Apparent octanol/water partition coefficients of pentachlorophenol as a function of pH

KLAUS L. E. KAISER AND ILZE VALDMANIS E~zuiron~nental Contarninants Division, National Water Research Institute, bur ling tor^. Ont., Canada L7R 4A6

Received October 6, 1981

KLAUS L. E. KAISER and ILZE VALDMANIS. Can. J. Chem. 60,2104 (1982). The apparent 1-octanol/water partition coefficient (log PAP,) of pentachlorophenol (PCP) varies in non-linear function with pH of

the aqueous solution. In the range of pH 1.2 to 13.5 extreme values of log PAP, 4.84 at pH 1.2 and log PA,, 1.3 at pH 10.5 were observed. In the alkaline regime, log PAP, increases strongly with the ionic strength. The ion-corrected partition coefficient of PCP was found to be log P 5.05 in good agreement with literature values.

KLAUS L. E. KAISER et ILZE VALDMANIS. Can. J. Chem. 60,2104 (1982). Le coefficient de partage apparent entre octanol-lleau, log Pap,, du pentachlorophtnol (PCP) varie d'une f a ~ o n non-lidaire avec

le pH de la solution aqueuse. Dans I'intervalle de pH de 1,2 13,5, les valeurs extrEmes sont log Pap, = 4,84 a un pH de 1,2 et de log Pap, = 1,3 a un pH de 10,5. En regime alcalin, le log Pap, augmente fortement avec la force ionique. Pour le PCP, on a trouvt que le coefficient de partage corrige pour les ions est le log P 5,05 qui est en bon accord avec les valeurs publiees dans le litttrature.

[Traduit par le journal]

Partition coefficients (log P) of organic solutes in gas chromatography on a 3% Dexsil-300 column at 175'C with an the l-octanol/water system are very useful electron capture detector. Other solvents were of pesticide

grade, acetic anhydride (ANALAR, double distilled), sulfuric physicochemical parameters for the correlation acid (ARISTAR). and Drediction of the solutes' biological activities (1). The fungicidal properties of pen~chlorophenol Standards and procedure (PCP) and its resistance to degradation make it an Octan01-Saturated water and water-saturated octanol were

prepared by stimng mixtures of 200 mL octanol with 10 mL important wood preservative (2)' it water and of 200 mL water with 2 mL octanol, respectively, for is frequently found to be present in industrial waste 1 h at high speed, centrifugating each mixture for 1.5 h at 2300 water effluents and has been observed as contami- rpm and removing the excess octanol, respectively water, with nant of water, sediment, and biota of many lakes a Pasteur pipette.

and rivers (2, 3). A 250 mL stock solution of pentachlorophenol (PCP) in water-saturated octanol was prepared with a PCP concentration The rates of biological magnification of conta- of l.o mglmL. This stock solution was diluted with

rninants correlate with their effective partition water-saturated l-octanol to stock solutions of 0.100 coefficients (4). For ionizing com~ounds, such as mnlmL PCP (A) and 0.010 m i l m ~ PCP ( B ) in octanol. As ~, - PCP, the effective or apparent partition cbefficient standards for the recovery tests: 10 mL volumes of 20.0 mglmL

(log PA,,) 1 is pH dependent. Therefore, it is of PCP (C) and 0.2 mglmL PCP (D) were also prepared. The pH of 2 L of octanol-saturated water was determined with

interest determine log P * ~ ~ for the wide range a pH meter and adjusted to the desired value with H,SO, or DH as found in the waste effluents and I-eceivinn NaOH solution. To each four of eight 200 mL aliauots in 250 mL waters. This paper reports on such measurements for PCP in 1-octanollwater over a range of pH 1.2 to 13.5.

Experimental Marerials

Pentachlorophenol (Aldrich, Gold Label, lot if081587 KC) was twice recrystallized from ethanollwater. 1-Octanol (MCB, Reagent Grade) was fractionated over a 0.6 m glass column; the distillate of bp 194 to 195°C was collected and found free of impurities by gas chromatography - flame ionization detector. Double distilled water was passed through a mixed bed of Amberlite XAD-4 and XAD8 resins to remove trace organic impurities. Purity was checked by extraction with hexane and

'Normally, the partition coefficients (log P) , also for ionizing compounds are determined for the non-ionized solute species only. The apparent partition coefficients (log P,,,), as reported here, represent the distribution of the sums of both ionized and non-ionized solute molecules between the two phases of the test system.

centrifuge flasks, 2.0 mL stock solution A, or i0.0 mL stock solution B were added. The flasks were covered with acetone-washed aluminum foil, stirred at high speed for 1 h, centrifugated at 2300 rpm for 1.5 h, and allowed to stand for 12 h. After removal of octanol layers with glass pipettes, the aqueous phases were transferred to 250 mL Erlenmeyer flasks and approximately 1.0g solid NaOH added to each. After dissolution, 1.5 mL acetic anhydride and 5.0 mL toluene were added to each. The flasks were covered with aluminum foil, sealed with screw top caps, stirred gently for 1 h, then at high speed for 0.2 h. The toluene layers were separated with glass pipettes and analyzed directly.

Analysis The toluene phases were analyzed for PCP-acetate by gas

chromatography of 1.0 p L volumes, in some cases after dilution with toluene, using a Tracor Microtek220 with a 1.8 m x 3.1 mm (i.d.) glass column packed with 3% Dexsil-300 on Gas Chrom 0 and 63Ni electron capture detector. The operating conditions were: carrier gas, prepurified N, at 66 mL/min flow; detector purge gas, 5% CH, in Ar at 10 mL/min flow; temperatures: injector, 240°C; column, 175°C isothermal; detector, 295°C. The

0008-40421821162 104-03$0 1 .OO/O 01982 National Research Council of CanadaIConseil national de recherches du Canada

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KAISER AND VALDMANIS 2105

chromatograms were recorded with both a Hewlett-Packard strip chart recorder and a Spectra-Physics SP-4000 data system.

In separate tests, the recovery of PCP as PCP-acetate was determined for several PCP concentrations in the aqueous phase. The recovery was found to be 35.5% with a standard deviation of 1.9%. All test data were corrected for this recovery rate and the concentrations of PCP in the octanol phases were determined by difference. As the determination of PA,, at each pH value, each octanol/water ratio, and each PCP concentration was made in duplicate, the mean of those data pairs were calculated for each test condition. The means and standard deviations reported in Table 1 are those of the four duplicate means at each pH level. The mean of the relative standard deviations of all 136 tests was found to be 10.7%.

Results and discussion The apparent octanollwater partition coeffi-

cients of PCP were determined at 16 selected pH values between pH 1.2 and 13.5. At each pH value, the four concentrations of 0.02,O. l,0.2, and 1.0 mg PCP per test system of 200 mL water and 2 or 10 mL octanol were tested in duplicate, In general, the duplicate measurements were within 5% of each other. Over the entire pH range, the apparent partition coefficient varies by more than three orders of magnitude between log PAP, 4.84 and 1.30. The means of the duplicate determinations for each pH value and test system are given in Table 1. Except possibly in the range of pH 1.2 to 4.7, no significant differences are found between the test systems with the lower ( A , B) and those with the higher ( C , D) PCP concentrations. This indicates

TABLE 1 . Apparent octanol/water partition coeffi- cient (log P,,,) of pentachlorophenol as function of

pH, PCP, and octanol concentrations

Test systema

pH A B C D Mean s.d.

1.2 4.74 4.81 4.83 4.99 4.84 0.11 2.4 4.68 4.64 4.77 4.79 4.72 0.07 3.4 4.56 4.52 4.76 4.64 4.62 0.11 4.7 4.51 4.26 4.90 4.62 4.57 0.27 5.9 3.82 3.68 3.68 3.78 3.72 0.07 6.5 3.55 3.38 3.57 3.74 3.56 0.15 7.2 3.34 3.36 3.23 3.36 3.32 0.06 7.8 3.27 3.11 3.17 3.23 3.20 0.07 8.4 3.03 3.11 3.04 3.23 3.10 0.09 8.9 2.74 2.81 2.71 2.74 2.75 0.04 9.3 1.66 1.19 1.58 1.37 1.45 0.21 9.8 1.34 1.07 1.70 1.33 1.36 0.26

10.5 1.43 1.31 1.40 1.07 1.30 0.16 10.5* 2.57 2.48 2.36 2.25 2.42 0.14 11.5 1.38 1.02 1.57 1.21 1.30 0.24 12.5 1.74 1.64 1.71 1.57 1.67 0.08 13.5 3.88 3.64 4.00 3.91 3.86 0.15

'Composition of the test systems in mg PCP, mL octanol (H,O presaturated), mL H,O (n-octanol presaturated): A , 0.02,2,200; B , 0.1, 10,200; C, 0.2, 2, 200; D, 1.0, 10,200.

bDeterminauon at elevated ionic strength of p = 1.0.

PENTACHLOROPHENOL meant s d

FIG. 1 . Apparent octanol/water partition coefficient of pentachlorophenol vs. pH of the aqueous phase. Means and standard deviations of eight determinations in four test systems at each pH value given. The single, high value at pH 10.5 observed at high ionic strength (p = 1.0).

the absence of any appreciable association of the solute molecules in the organic phase as generally known to occur at higher concentrations. There- fore, the results for the four test systems were combined to the means + s.d. at each of the pH values and are also given in Table 1.

As is to be expected, the apparent partition coefficient of PCP varies strongly with the pH of its aqueous solution. Figure 1 shows the means and standard deviations of log P vs. pH, using the data of Table 1. In the range of pH 1 to 5, the apparent partition coefficient declines slightly from log PAP, 4.84 to x4.5. At increasing values above pH 5, the apparent partition coefficient of PCP is declining more steeply, presumably this is caused by the ionization of this weak acid (pKa 4.8). At pH ~ 9 , another strong decrease of log PAP, is found for a relatively small increase in the pH. The cause of this drop, approximately 1.5 log P units, is not clear at this time. According to the general relation between ionization, pKa and pH (5), at pH 9, PCP is ionized in excess of 99.99% compared with 99.94% of pH 8. This minor change in ionization cannot account for the observed change in log PAP,. Hypotheses which may explain the phenomenon include a change in the specific PCP-octanol solvation complex (6) or the formation of PCP-sodium complexes. The apparent partition coefficient of PCP reaches a minimum of log PAP, x 1.3 at approximately pH 10.5. Further increase of pH results in a steeply increasing coefficient. At the highest pH investigated, pH 13.5, the partition

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2106 CAN. J. CHEM. VOL. 60, 1982

coefficient rose to log PAP, 3.9. This strong rise appears to result from the increase in ionic strength of the aqueous solution with pH. This interpreta- tion is supported by a separate determination of the partition coefficient at pH 10.5 at the higher ionic strength of p 1.0 which results in log PAP, x2 .4 compared to log PAP, 1.3 at p xO.01.

In literature, several values are reported for the ion-corrected partition coefficient of PCP. Values for the 1-octanol/water system are log P 5.01 and logP 5.12(1), logP5.86(7), and logP 3.81 (4). We observed the highest log PAP, values in the range of pH 3.4 to 1.2. In this pH range, results of the test system D (Table 1) are likely to be more accurate than those of A to C due to the analytical constraints as the PCP concentration in the aqueous phase approached 10-lo M (0.2 pg L-I). As it has been suggested to measure log P at least 4 pH units away from the pK values (I), it may be reasonable to calculate the partition coefficient for the non-ionized PCP for pH 0.8 from the data for the test system D at the three lowest pH values measured (Table 1). This extrapolation results in log P 5.05 which appears in good agreement with the data presently considered to be most reliable (1).

Our results demonstrate the strong dependence of the apparent partition coefficient of PCP on the pH and, in basic medium, on the ionic strength. As pH and ionic strengths of effluents containing PCP as well as those of the receiving waters vary widely, the environmental effects and pathways of ionizing

compounds such as PCP may vary accordingly. In particular, it would appear that the bioconcentra- tion factors, which generally correlate well with the partition coefficients (4, 8), could vary between different aquatic systems by several orders of magnitude. Specifically, the observed variation of log PAP, of PCP will affect fugacity calculations (9) and may also explain the disagreement between the experimental bioconcentration factor and that calculated from the partition coefficient of the un-ionized PCP moiety (8, 10).

1. C. HANSCH and A. J . LEO. Substituent constants for correlation analysis in chemistry and biology. John Wiley and Sons, New York, 1979 and references therein.

2. P. A. JONES. Chlorophenols and their impurities in the Canadian Environment. Environment Canada, Economic and Technical Review Report EPS 3-EC-81-2. 1981.

3. D. KONASEWICH, W. TRAVERSY, and H. ZAR. Great Lakes Water Quality. Status Report on organic and heavy metal contaminants in the Lakes Erie, Michigan, Huron and Superior basins to the Implementation Committee of the Great Lakes Water Quality Board. International Joint Commission. Windsor (1978).

4. P.-Y L u and R. L. METCALFE. Environ. Health Persp. 10, 269 (1975).

5. R. F. REKKER. The hydrophobic fragmental constant. Elsevier Scientific Publ. Co., Amsterdam. 1977.

6. Y. I. KORENMAN. RUSS. J. Phys. Chem. 46,42 (1972). 7. L. S. YAGUZHINSKII, E. G. SMIRNOVA, L. RATNIKOVA,

G. M. KOLESOVA, and I. P. KRASINSKAYA. J. Bioenerg. 5, 163 (1973).

8. D. MACKAY. Environ. Sci. Technol. 16, 274 (1982). 9. D. MACKAY and S. PATERSON. Environ. Sci Technol. 15,

1006 (1981). 10. G. D. VEITH, D. L. DEFOE, and B. V. BERGSTEDT. J. Fish.

Res. Board Can. 36, 1040 (1979).

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