Polarographic Determination of O,O-Diethyl O-p-Nitrophenyl

Polarographic Determination of O,O-Diethyl O-p-Nitrophenyl Thiophosphate (Parathion) C. V. BOWEN and FRED I. EDWARDS, JR.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1950 | doi: 10.1021/ba-1950-0001.ch034

Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Beltsville, Md.

Parathion may be determined quantitatively by means of the polarograph. The electrolysis is carried out in an acetone-water solution with 0.05 Ν potas sium chloride as the electrolyte, and 0 . 0 1 % gelatin as the suppressor at 25° ± 0.5° C. An accuracy of ± 1% is obtained. Several commercial products were analyzed.

T h e only reported method for the estimation of 0,0-diethyl O-p-nitrophenyl thiophos phate (parathion) is a colorimetric procedure (Î) based upon the reduction of the nitro group to an amino group with subsequent diazotization and coupling with iV-(l-naphthyl)ethylenediamine to produce a color that may be measured. This procedure has had a p plication i n the determination of spray and dust residues where 9 0 % recovery is satisfactory, but is not suitable for use i n the assay of technical materials. Consequently, reliable and sensitive methods of analysis are greatly needed for this new and highly toxic material i n insecticidal formulations. Because the reduction occurs so readily with zinc in the above-mentioned procedure and nitrobenzene (6) was the first organic compound t o be investigated with the polarograph, it was considered probable that parathion would be easily reduced at the dropping mercury electrode and thus be determined b y this means. Apparatus A Sargent M o d e l X X I polarograph was used i n this investigation. The reduction was carried out i n an Η-type electrolysis cell with a saturated calomel reference cell i n one arm (5). A thermostatically controlled water bath maintained the cell at 25° =*= 0.5° C . During the recording of the polarogram the air stirrer was stopped i n order to eliminate vibration and the heating system was disconnected at the bench outlet to remove the possibility of stray current effect (3). I t was observed that other operating electrical appliances, such as hot plates on the same bench, had a stray current effect on the polarograph. Preparation of Standard Curves The 0,0-diethyl O-p-nitrophenyl thiophosphate used i n the preparation of the standard curves was obtained b y isolation from a high-purity technical parathion accord ing to the method devised by Edwards and H a l l (#). It was a crystalline material that melted sharply at 6° C . The physical constants were in agreement with those published by Fletcher et al. (4). A sample of 0.4863 gram of this purified 0,0-diethyl O-p-nitrophenyl thiophosphate was dissolved i n acetone to make 1 liter of standard solution. A 20-ml. aliquot, con taining 9.73 mg., was placed i n a 100-ml. volumetric flask and 30 m l . of acetone were added. Then 0.35 gram of potassium chloride and 0.6 gram of acetic acid were dissolved in about 25 m l . of water and added to the acetone solution; 0.01 gçam of gelatin was dissolved i n a few milliliters of water b y warming, cooled, and added to the above, and the 198

In AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.

199"


BOWEN AND EDWARDS—-POLAROGRAPHIC DETERMINATION OF PARATHION

solution was brought to the mark with water. (This solution is 0.05 Ν with respect to p o tassium chloride, and 0.1 Ν with respect to acetic acid, and contains 0.01% of gelatin and 5 0 % of acetone i n water.) The acetic acid was added to prevent any hydrolysis of the ester during the electrolysis. The sample side of the Η cell was emptied and rinsed by means of suction without being removed from the thermostat bath. The used mercury was retained i n the suction flask. The cell was rinsed well with acetone and then with a portion of the sample solu tion before being filled with the sample solution. Prior to the electrolysis oil-pumped nitrogen was bubbled through the sample solution for 10 minutes to remove dissolved oxygen. The nitrogen was passed through a 1 to 1 acetone-water solution before i t reached the sample solution. F o r electrolysis the dropping mercury electrode was placed firmly in the cell and the polarograph set to record the wave at 0 to —1.5 volts at a sensi tivity of 0.020 microampere with maximum damping. Waves were recorded i n duplicate for 0.020-, 0.030-, and 0.040-microampere sensitivity to allow for considerable leeway i n the size of the sample. The sensitivities of the polarograph refer to the microamperes corresponding to 1-mm. deflection of the recorder. Polarographic waves were obtained i n the same manner for 7.29-, 4.86-, and 2.43-mg. samples of parathion. Figure 1 shows the average wave height for from 2 to 10 determinations at each concentration plotted against the concentration to give the standard curves.

1 2

Figure 1.

3 4 5 6 7 8 PARATHION - m g / ί θ θ ml.

9

10

Standard Curves for Parathion Determination

Sensitivity, microampere per millimeter. A, 0.020; β, 0.030; C, 0.040

After some of the standardization polarograms had been obtained, i t was decided that considerable time could be saved b y using an aqueous stock solution of twice the nor mality of potassium chloride and acetic acid desired i n the sample to be analyzed instead of weighing these materials for each determination. The gelatin was weighed fresh each day.

Analysis of Commercial Products Technical parathion samples were analyzed i n the same manner as the standard samples (Table I). F o r dust formulations at least 1 gram was taken for a sample and In AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.

200

ADVANCES

IN CHEMISTRY SERIES

made to volume with acetone to obtain an extract containing approximately 1 mg. per m l . of parathion based on the manufacturer's claims. The sample was shaken intermittently for 1 hour, and allowed to stand for 10 minutes, and a portion was centrifuged i n a glass-stoppered tube until clear. A n aliquot of this clear solution to contain approximately 10 mg. was taken and the procedure described was followed. The recovery of parathion from dusts b y this procedure was checked by extracting two of these commercial dusts i n a Soxhlet apparatus. The results were found to be within the limits of accuracy of the method, as shown i n Table I I . Dusts of known parathion content prepared i n this laboratory were analyzed after Soxhlet extraction, and the recovery as shown i n Table I I was found to give results also within the limits of accuracy of the method.


Table I.

Analysis of Commercial Samples of Technical Parathion Sample No.

Parathion Found, %

1

84.7 83.9 85.3 Av. 84.6

2

92.3 91.5 92.0 Av. 91.9

3

98.1 97.5 97.7 Av. 97.8

4

93.1 92.0 93.2 Av. 92.8 94.4 93.5 95.3 Av. 94.4

5

Table II.

Analysis of Parathion Formulations Parathion

Material

Present, %

Commercial dusts Sample 1 (25%)

...

Sample 2 (25%)

...

Sample 3(1%)

...

Wettable powders Sample 1 (25%)

...

Sample 2 (25%)

...

Synthetic dusts Sample 1

10.0

Parathion Found Soxhlet extraction, % ...

24.4 24.1 24.4 Av. 24.3 ...

...

23.3 23.3 23.6 Av. 23.4

Flask extraction, % 24.0 24.0 24.1 A v . 24.0 24.0 24.1 24.0 A v . 24.0 0.97 0.92 0.94 Av. 0.94 24.0 23.7 23.9 Av. 23.9 23.9 23.7 23.8 A v . 23.8

... Av.

Sample 2

12.2

Sample 3

29.1

12.2 12.0 12.1 A v . 12.1 29.2 29.3 29.3 Av. 29.3


9.7 9.7 9.8 9.7

BOWEN AND EDWARDS—POLAROGRAPHIC

DETERMINATION OF PARATHION

201


Discussion Normal curves for the polarograms were obtained with the technical materials as well as with the purified sample. The decomposition potential of —0.30 volt and a half-wave potential of —0.39 volt were obtained against the saturated calomel electrode. p-Nitrophenol, which is a major contaminant of the technical parathion, does not re duce at the dropping mercury electrode until after the parathion has been completely re duced, and consequently does not interfere with the curve obtained i n the analysis. The decomposition and half-wave potentials for p-nitrophenol under the conditions for the determination of parathion were found to be —0.45 and —0.68 volt, respectively. Diethyl p-nitrophenyl phosphate, the oxygen analog of parathion, was investigated to ascertain whether it would interfere, if present, i n the determination of parathion. It was found, however, under the conditions used i n this method to have a decomposition potential of —0.37 volt and a half-wave potential of —0.47 volt. A mixture consisting of one-third parathion and two-thirds oxygen analog, instead of giving the anticipated broken wave beginning at the decomposition voltage for para thion, gave a normal curve with a decomposition potential of —0.34 volt. This i n d i cates an interference if small amounts of the oxygen analog should be present, a situation not likely to occur with present methods of synthesis (4) · The polarographic method of analysis of parathion as described here has an accuracy of ± 1%, and 2 mg. of Ο,Ο-diethyl O-p-nitrophenyl thiophosphate per 100 m l . of solution are apparently a minimum concentration for the sensitivities investigated. However, the polarograph used is equipped with resistors, so that a sensitivity of 0.003 microampere per millimeter may be used. A t this sensitivity it would be possible to obtain a sufficient wave height to determine parathion at a concentration of less than 1 p.p.m.

Literature Cited Averell, P. R., and Norris, M . V., Anal. Chem., 20, 753-6 (1948). Edwards, F . I., and Hall, S. Α., Ibid., 21, 1567 (1949). English, F . L . , Ibid., 20, 889-91 (1948). Fletcher, J. H . , Hamilton, J . C., Hechenbleikner, I., Hoegberg, Ε. I., Sertl, B. J., and Cassaday, J. T . , J. Am. Chem. Soc., 70, 3943-4 (1948). (5) Kolthoff, I. M . , and Lingane, J . J . , "Polarography," New York, Interscience Publishers, 1946. (6) Shikata, M . , Trans. Faraday Soc., 21, 42-52 (1925). (1) (2) (3) (4)


Polarographic Determination of O,O-Diethyl O-p-Nitrophenyl

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