ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
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Potentiometric Determination of Phenoxyalkylcarboxylic Acids in Pesticides N. Ciocan Central Laboratory for Pesticides Control, Sos. Afumati 11, Bucharest 72964, Romania
G. E. Baiulescu" National Institute of Chemistry, Spl. Independentei 202, Bucharest 77208, Romania
It is well known t h a t attachment of -OCH,COOH, -OCH(CH,)COOH, and -O(CH2)3COOH groups to aromatic rings leads to a n important class of herbicides. T h e repeated extractions of these herbicides with diethyl ether in acid medium, followed by the gravimetric and volumetric analyses, constitute a method used for the determination of phenoxyalkylcarboxylic acids in pesticide formulations. This method is recommended by F A 0 and CIPAC and used by all producers and users in spite of its disadvantages, such as lack of specificity, reduced sensitivity, consumation of a great quantity of ether, etc. T h e method and its various modifications (1-4) necessitate the use of macrosamples (10-80 g) and determinations of equivalent weights and melting points to prove the purity of the ethered extract. The analysis of pesticides that contain mixtures of phenoxyalkylcarboxylic acids with other extractable compounds in diethyl ether becomes even more complicated. We propose below a method that eliminates the disadvantages mentioned above.
EXPERIMENTAL Apparatus. The potential was measured vs. a double-junction Orion electrode (No. 90-02-00) with potassium nitrate solution in the other compartment using an Orion Research Digital Ionalyzer 801 A. The solutions were magnetically stirred. Membrane Electrode. As indicator electrode, a Hg?+ sensitive
extractive electrode was used. This electrode has been described elsewhere ( 5 , 6). Reagents. Analytical grade reagents were used, supplied by Chemapol: 2,4-dichlorophenoxyaceticacid (2,4-D),Bayer: 4chloro-2-methylphenoxyacetic acid (MCPA), Serva: 2,4,5-trichlorophenoxyacetic acid (2,4,5-T),VEB Electrochemisches Kombinat Bitterfeld: dimethylamine salt of 2,4-dichlorophenoxyacetic acid. All other reagents were from Reactivul Bucharest. The Hg2(N03)2solution, -0.1 N, was prepared by grinding mercurous nitrate with metallic mercury. Then 20 mL of 2 ru' "OB and 500 mL of distilled water were added. The solution was filtered and diluted to 1000 mL with distilled water. Metallic mercury was added as a preservative. Procedure. The following procedure was used for all determinations: weigh precisely A g of substance corresponding to 0.14.8 g of phenoxyalkylcarboxylic acid. Add 10-20 mL of methyl alcohol and 40-60 mL of distilled water. Neutralize with 0.1 N HN03 vs. methyl red (addition of excess acid leads to precipitation of organic acid). A solution of 0.1 N Hg2(?J03)2is added. The amount added depends on the amount of phenoxyacetic acid presumed to be in the sample. An excess of about 1-30 mL should be present. The less excess present, the more rapid the washing of the precipitate mentioned below. Leave the sample for 10 minutes with occasional shaking. Filter through wide pore filter paper and wash the precipitate with distilled water until complete elimination of Hg,*+ (the wash water does not react with KBr or diphenylcarbazonej. The precipitate is transferred quantitatively with 100-250 mL of 1-2 N HN03 into the beaker in which the
Table I. Analysis of 2-Methyl-4-chlorophenoxyacetic Acid (MCPA), 2,4-DichlorophenoxyaceticAcid (2,4-D), 2,4,5Trichlorophenoxyacetic Acid (2,4,5-T) and Dimethylamine Salt of 2,4-Dichlorophenoxyacetic Acid (DMA) Sample No. 1
2 3 4 5
MCPA, g Taken Found 0.1000 0.0998 0.1404 0.1408 0.1805 0.1800 0.2206 0.2206 0.2610 0.2607
2 , 4 - ~g, Taken Found 0.1105 0.1100 0.1547 0.1543 0.1989 0.1987 0.2431 0.2433 0.2873 0.2871
DMA, g Taken Found 0.1250 0.1247 0.1750 0.1754 0.2256 0.2250 0.2750 0.2750 0.3250 0.3257
2,4,5-T, g Taken Found 0.1010 0.2222 0.1414 0.2805 0.2930
0.1013 0.2220 0.1418 0.2811 0.2925
Table 11. Some of the Herbicides Analyzed by Potentiometric Titration (For Each Product Five Analyses Were Performed ) No.
Commercial name
Taken, g
Liquid
0.35-1.10
Active ingredients
Results
33% dimethylamine salt of 2,4-dichloro29.5 t 0.1 phenoxyacetic acid 40% ammonium salt of 2-methyl-4-chloro37.2 i 0.2 2 Dikotex 40b Liquid 0.30-1.20 phenoxyacetic acid 3 Butyrac 175c Liquid 0.50-1.90 23% dimethylamine salt of 4-(2,4-di23.1 i 0.1 ch1orophenoxy)butyric acid 4 Kurond Solid 0.30-0.80 45% 2-(2,4,5-trichlorophenoxy)45.0 t 0.1 propionic acid 24.8 + 0.15 5 BanvelMe Liquid 0.40-1.00 25% 2-methyl-4-chlorophenoxyaceticacid, 2% 3,6-dichloro-o-anisinic acid 6 Aniten Mf Liquid 0.40-1.20 250 g/L 2-methyl-4-chlorophenoxpaceticacid, 250 t 1.5 8 0 g/L 9-hydroxyfluoren-9-carboxylicacid 7 DM 68g Liquid 0.30-1.00 25% (4-chloro-2-ri~ethylphenoxy)-2-propionic acid. 25.0 r 0.1 13.5% 2,4-dinitro-6-tert-butyl phenol a Romanian product; Czechoslovakian product; Amchern, USA; Dow-Chemical, USA; e Chemie Linz, Austria; Cela Merck, W. Germany; g Pepro, France. 1
DMA'
State
0003-2700/78/0350-1407$01 .OO/O
1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
precipitation was done. Titrate potentiometrically with B mL of 0.1 N KBr standard solution. The phenoxyalkylcarboxylicacid content with the molecular weight, mol wt, is given by:
% Acid =
4
4
mol wt.B lOOA
RESULTS AND DISCUSSION T h e method is based on the fact that phenoxyalkylcarboxylic acids react with Hg?+ giving insoluble precipitates, (ArOCOO)pHg2. T h e determination of mercurous mercury contained in the precipitate leads to the evaluation of the concentration of phenoxyalkylcarboxylic acids in the sample. T h e titration curve allows a very accurate localization of the equivalence point, with potential jumps exceeding 150 mV (Figure 1). T h e method is particularily precise. As an illustration, Table I shows the results obtained with analytical standards. The method can be extended to the analysis of samples with a minimum content of 0.01 g of phenoxyalkylcarboxylic acid using 0.01 N KBr solutions as titrant. The method is applicable to the determination of phenoxyalkylcarboxylic acids in all commercial products. Table I1 presents some products analyzed using the method previously described as well as the analytical results. Halides, pseudohalides, sulfides, carbamates, amines, oxidants and reductants, different cations, etc. do not interfere. Organic acids that give precipitates with Hg2*+in slightly acidic medium (phthalic, oxalic) are not found in herbicides. Because of its selectivity, the method is applicable to the analysis of a wide range of herbicides containing phenoxyalkylcarboxylic acids alone or in a mixture with other acidic organic compounds extractable in diethyl ether. If the acid group is esterified, it becomes necessary to hydrolyze it beforehand with hydroxides (1-4).
2
4
6 r-
8 10 KBr
1s
14
Figure 1. Titration curve of mercurous 2.4-dichlorophenoxyacetate in 250 mL of 2 N HNO, with 0.1 N KBr Because of the performances of the method in which the precision, selectivity, and rapidity is concerned, it has been used by us to establish the quality of all commercial products containing phenoxyalkylcarboxylic acids.
LITERATURE CITED (1) F A 0 Phnt Prot. Bull., 11, NO. 2-5. 1963. (2) "CIPAC Handbook", G.R. Raw, Ed.. Vol. I , "Analysis of Technical and Formulated Pesticides". 1970. (3) "Pesticides Plant Crowth kegubtors and Food Additives", Vol. I V , V, Gunter Zweig, Ed., Academic Press, New York. N.Y., 1967. (4) The Dow Chemical Company, Analytical Method No. 20959. 1974. ( 5 ) N. Ciocan, Rom. Patent 81211 (1975). (6) G. E. Baiulescu and N. Ciocan, Talanta, 24, 37 (1977).
RECEIVED for review November 23, 1977. Accepted April 20, 1978.
