to be tested, using the required concentrations in water, and shall not contain any non-dissolved substances. Since the surface tension of a solution in the measurement vessel alters over a period of time, several measurements shall be made at various times and a curve plotted showing surface tension as a function of time. When no further change occurs, a state of equilibrium has been reached.
Dust and gaseous contamination by other substances interfere with the measurement. The work shall therefore be carried out under a protective cover. The solutions to be measured shall be transferred to the carefully cleaned measurement vessel, taking care to avoid foaming, and subsequently the measurement vessel shall be placed onto the table of the test apparatus.
The table-top with measurement vessel shall be raised until the ring is immersed below the surface of the solution to be measured. The liquid layer attached to the ring must not separate from the ring. After completing the measurements, the ring shall be immersed below the surface again and the measurements repeated until a constant surface tension value is reached. The time from transferring the solution to the measurement vessel shall be recorded for each determination. Readings shall be taken at the maximum force required to detach the ring from the liquid surface.
This will yield a value which applies only approximately and therefore requires correction. Harkins and Jordan 4 have empirically determined correction factors for surface-tension values measured by the ring method which are dependent on ring dimensions, the density of the liquid and its surface tension. Since it is laborious to determine the correction factor for each individual measurement from the Harkins and Jordan tables, in order to calculate the surface tension for aqueous solutions the simplified procedure of reading the corrected surface-tension values directly from the table may be used.
Interpolation shall be used for readings ranging between the tabular values. This table has been compiled on the basis of the Harkins-Jordan correction. The table provides corrected values for surface-tension measurements taken after calibration with weights or calibration with water. Alternatively, without the preceding calibration, the surface tension call can be calculated according to the following formula:.
It is useful to have preliminary information on the structural formula, the vapour pressure, the dissociation constant and the hydrolysis as a function of pH of the substance to perform this test. The two test methods described below cover the whole range of solubilities but are not applicable to volatile substances:.
The water solubility of the test substance can be considerably affected by the presence of impurities. The solubility in water of a substance is specified by the saturation mass concentration of the substance in water at a given temperature. The solubility in water is specified in units of mass per volume of solution.
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The approximate amount of the sample and the time necessary to achieve the saturation mass concentration should be determined in a simple preliminary test. This method is based on the elution of a test substance with water from a micro-column which is charged with an inert support material, such as glass beads or sand, coated with an excess of test substance. The water solubility is determined when the mass concentration of the eluate is constant.
This is shown by a concentration plateau as a function of time. In this method, the substance solids must be pulverised is dissolved in water at a temperature somewhat above the test temperature. When saturation is achieved the mixture is cooled and kept at the test temperature, stirring as long as necessary to reach equilibrium. Alternatively, the measurement can be performed directly at the test temperature, if it is assured by appropriate sampling that the saturation equilibrium is reached.
Subsequently, the mass concentration of the substance in the aqueous solution, which must not contain any undissolved particles, is determined by a suitable analytical method.
This depends upon the method of analysis, but mass concentration determinations down to 10 -6 grams per litre can be determined. The chosen temperature should be kept constant in all relevant parts of the equipment. After each addition of the indicated amount of water, the mixture is shaken vigorously for 10 minutes and is visually checked for any undissolved parts of the sample.
If, after addition of 10 ml of water, the sample or parts of it remain undissolved, the experiment has to be repeated in a ml measuring cylinder with larger volumes of water. At lower solubilities the time required to dissolve a substance can be considerably longer at least 24 h should be allowed. The approximate solubility is given in the table under that volume of added water in which complete dissolution of the sample occurs. If the substance is still apparently insoluble, more than 24 h should be allowed 96 h maximum , or further dilution should be undertaken to ascertain whether the column elution or flask solubility method should be used.
The support material for the column elution method should be inert. Possible materials which can be employed are glass beads and sand. A suitable volatile solvent of analytical reagent quality should be used to apply the test substance to the support material. Water which has been double distilled in glass or quartz apparatus should be employed as the eluent. A suitable, weighed amount of test substance is dissolved in the chosen solvent. An appropriate amount of this solution is added to the support material.
The solvent must be completely evaporated, e. The loading of support material may cause problems erroneous results if the test substance is deposited as an oil or a different crystal phase. The problem should be examined experimentally and the details reported. The loaded support material is allowed to soak for about two hours in approximately 5 ml of water, and then the suspension is added to the microcolumn.
Alternatively, dry loaded support material may be poured into the microcolumn, which has been filled with water, and then equilibrated for approximately two hours. The elution of the substance from the support material can be carried out in one of two different ways:. A schematic arrangement of a typical system is presented in figure 1. A suitable microcolumn is shown in figure 2, although any size is acceptable, provided it meets the criteria for reproducibility and sensitivity.
The column should provide for a headspace of at least five bed volumes of water and be able to hold a minimum of five samples. Alternatively, the size can be reduced if make-up solvent is employed to replace the initial five bed volumes removed with impurities. The pump is connected with polytetrafluoroethylene P. The column and pump, when assembled, should have provision for sampling the effluent and equilibrating the headspace at atmospheric pressure. The flow through the column is started. The first five bed volumes minimum are discarded to remove water-soluble impurities.
These samples should be separated from each other by time intervals corresponding to the passage of at least 10 bed volumes of the eluent. Apparatus see figures 4 and 3. Levelling vessel: the connection to the levelling vessel is made by using a ground glass joint which is connected by PTFE tubing. Successive eluate fractions should be collected and analysed by the chosen method. In both cases using a recirculating pump or a levelling vessel , a second run is to be performed at half the flow rate of the first. If the results of the two runs are in agreement, the test is satisfactory; if there is a higher apparent solubility with the lower flow rate, then the halving of the flow rate must continue until two successive runs give the same solubility.
In both cases using a recirculating pump or a levelling vessel the fractions should be checked for the presence of colloidal matter by examination for the Tyndall effect light scattering. Presence of such particles invalidates the results, and the test should be repeated with improvements in the filtering action of the column. The pH of each sample should be recorded.
A second run should be performed at the same temperature. The quantity of material necessary to saturate the desired volume of water is estimated from the preliminary test. The volume of water required will depend on the analytical method and the solubility range. About five times the quantity of material determined above is weighed into each of three glass vessels fitted with glass stoppers e. The chosen volume of water is added to each vessel, and the vessels are tightly stoppered.
A shaking or stirring device capable of operating at constant temperature should be used, e. After one day, one of the vessels is removed and re-equilibrated for 24 hours at the test temperature with occasional shaking. The contents of the vessel are then centrifuged at the test temperature, and the concentration of test substance in the clear aqueous phase is determined by a suitable analytical method.
If the concentration results from at least the last two vessels agree with the required reproducibility, the test is satisfactory. The whole test should be repeated, using longer equilibration times, if the results from vessels 1, 2 and 3 show a tendency to increasing values. In order to estimate the rate of establishment of the saturation equilibrium, samples are taken until the stirring time no longer influences the concentration of the test solution.
A substance-specific analytical method is preferred for these determinations, since small amounts of soluble impurities can cause large errors in the measured solubility. Examples of such methods are: gas or liquid chromatography, titration methods, photometric methods, voltammetric methods.
The mean value from at least five consecutive samples taken from the saturation plateau should be calculated for each run, as should the standard deviation. The results should be given in units of mass per volume of solution. Column elution method with recirculating pump. Column elution method with levelling vessel. It is useful to have preliminary information on structural formula, dissociation constant, water solubility, hydrolysis, n-octanol solubility and surface tension of the substance to perform this test.
Measurements should be made on ionisable substances only in their non-ionised form free acid or free base produced by the use of an appropriate buffer with a pH of at least one pH unit below free acid or above free base the pK. This test method includes two separate procedures: the shake flask method and high performance liquid chromatography HPLC. The former is applicable when the log P ow value see below for definitions falls within the range - 2 to 4 and the latter within the range 0 to 6. Before carrying out either of the experimental procedures a preliminary estimate of the partition coefficient should first be obtained.
The shake-flask method applies only to essentially pure substances soluble in water and n-octanol. It is not applicable to surface active materials for which a calculated value or an estimate based on the individual n-octanol and water solubilities should be provided. The HPLC method is not applicable to strong acids and bases, metal complexes, surface-active materials or substances which react with the eluent.
For these materials, a calculated value or an estimate based on individual n-octanol and water solubilities should be provided. The HPLC method is less sensitive to the presence of impurities in the test compound than is the shake-flask method. Nevertheless, in some cases impurities can make the interpretation of the results difficult because peak assignment becomes uncertain. For mixtures which give an unresolved band, upper and lower limits of log P should be stated.
The partition coefficient P is defined as the ratio of the equilibrium concentrations c i of a dissolved substance in a two-phase system consisting of two largely immiscible solvents. In the case n-octanol and water:. The partition coefficient P therefore is the quotient of two concentrations and is usually given in the form of its logarithm to base 10 log P. In order to correlate the measured HPLC data of a compound with its P value, a calibration graph of log P versus chromatographic data using at least six reference points has to be established.
It is for the user to select the appropriate reference substances. Whenever possible, at least one reference compound should have a P ow above that of the test substance, and another a P ow below that of the test substance. For log P values less than 4, the calibration can be based on data obtained by the shake-flask method. For log P values greater than 4, the calibration can be based on validated literature values if these are in agreement with calculated values. For better accuracy, it is preferable to choose reference compounds which are structurally related to the test substance.
Extensive lists of values of log P ow for many groups of chemicals are available 2 3. If data on the partition coefficients of structurally related compounds are not available, then a more general calibration, established with other reference compounds, may be used. A list of recommended reference substances and their P ow values is given in Appendix 2. In order to determine a partition coefficient, equilibrium between all interacting components of the system must be achieved, and the concentrations of the substances dissolved in the two phases must be determined.
A study of the literature on this subject indicates that several different techniques can be used to solve this problem, i. HPLC is performed on analytical columns packed with a commercially available solid phase containing long hydrocarbon chains e. C 8 , C 18 chemically bound onto silica. Chemicals injected onto such a column move along it at different rates because of the different degrees of partitioning between the mobile phase and the hydrocarbon stationary phase.
Mixtures of chemicals are eluted in order of their hydrophobicity, with water-soluble chemicals eluted first and oil-soluble chemicals last, in proportion to their hydrocarbon-water partition coefficient. The partition coefficient is deduced from the capacity factor k, given by the expression:. Quantitative analytical methods are not required and only the determination of elution times is necessary. In order to assure the accuracy of the partition coefficient, duplicate determinations are to be made under three different test conditions, whereby the quantity of substance specified as well as the ratio of the solvent volumes may be varied.
In order to increase the confidence in the measurement, duplicate determinations must be made. The measuring range of the method is determined by the limit of detection of the analytical procedure. This should permit the assessment of values of log P ow in the range of - 2 to 4 occasionally when conditions apply, this range may be extended to log P ow up to 5 when the concentration of the solute in either phase is not more than 0,01 mol per litre. Typical correlations can be found in the literature 4 5 6 7 8.
Higher accuracy can usually be achieved when correlation plots are based on structurally-related reference compounds 9. The Nernst Partition Law applies only at constant temperature, pressure and pH for dilute solutions. It strictly applies to a pure substance dispersed between two pure solvents. If several different solutes occur in one or both phases at the same time, this may affect the results. Dissociation or association of the dissolved molecules result in deviations from the Nernst Partition Law.
Such deviations are indicated by the fact that the partition coefficient becomes dependent upon the concentration of the solution. Because of the multiple equilibria involved, this test method should not be applied to ionisable compounds without applying a correction. The use of buffer solutions in place of water should be considered for such compounds; the pH of the buffer should be at least 1 pH unit from the pKa of the substance and bearing in mind the relevance of this pH for the environment. The partition coefficient is estimated preferably by using a calculation method see Appendix 1 , or where appropriate, from the ratio of the solubilities of the test substance ill the pure solvents Water: water distilled or double distilled in glass or quartz apparatus should be employed.
For ionisable compounds, buffer solutions in place of water should be used if justified. Pre-saturation of the solvents. Before a partition coefficient is determined, the phases of the solvent system are mutually saturated by shaking at the temperature of the experiment. To do this, it is practical to shake two large stock bottles of high purity analytical grade n-octanol or water each with a sufficient quantity of the other solvent for 24 hours on a mechanical shaker and then to let them stand long enough to allow the phases to separate and to achieve a saturation state.
Preparation for the test. The entire volume of the two-phase system should nearly fill the test vessel. This will help prevent loss of material due to volatilisation. The volume ratio and quantities of substance to be used are fixed by the following:. Three tests are carried out. In the first, the calculated volume ratio of n-octanol to water is used; in the second, this ratio is divided by two; and in the third, this ratio is multiplied by two e. A stock solution is prepared in n-octanol pre-saturated with water. The concentration of this stock solution should be precisely determined before it is employed in the determination of the partition coefficient.
This solution should be stored under conditions which ensure its stability. Establishment of the partition equilibrium. Duplicate test vessels containing the required, accurately measured amounts of the two solvents together with the necessary quantity of the stock solution should be prepared for each of the test conditions. The n-octanol phases should be measured by volume. The test vessels should either be placed in a suitable shaker or shaken by hand. When using a centrifuge tube, a recommended method is to rotate the tube quickly through o about its transverse axis so that any trapped air rises through the two phases.
Experience has shown that 50 such rotations are usually sufficient for the establishment of the partition equilibrium. To be certain, rotations in five minutes are recommended. When necessary, in order to separate the phases, centrifugation of the mixture should be carried out. This should be done in a laboratory centrifuge maintained at room temperature, or, if a non-temperature controlled centrifuge is used, the centrifuge tubes should be kept for equilibration at the test temperature for at least one hour before analysis.
For the determination of the partition coefficient, it is necessary to determine the concentrations of the test substance in both phases. This may be done by taking an aliquot of each of the two phases from each tube for each test condition and analyzing them by the chosen procedure. The total quantity of substance present in both phases should be calculated and compared with the quantity of the substance originally introduced.
The aqueous phase should be sampled by a procedure that minimises the risk of including traces of n-octanol: a glass syringe with a removable needle can be used to sample the water phase. The syringe should initially be partially filled with air. Air should be gently expelled while inserting the needle through the n-octanol layer.
An adequate volume of aqueous phase is withdrawn into the syringe. The syringe is quickly removed from the solution and the needle detached. The contents of the syringe may then be used as the aqueous sample. The concentration in the two separated phases should preferably be determined by a substance-specific method.
Examples of analytical methods which may be appropriate are:. A liquid chromatograph, fitted with a pulse-free pump and a suitable detection device, is required. The use of an injection valve with injection loops is recommended. The presence of polar groups in the stationary phase may seriously impair the performance of the HPLC column. Therefore, stationary phases should have the minimal percentage of polar groups Commercial microparticulate reverse-phase packings or ready-packed columns can be used.
A guard column may be positioned between the injection system and the analytical column. Isocratic elution should be employed. For compounds of high log P it may be necessary to shorten the elution time and those of the reference compounds by decreasing the polarity of the mobile phase or the column length.
Substances with very low solubility in n-octanol tend to give abnormally low log P ow values with the HPLC method; the peaks of such compounds sometimes accompany the solvent front. This is probably due to the fact that the partitioning process is too slow to reach the equilibrium in the time normally taken by an HPLC separation. Test and reference compounds should be soluble in the mobile phase in sufficient concentrations to allow their detection. Only in exceptional cases may additives be used with the methanol-water mixture, since additives will change the properties of the column.
For chromatograms with additives it is mandatory to use a separate column of the same type. If methanol-water is not appropriate, other organic solvent-water mixtures call be used, e. The pH of the eluent is critical for ionisable compounds. It should be within the operating pH range of the column, which is usually between 2 and 8.
Buffering is recommended. HPLC measurements with silica-based stationary phases above pH 8 are not advisable since the use of an alkaline, mobile phase may cause rapid deterioration in the performance of the column. The reference compounds should be the purest available. Compounds to be used for test or calibration purposes are dissolved in the mobile phase if possible. The dead time to can be determined by using either a homologous series e. For calculating the dead time to by using a homologous series, a set of at least seven members of a homologous series is injected and the respective retention times are determined.
The dead time to is then given by:. The next step is to construct a correlation plot of log k values versus log p for appropriate reference compounds. In practice, a set of between 5 and 10 standard reference compounds whose log p is around the expected range are injected simultaneously and the retention times are determined, preferably on a recording integrator linked to the detection system.
The corresponding logarithms of the capacity factors, log k, are calculated and plotted as a function of the log p determined by the shake-flask method. The calibration is performed at regular intervals, at least once daily, so that possible changes in column performance can be allowed for. The test substance is injected in as small a quantity of mobile phase as possible. The retention time is determined in duplicate , permitting the calculation of the capacity factor k. From the correlation graph of the reference compounds, the partition coefficient of the test substance can be interpolated.
For very low and very high partition coefficients, extrapolation is necessary. In those cases particular care has to be taken of the confidence limits of the regression line. The reliability of the determined values of P can be tested by comparison of the means of the duplicate determinations with the overall mean. If there is a suggestion of concentration dependency of the partition coefficient, this should be noted in the report,.
Hansch and A. Hansch, chairman, A. Leo, dir. Leo, C. Hansch and D. Elkins, Partition coefficients and their uses. Chemical products for industrial use — Determination of partition coefficient — Flask shaking method. Hansch, A. Leo, S. Unger, K. Kim, D. Nikaitani and E. Lien, J. Neely, D. Branson and G. Blau, Environ. Seydel and K. Nirrlees, S. Noulton, C. Murphy, P. Taylor; J. A general introduction to calculation methods, data and examples are provided in the Handbook of Chemical Property Estimation Methods a.
Preliminary estimate of the partition coefficient. The value of the partition coefficient can be estimated by the use of the solubilities of the test substance in the pure solvents: For this:. Principle of the calculation methods. All calculation methods are based on the formal fragmentation of the molecule into suitable substructures for which reliable log P ow -increments are known. The log P ow of the whole molecule is then calculated as the sum of its corresponding fragment values plus the sum of correction terms for intramolecular interactions.
Lists of fragment constants and correction terms ate available b c d e ;. Some are regularly updated b. In general, the reliability of the calculation method decreases with increasing complexity of the compound under study. In the case of simple molecules with low molecular weight and one or two functional groups, a deviation of 0,1 to 0,3 log P ow units between the results of the different fragmentation methods and the measured value can be expected. In the case of more complex molecules the margin of error can be greater. This will depend on the reliability and availability of fragment constants, as well as on the ability to recognise intramolecular interactions e.
In the case of ionising compounds the correct consideration of the charge or degree of ionisation is important. They are used for the calculation of log P ow for aromatic molecules or substructures. According to Rekker g the log P ow value is calculated as follows:. The correction terms can be expressed as an integral multiple of one single constant C m so-called magic constant.
The determination of the interaction terms is carried out according to set rules described in the literature e h i. According to Hansch and Leo c , the log P ow value is calculated from:. Derived from experimental P ow values, a list of atomic and group fragmental values and a list of correction terms F j so-called factors were determined by trial and error.
The correction terms have been ordered into several different classes a c. It is relatively complicated and time consuming to take into account all the rules and correction terms. Journal of animal Research 5 4 : Seroprevalence of Newcastle disease, chicken infectious anemia and avian influenza in indigenous chickens in Grenada, West Indies.
Journal of Animal Research 5 1 : Seroprevalence of Neospora caninum in sheep and goats from Grenada, West Indies. Open J vet Med. Seroprevalence of Toxoplasma gondii in sheep and goats in Grenada, West Indies. Serological evidence of antibodies to Neospora caninum in stray and owned Grenadian dogs. Tropical Biomedicine. Open Journal of Veterinary Medicine 5: Prevalence and antimicrobial resistance of Campylobacter species isolated from backyard chickens in Grenada.
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Journal of Reproductive Medicine. Benjamin Hobson : His work as a medical missionary and influence on the practice of medicine and knowledge of anatomy in China and Japan. Clin Anat. Johann Conrad Brunner and the first description of syringomyelia. Childs Nerv Syst. Friedrich Trendelenburg: historical background and significant medical contributions. Fat herniation through the canal of Schwalbe. Folia Morphol 73 4 A review of the anatomy and clinical significance of adrenal veins. Medical Science Monitor , 20 A historical perspective: Bernhard von Langenbeck German surgeon Cesmebasi A.
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Common cancers in centenarians. Kapur S. Malignant mesenteric perivascular epithelioid cell neoplasm presenting as an intra-abdominal fistula in a year-old female Case Reports in Oncological Medicine Article Number The topographic anatomy of the masseteric nerve: A cadaveric study with an emphasis on the effective zone of botulinum toxin A injections in masseter.
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Minor revisions. Vet Srg May Photobacterium damselae in bigeye scad, Selar crumenophthalmus Bloch, from Grenada, West Indies. Journal of Fish diseases. Kabuusu, A. Open J. Vet Med. Kalchofner Guererro, KS. Tierarztliche Praxis in press. Mycobacterium avium subspecies para tuberculosis confirmed following serological surveillance of small ruminants in Grenada, West Indies Journal of Veterinary diagnostic investigation 25 4 Macpherson CNL. Epidemiology and Public Health importance of toxocariasis: a zoonosis of global importance. CAB Reviews, 8: Int J Parasitol, 43 Immunol microbial Infect Dis.
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