Determination of Phosphorus in Hexaethyl Tetraphosphate and Tetraethyl Pyrophosphate MARTIN J.ICOBSON AND S. A. HALL Bureau of Entomology and Plant Quarantine, Beltsville, M d . From the many published methods for determining phosphorus, a rapid and reliable procedure has been chosen that is especially applicable to technical grades of tetraethyl pyrophosphate, including so-called hexaethyl tetraphosphate. Conversion of the organic phosphorus by a1kali:nitrate fusion and dilute nitric acid digestion is followed by a colorimetric determination in which the yellow mol3 bdivanadophosphoric acid method of 4Iisson is used, as modified by Kitaon and hlellon.
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KCREASEU interest in the use of hexaethyl tetraphosphate and tetraethyl pyrophosphate as agricultural insecticides and the variety of procedures used in their commercial manufacture have made rapid and reliable analytical methods for their determination highly desirable. This paper describes a method for determining the phosphorus content of these materials. Of the known methods for the conversion of organic phosphorus compounds to orthophosphates, the alkali-fusion method described by Clark (I), with certain modifications, has been found by the authors most suitable from the standpoint of ease and rapidity. In order t o avoid time-consuming operations to attain constant weight, gravimetric determination of the phosphate was discarded in favor of a colorimetric procedure. The bluecolor method of Dickman and Bray (@ was inapplicable because of interference of the nitrate ion from the nitric acid used in conversion of pyrophosphate to orthophosphate. However, the yellow molybdivanadophosphoric acid method of Xisson (5), as modified by Kitson and Mellon (4, proved both rapid and reliable. The entire procedure, from the weighing of the sample to reading of the color, can be run in about an hour.
graph for reference of all analyses made on the same instrument with the same reagents. Fusion of Unknown Sample. From 40 to 50 mg. of hexaethyl tetraphosphate or tetraethyl pyrophosphate were weighed by means of a weighing pipet (specially designed to handle hygroscopic liquids) (Figure 1) into a small platinum crucible of about 7-ml. capacity, and approximately 0.9 gram of the alkali-nitrate fusion mixture was added. A small glass scoop holding the required amount was found very convenient for adding the fusion mixture. The crucible was covered and gently heated over a small burner until foaming ceased, and then more heat was applied until the mass became a clear liquid. The progress of the fusion, which usually requires 5 or 6 minutes, was followed by gently removing the crucible cover a t about 2-minute intervals. Upon cooling, the mass was dissolved from the crucible and cover with 75 ml. of hot distilled water, transferred to a 125-ml. Erlenmeyer flask, acidified to Congo red Kith concentrated nitric acid, and slowly simmered until the volume was 50 ml. (about 25 minutes), After cooling, the solution was made just acid to litmus by careful addition of ammonium hydroxide, transferred quantitatively to a 100-ml. volumetric flask, made up to the mark with water, and shaken. A 5-ml. aliquot of this solution was transferred to a 50-ml. volumetric flask, 5 ml. each of 1 to 2 nitric acid, ammonium vanadate, and ammonium molybdate were added, and the solution was made up to the mark and allowed to stand.
REAGENTS
Sodium Hydroxide-Potassium Sitrat,e Fusion Llixture. Four parts of powdered sodium hydroxide and one part of powdered potassium nitrate were intimately mixed in a mortar. Potassium hydroxide may be used in place of sodium hydroxide. B. Sitric acid, C.P. grade, 70%. C. Xitric acid, dilute (1to 2 ) . D. Ammonium hydroxide, C.P. grade, 28 to 297,. E. Ammonium vanadate (0.25y0).In 500 ml. of warm water 2.5 grams of ammonium vanadate were dissolved, the solution was cooled, 20 nil. of concentrated nitric acid rvere added, and the mixture was diluted to 1 liter. F. hmlnonium molybdate (57,). Fifty grams of ammonium molybdate iwre dissolwd in 1 liter of tvarni water. A.
PROCEDURE
Preparation of Standard Graph. Twice-recrystallized potassium dihydrogen phosphate (43.9 mg.) was weighed out, on a small piece of cigaret paper and transferred to a 100-ml. volumetric flask, and the flask was made up to the mark with water and shaken. This solution therefore contained 0.1 mg. of phosphorus per milliliter. Aliquots of 1 to 10 ml. were measured into a 50ml. volumet,ric flask from a 10-ml. buret that could be estimated to 0.01 ml. Five milliliters each of the 1 to 2 nitric acid, ammonium vanadate, and ammonium molybdate were added in that order, and the flask was made up to the mark with water and shaken. The solution was allowed to stand for a t least 10 minutes, and then a portion was poured into a clean, dry photometer test tube and the color was measured in a photometer. The color was measured in this laboratory in a photoelectric photometer, employing a No. 46 blue filter (optical centroid at about 460 millimicrons) and a solution of reagents C, E, and F as a blank to balance The results were plotted on semilogaa t 1 0 0 ~transmittance. o rithmic paper as per cent transmittance against concentration of phosphorus and the straight line obtained provided a standard
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Figure 1. Weighing Pipet 1. 2. 3.
,
Glass tube with fine capillary tip Wire holder Rubber bulb
The percentage of light transmittqnce was then measured in the photometer and referred to the standard graph. The concentration of phosphorus per nlilliliter of solution was read from the graph, and the percentage of phosphorus in the unknown sample was calculated as follows: %ofP= Table I.
amount of P x 2000 mg. of unknown sample
Percentage of Phosphorus in Samples Tested
Product Triethyl orthophorphate, (CzHdsPO4 (purified) H e x a e t h ~ tetraphosphate, l (CzHs)6P4Ola ~ ~ ~ $ ' $ ~ ~ k b Tetraethyl pyrophosphate, (C9Ha)aP207 Schrader C Woodstockd Purified
C
736
c
Calcd. 17.01
Found 17.06
24.47 24.47
26.28
21.35 21.35 21.35
23.96 22.25
25.20
21.86
~POCls~+ 5(CzHa)sPOn ~ 1 + ~ 3CzHoC1 f 2 4- 3(CzHd4PnOl (C2Hs)aP401a ~ ~ ~ ~ ~ : f . ~ + 4(CzHs)sPO4 d 3(CzHs)rPz07
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8
137
Fused mixtures of alkali hydroxides and nitrates attack platiouni and prolonged fusion in platinum, especially at higher temperatures, is very undesirable. I n the authors' experience, however, the brief fusion of the small quantity of fusion mixture reYulted in a loss in weight of the crurible of only a few milligrams after 35 separate fusions. Gold is much less attacked than platinum by fused alkali hydroxides and nitrates. If the fusion is to be repeated many times, the use of a gold crucible might be orefi.rahlr
phorus is on the basis that the reactions shown in the table were carried out quantitatively, except in the cases of the true compounds triethyl orthophosphate and tetraethyl pyrophosphate (purified), The samples of hexaet,hyl tetraphosphate and tetraethyl pyrophosphate t,ested were high in phosphorus content, a fact that is further borne out by the values for refractive index and ethoxyl determination reported previously (9) for these samples.
DISCUSSION OF RESULTS
(1) Clark, E. P., "Semimicro Quantitative Organic hnalnis," p. 6 5 . Sew Yolk, Academic Press, 1943.
Triethyl 01 thophosphate, purified by fractionation, Tvas ubed to Inheck the accuracy of the procedure before any unknown samples were run. The percentage of phosphorus found for all materials tested is shown in Table I. In each case the average value found was the result of a t least three determinations, and excellent cherks were obtained in all cases. Theoretical per rent phos-
(2) Dickman, S. R., and Bray, R . H., ISD. EXG.CHEM.,ANAL.ED.. 12, 665 (1940). (3) Hall, S. A., and Jacohsoii, XI., Ind. Eng. Chem., 40, 694 (1948). (4) Kitson, R. E., and Mellon, M . G., IND. ENO.CHEM..A N A L . E D . . 16, 379 (1944). ( 5 ) Miason, G., Chem.-Ztg., 32, 633 (1!>08).
LITERATURE CITED
R E C E I V KSnr.eniher D 14, 1947.
Analysis of 1,2,3,4,5,6=Hexachlorocyclohexane for Gamma Isomer A Polarographic Method GEHRIT E. I .
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m-Ic;T
Pont de ,Venwrcrs & Company, Grurselli Chemic& Depurtmerrt, Clezelund, Ohio
4 polarographic method is described for the determination of the gamma isomer Under the content of the new- insecticide-1,2,3,4,5,6-hexachlorocyclohexane. conditions employed, the gamma isomer is the only one of the five isomers (alpha, beta, gamma, delta, and epsilon) that is reduced at the dropping mercury cathode. Comparative results are given for the bioassay method and for the method described in this paper.
K
ECENTLT 1,2,3,4,5,6-hexachlorocyclohoxane (benzene hexachloride) has been announced as an insecticide that promises to become of considerable importance to agriculture. The present technical material is a mixture of a t least five of the sixteen possible stereoisomers, of which the so-called gamma isomer is bv far the most biologically active. The activity of the ganima isomer is so marked and superior to that of the other isomers that the evaluation of hexachlorocyclohexane is usually evpressed in terms of the per cent gamma content, and the latter is regarded as a n index of the biological activity ( 5 ) . Development work in both the manufacturing and the biological fields has been hampered by lack of rapid analytical method9. Analysis by biological assay has been employed and should probably be considered as the final test of any insecticidal material. However, this method does not lend itself to high accuracy and is further limited by the requirements of a rather specialized technique and the fact that it is time-consuming. Daasch (1)has recently described an infrared method which should prove valuable, particularly as it permits the determination of all five isomers. The present paper describes a polarographic procedure for the determination of gamma isomer. It is based upon the observation that the gamma isomer is the only one of the five isomers (alpha, beta, gamma, delta, and epsilon) that is reduced a t the dropping mercury electrode under the conditions employed. Apart from a brief reference by Keller et al. (b), no description of thi.: approach has appeared in the literature. REAGENTS AND MATERIALS
All reagents were of C.P. quality. The hexachlorocyclohexane isomer samples used in this work
possessed the following melting points: alpha isomer 157-158" C. ; beta isomer 180-196" C. (sublimes); gamma isomer 1 1 2 . g 114' C.; delt'a isomer 138-139" C.; epsilon isomer 217-219" .C. (obtained through the courtesy of the Don, Chemical Co., Midland, Mich.). These values are in good agreement with those obtained by other workers ( 4 , 5 ) . The values for alpha, gamma, and delta isomers are corrected capillary melting point values. The sublimation range value for the beta isomer was obtained by using tt Fisher-Johns melting point block. These isomers were wparated from the commercial material by the recrystallization process described by Slade ( 5 ) . A sample of one of the hept'achlorocyclohexane isomers (melting point 85-86 ' C.) was also available for study (obtained through the courtesy of U.S. Department of Agriculture, Beltsville, &Id.), The potassium chloride-sodium acetate buffer solution was prepared by dissolving 2 grams of potassium chloride and 20 grams of sodiuni acetate trihydrate in 50 ml. of water. X solution of 0.10 grain of carpenter's glue in 20 ml. of hot water was prepared separately, then added to the sodium acetate solution and diluttd to 100-ml. volunics. Thii: solution vias prepared fresh, daily. Stock solutions of t,hebeta, delta, arid epsilon isomers were prepared by dissolving 0.4000 gram of t,he isomer in 80 ml. of acetone and diluting to 100-ml. volume with distilled water. A gamma isomer stock solution was prepared by transferring 0.060 g a m of gamma isomer to a 100-ml. volumetric flask, adding 60 nil. of acetone, and diluting to the mark with dist,illed water. The isomer-mix stock solution was prepared by weighing out 0.280 gram of alpha isomer, 0.040 gram of beta isomer, and 0.040 gram of delta isomer, dissolving in 80 ml. of acetone, and diluting t o 100 ml. with distilled water. This solution together with varying amounts of the gamma stock solution was intended to approximate t,he composition of commercial hexachlorocyclohexane. The addition of t,he epsilon isomer was omitted because of the lack of a suitable amount of this material. The potassium chloride-alcohol-water mixture was prepared hy dissolving 0.010 gram of carpenter's glue in 20 ml. of hot water. diluting to 110 ml. with distilled water, adding 2 grams of
