times the original volume of the test solution, and 1 gram of potassium iodide added. The solution is then titrated with suitable strength of sodium thiosulfate, The dead-stop method is used to detect the end point. However. in the absence of suitable electrical equipment, the titration may be conducted in the presence of a small amount of chloroform, the end Doint being ivhen the last violet color disappears from the Organic layer. Alternatively, the end point may
be observed by using starch if the final dilution is great enough to avoid oxidation by air. I n the presence of an oxidizing agent, sufficient sodium sulfite is added to ensure complete reduction, warming if necessary. After cooling, iodide is estimated.
(4)Hirozawa, S. T., Brasted, R. C., ANAL. CHEM.25, 221 (1953). (5) Johannesson, J. K., Ibicl., 28, 1475 f19B6).
(6j-0esperJ R. E., ‘(Newer Methods of Volumetric ChemiLal Analysis,” p. 71, Van Kostrand, Yew York, 1938. ( 7 ) Reiss, R., Arzneimittel-Forsch. 6 (2), 77 (1956).
LITERATURE CITED
(1) Bell,
(2) Carson, W.S., Jr., AXAL. CHEM.23, 1016 (1951). (3) Cumming, K. AI,, Alexander, W,-4., Analyst 68, 273 (1943).
R. P., Gellis, E., J. C h e m SOC.
( L o n d o n ) 1951,2735.
RECEIVEDfor review ilugust 25> 1957. Accepted May 6, 1958.
Colorimetric Determination of Tetra methy1phosphoniu m Ion JAMES KOLMERTEN and JOSEPH EPSTEIN Sanitary Chemistry Branch,
U.S. Army Chemical Warfare Laboratories, Army Chemical Center, Md.
b Micro quantities of tetramethylphosphonium chloride (TMPC) can b e estimated colorimetrically as the orthophosphate ion b y oxidation in dilute aqueous solution with ammonium persulfate. Chloride ion interferes with the oxidation in acid or neutral solution, but not in alkaline solution.
T
conversion of various orgenophosphorus compounds to the orthophosphate ion in dilute aqueous solution with ammoniuni persulfate is under investigation in these laboratories in connection with the analytical program. The usefulness of ammonium persulfate for conversion of the nerve gases Sarin (isopropylmethylphosphonofluoridate) and Tabun (ethyl dimethylphosphoramido-cyanidate) to the orthophosphate ion was discovered approximately 10 years ago (S), and has, since then, been extended because of its ability to oxidize easily many phosphite, phosphonate, and phosphate esters. The orthophosphate ion resulting from the oxidation can be estimated colorimetrically by a n y of several methods. I n the present work, it has been found that tetramethylphosphonium chloride (TMPC), a compound reputedly very resistant to oxidation ( I ) , can be converted to the orthophosphate ion in dilute aqueous solution, quantitatively and under normal laboratory conditions. The phosphomolybdate method of Dickman and Bray ( 2 ) has been used b y the present authors to estimate the orthophosphate concentration. Although tetramethylphosphonium chloride in distilled water can be quantitatively oxidized to the orthophosphate ion, the conversion is incomplete when
Table
ANALYTICAL CHEMISTRY
Recoveries of TMPC Orthophosphate ton
as
(After treatment J\ith ammonium persulfate solutions) TMPC, Mg. XaC1, Mg. Added Recovered
HE
1536
1.
0
0 1030 0 1045 A 0 0056”
0 1 40 2 82 4 68
0 0 0 0
2760 2760 2760 2760
0 0 0 0
2590 1905 1716 0469
Average of seven individual determinations. 0
the solution contains a n excess of chloride ion (Table I). The less than theoretical recovery n’as shown not to be due to the interference of chloride ion in the formation of the molybdenum blue color from orthophosphate ion. From 98 to 99% recovery of orthophosphate ion (0.0738 mg.) was achieved even when as much a s 700 nig. of sodium chloride was added. It is believed that chloride ion interferes in the initial oxidation of tetramethylphosphonium chloride, and the interference may be overcome b y performing the oxidation in strongly alkaline solution (Table 11). The initial concentration of sodium hydroxide needed for complete oxidation is dependent upon the quantity of chloride ion in the sample. For example, theoretical conversion to orthophosphate b y ammonium persulfate in this procedure is obtained when the initial sample contains 0.1 mg. of tetramethylphosphonium chloride and 6 mg. of sodium chloride, and is 0.471M with respect to sodium hydroxide. On the other hand, theoretical recoveries (within the limits of experimental
error) are obtained even with solutions containing as much as 90 mg. of sodium chloride in a sample of 0.1 mg. of tetramethylphosphonium chloride, if the initial concentration of sodium hydroxide is 1.0~14. Other phosphorus compouiids in which the phosphorus atom is contained in the anion nioiety of the compound, or neutral phosphorus compounds, can be separated from tetramethylphosphonium chloride b y using cationic exchange resins ( I ) . The tetramethylphosphonium ion, which is exchanged and affixed to the resin, is then eluted with hydrochloric acid. Results of experiments using a n ion exchanger (Dowex 50, hydrogen state) are shown in Table 111. REAGENTS AND APPARATUS
Ammonium molybdate-hydrochloric acid reagent and stannous chloride reagent were prepared according to the method of Dickman and Bray ( 2 ) . Ammonium persulfate reagent, 0.25M aqueous solution of ammonium persulfate (ACS grade). Sodium sulfite reagent, 0.25M aqueous solution of anhydrous sodium sulfite (A4CSgrade). Potassium phosphate (dibasic, anhydrous), Fisher, certified reagent, dried to constant weight at 11.5’ C. and stored in a desiccator. A stock solution of this 0.01 material (0.05622 mg. per ml. mg. of phosphorus per ml.) was used in preparation of a calibration curve. Sodium hydroxide reagent, i . O N aqueous solution, stored in a paraffinlined bottle or in borosilicate glass. Klett-Summerson photoelectric colorimeter, S o . 60 filter. PROCEDURE
Calibration Curve. Aliquots of t h e
stock phosphate solution (1 t o 5 ml.) were pipetted into 50-ml. volumetric flasks, a n d each was diluted t o 35 ml. with distilled water. T e n milliliters of t h e ammonium molybdate-hydrochloric acid reagent was added, followed b y 5 ml. of t h e stannous chloride reagent. After mixing, t h e contents were diluted t o t h e mark and mixed well b y inversion. After 4 minutes, or not longer than 20 minutes, the intensity of the color was read on a Klett-Summerson photoelectric colorimeter equipped with a No. 60 filter and zeroed against distilled water. Treatment of Unknown. One milliliter of a solution containing approximately 0.1 mg. of tetramethylphosphonium chloride is pipetted into a test tube. Alkali chlorides equivalent t o as much as 90 mg. of sodium chloride m a y be present in t h e sample. T o the tube are added 0.5 ml. of 7 M sodium hydroxide solution and 2 ml. of t h e ammonium persulfate reagent.
Table 11. Recovery of TMPC as Orthophosphate Ion in Presence of Sodium Chloride"
(After treatment in alkaline solution with ammonium persulfate) NaOH Concn.,b TMPC, Mg. hIoles/Liter Added Recovered 0.1032 0,0108 0.036 0.0730 0.30 0.1035 0.1032 0.0708 0.36 0.1035 0,0788 0.40 0.50 0.1035 0.0932 0.1035 0.0954 0.60 0.1035 0.0966 0.70 0.1010 0.90 0.1035 0.1932 1.00 0.2070 0.1035 0,1088 1.40 0.1035 0.1056 1.80 a NaC1,24 mg. * Initial concentration of NaOH in reaction mixture.
Table 111. Recoveries of TMPC after Resin Treatment" Found,. Mg. Solution Concn., Effluent Eluent Run Contentsb P.P.hI. .4dded, Mg. ... 22.2 23.1 1 TMPC 23 1 ... 22.2 24.3 486 2 TMPC ... 3.42 3.61 241 3 TMPC ... 3.34 4 TMPC 3.61 24 1 ... 4.18 5 TAIPC 2.5 4.88 NaCl 250 6 TMPC 3.30 206 3.10 260 DEEP D.4P 423 -I TMPC .. 2.31 60 2.38 DEEP EMPA 4.08 .. TEP 50J DAP 1.23 1.27 ... 56 8 DEEP 681 EMPA 4.63 0.24 as Po4--68 i TEP 80) DAP 56 1.23 1.50 a Resin column 6 to 12 inches in a 25- or 50-ml. buret. Flow rate varied from 0.10 to 3.7 ml. per minute. b TMPC, tetramethylphosphonium chloride; DEEP, ethyl ethanephosphonate; EMPA, ethyl ethanephosphonic acid; TEP, ethyl phosphite; DAP, dipotassium hydrogen phosphate. c ilnalyses on effluent and on eluent were for orthophosphate ion content. All solutions were oxidized with ammonium ersulfate prior to development of the phosphomolybdate color except in runs 6,7, an88, where analyses were also made for orthophosphate ion without oxidation to determine DAP content. The column, after water washing, was eluted with 50 to 100 ml. of 5N HCl. The eluent was evaporated to dryness and tested for TMPC according to the procedure given herein.
E\
The tube is then placed in a boiling water bath for 1 hour. A t the end of this time, 2 ml. of the sodium sulfite reagent is added and the contents are again heated in a boiling mater W t h for a n additional 45 minutes. The tube is then cooled and 2 drops of quinaldine red indicator are added. A stable pink color should result. If i t does not develop, a few drops of 7.5M ammonium hydroxide solution are added until the pink color appears. The contents of the tube are then quantitatively trans-
ferred to a 50-nil. volumetric flask, diluted to 35 nil. with distilled water, and treated as described under Calibration Curve. If orthophosphate compounds are known to be present. a blank determination as follow is recommended: One milliliter of distilled water is heated with ammonium persulfate in alkaline solution as in the treatment of the unknown. Following the sodium sulfite addition and subsequent heating. 1 ml. of the unknown is added. After
any necessary p H adjustment, the solution is transferred to a 50-ml. volumetric flask, and made u p to 35 ml. The sample is then treated as described under Calibration Curve. The blank is subtracted from the unknown and the concentration of phosphorus is read directly from the calibration curve. The concentration of tetraniethylphosphonium chloride per milliliter in the original sample is obtained b y multiplying the phosphorus content in the final sample by 4.086. DISCUSSION
Ancillary experiments indicated that the interference of chloride ion in the oxidation of tetramethylphosphonium chloride b y animonium persulfate is due to a simultaneous, competing reaction of chloride ion with ammonium persulfate in acid and neutral media, and that the role of hydroxyl ion is to prevent this reaction. It was demonstrated. for examde, that, in neutral and acid solution. volatile aroduct was formed (most probably, chlirine) which was capable of oxidizing iodide ion to iodine. Under the same experimentaI conditions, no such material was formed in the absence of chloride ion. It is probable that there is either no reaction between chloride and ammonium persulfate in alkaline solution, or a very slow one. If there were a reaction, i t might be expected that sodium hypochloride would be one of the products formed. I t x a s established t h a t sodium hypochlorite \vould not convert tetramethylphosphoniuni chloride in alkaline solution to the orthophosphate ion. As this reaction does take place in alkaline solution with quantities of sodium chloride sufficient to use all the available oxygen in the ammonium persulfate, it would appear reasonable t o conclude that sodium hypochlorite is formed, if a t all, in negligible quantities. Boiling a n alkaline tetramethylphosphonium chloride solution does not result in the formation of a compound more easily oxidizable to the orthophosphate ion than tetramethylphosphonium chloride itself. It thus appears that the role of hydroxyl ion in this procedure is to prevent the oxidation of chloride ion by ammonium persulfate.
a
LITERATURE CITED
(1) Anderson, C. J., Keeler, R. A,, ANAL.CHEM.26, 213 (1954). (2) Dickman, S. R., Bray, R. H., IXD. ESG. CHEY., ASAL. -ED. 12, 665
(1940). (3) Epstein, J., Koblin, A,, Division of Analytical Chemistry, Symposium on Air Pollution, 130th Meeting, .4CS, Atlantic City, September 1956.
RECEIVED for review October 11, 1957. Accepted April 19, 1958.
VOL. 30, N O . 9, SEPTEMBER 1958
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