Volumetric Determination of Some Organophosphorus Halidates and Pyroester Compounds Using a Peroxide Reagent SAMUEL SASS, IRWIN MASTER, PAUL M. DAVIS, and NATHAN BEITSCH Chemical Research Division, Chemical Warfare laboratories, Army Chemical Center,
b A method has been developed for the semimicro- to macroanalysis of organophosphorus fluoridate, chloridate, and pyroester compounds using a peroxide reagent in an alkaline medium. The method, which depends on the formation of a perphosphorus acid by reaction of peroxide with phosphorus halidate or pyroester with the subsequent determination of excess peroxide, has shown a precision and accuracy within 1%.
A
existed for a specific semimicro- or macromethod that could be used to assay phosphonohalidates in the presence of phosphorus esters and acids. It was previously shown that a peroxide-amine reaction could be used for the colorimetric ($, 4 ) or fluorometric (3) estimation of organophosphorus halides, pyrophosphates, and acylating compounds via a perphosphorus-type mechanism. Peroxide in an alkaline medium forms a peracid compound when reacted with paraoxon (1). It was found in these laboratories that a stoichiometric quantity of peroxide is consumed when reacted with semimicro to macro quantities of oxy-alkyl phosphorus halidate or anhydride. I n the method described below, a weighed quantity of organophosphorus compound is pipetted into an excess of alkaline peroxide (pH lo), the solution is acidified with sulfuric acid after 2 to 4 minutes, then potassium iodide is added, and the iodine is titrated with standard sodium thiosulfate. The peroxide reaction ( 2 ) with the phosphorus compound follows. KEED
These and related compounds are highly toxic! Sampling and other work involving the intact molecule should be done in a well-ventilated hood. REAGENTS AND APPARATUS
Sodium pyrophosphate peroxide (Na4P207. 2H202),Buffalo Electrochemical Corp. (Becco). Reagent prepared by dissolving 8.5 to 9.0 grams of sodium pyrophosphate peroxide and 5.68 grams of sodium borate (Na2B407. 10HzO),c.P., in 500 ml. of water. Add 12 ml. of 10% sodium hydroxide solution (or enough to buffer the solution a t p H lo), and dilute to a liter with water. 2-Propanol, loo%, dried over aluminum isopropoxide and distilled. Hydrogen fluoride, anhydrous, 5% solution in ethyl acetate. Ethyl acetate, Fisher certified reagent, dried and distilled over phosphorus pentoxide (less than 0.01% water). Polyethylene bottle with screw cap, 30-ml. capacity. Jt7eighing bottle, micro-1-ml. Teflon or glass cup, 4-mi. capacity. PROCEDURES
W e t M e t h o d for Fluoridates and Pyrophosphonates. Weigh 0.18- to
0I R'
0 lo+ OOH- pH -
r
R-P-00H OR'/ I
0
0 9
+ HzO + R-P-0-r I
I n the absence of chloridate or difluoridate the original dilution of pyrophosphorus ester or alkyl phosphorus fluoridate can be made in dry 2-propanol. Ester phosphorus chloridates can be determined, using dilution in dry 2-propanol, by rapidly pipetting the diluted chloridate into alkaline peroxide. However, although results are fairly reproducible, the method is empirical and results must be corrected for the quantity of chloridate lost because of esterification and hydrolysis. M e t h o d for Alkyl Phosphono- and Dialkyl Phosphorochloridates (Fluorination). Weigh to the nearest milli-
gram a 2-gram sample of chloridate into a previously tared polyethylene bottle containing a weighed 15-ml. volume of hydrogen fluoride solution. Swirl the bottle to mix the contents thoroughly and allow to stand for 0.5 hour. Swirl the container and weigh a 0.5000- to 1.500-gram sample into a glass ampoule. Add the sample to a 250-ml. glass-stoppered Erlenmeyer flask containing 50 ml. of 50y0 2-propanol. Pipet a 20-ml. sample and continue as described for the wet method. It'hen calculating, correct the volume of the 50% 2-propanol solution (50 ml.) for the additional volume imparted by the added fluorinated sample using weight directly as volume. Calculate the per cent of phosphorus chloridate as follows: Weight of chloridate sample X 100 D = combined weights of hydrofluoric acid and chloridate
OOH-
1
+ X-
OR'
X = chlorine, fluorine, or the phosphorus group as in pyrophosphates and pyrophosphonates; R = alkyl or alkoxyl; R' = alkyl.
proximately 2 to 4 minutes with occasional swirling. Add 20 ml. of 50% 2-propanol to the blank flask and treat similarly. Add 30 ml. of water, 10 ml. of 18N sulfuric acid, and 3 grams of potassium iodide in that order with mixing. Allow the solutions to stand in the dark for 10 minutes and titrate the liberated iodine with 0.lN sodium thiosulfate solution. Subtract the sample titer from the reagent titer. Calculate per cent fluoridate or pyroester using an equivalent weight, one fourth of the molecular weight.
0.24-gram samples in glass ampoules t o the closest tenth of a milligram. Place the ampoule in a 100-ml. volumetric flask containing approximately 20 ml. of 50% 2-propanol. Break the ampoule with a glass rod, swirl the flask, and dilute the solution t o 100 ml. with more 50% 2-propanol. By pipet or buret add 50 ml. of peroxide reagent to 500-ml. iodine flasks, one of which will be used for a blank. Run a blank with each series of samples.
0
r R-P--X
Md.
Pipet 20 ml. of sample solution into the iodine flask while continuously swirling the flask. Stopper, mix thoroughly, and allow the flask to stand for ap-
where = Per cent chloridate in hydrofluoric acid solution (15 ml.) 50.0 W
+
20.0
VOL. 32, NO. 2, FEBRUARY 1960
285
molecular weight
x
0 . 1 x ( B - C)
x n;
w where
F B
per cent chloridate titrated milliliters of sodium thiosulfate wed in tions C = milliliters of sodium thiosulfate solution used in sample titrations = =
Table 1.
Ai
normality of sodium thiosulfate solution T V = weight of fluorinated aliquot (ampouled sample) F then per cent chloridate = - x 100 D =
To correct for pyrophosphorus ester as impurity in chloridate use the following:
yo chloridate
=
(% chloridate from fluo-
Analytical Recovery and Purity Obtained Using Peroxide Method Sample, Gram Purity ReCalcuRecovery, Found, covered lated 70 %
Is0 ropy1 methylphosphonofluoridate 8alculated purity, 99.0
Calculated purity, 74.8 Diisopropyl phosphorofluoridate Calculated purity, 98.0
0.2346 0,1945 0.1948 0.2266 0,2217 0.1438 0.1421 0.1350
0.2337 0,1932 0,1937 0.2266 0.2204 0,1438 0.1410 0,1345
0,2132 0.2127 0.1874 0.1887 0.1961 0.1971 Calculated purity, 79.2 0.1780 0.1766 0.1668 0.1685 0.1565 0.1555 0,2251 0.2248 IEopropyl ethylphosphonofluoridate 0,1971 0,1984 Calculated purity, 93.9 0.2111 0.2091 0.2407 0.2400 Tetraethyl pyrophosphatea 0,2320 0.2339 Calculated purity, 96.8 0,2309 0.2287 0.1931 0.1943 Calculated purity, 83.0 0,1880 0.1892 0,2131 0.2144 Tetraisopropyl pyrophosphate" 0.2158 0.2172 Calculated purity, 80.1 0.2037 0.2058 0.2441 Tetra n-butyl pyrophosphate5 0.2452 0.2269 0.2271 Calculated purity, 89.0 0.2331 0,2352 0.2228 0.2233 Bis (ethyl methylphosphonic) anhydride 0,2102 0.2123 Calculated purity, 90.4 0.1962 0.1975 0.2244 0.2225 Bis (isopropyl methylphosphonic) anhydride 0.2166 0.2171 Calculated purity 92.8 0.2193 0.2183 5 Dry method can be used to preclude hydrolysis during sampling.
99.6 99.3 99.4 100.0 99.4 100.0 99.2 99.6
98.6 98.3 98.4 99.0 98.9 74.8 74.2 74.5
99.8 99.3 99.5 99.2 99.0 99.4 99.9 99.3 99.1 99.7 99.2 99.1 99.4 99.4 99.4 99.4 99.0 99.6 99.9 99.1
97.8 97.3 E7.5 18.6 78.3 78.7 93.8 93.3 93.0 96.5 96.0 95.9 82.4 82.5 79.6 79.2 79.3 88.6 88.2 88.9 90.2 89.5 89.8 92.0 92.6 92.4
99.8
99.0 99.3 99.2 99.8 99.5
Analytical Recovery of Phosphoro- and Phosphonochloridates Fluorination Purity Purity ReSample, Gram covery, calcd., found, Calcd. Recovered 70 70 5% 94.1 Diisopropyl phosphoro0,1854 0.1850 99.8 96.0 95.8 95.4 05.4 chloridate 0,1692 0.1682 99.4 93.8 0.1521 0.1520 99.9 95.9 79.1 78.6 77.9 0.1439 0.1430 99.4 i8.3 78.4 0.1461 0.1448 99.1 78.5 77.0 99.2 0.1367 0.1356 96.6 96.6 95.8 Isopropyl methylphosphono- 0.1488 0.1488 100.0 96.1 96.0 99.5 chloridate 0.1534 0.1526 94.9 95.9 99.5 0.1675 0.1666 85.0 85.5 85.3 99.8 0.1607 0.1603 84.9 85.5 0,1543 0.1643 100.0 85.1 84.6 99.5 0.1500 0.1493 65.8 65.4 64.8 99.4 0.1310 0.1302 65.7 99.8 0.1263 0.1261 64.0 65.3 64.3 99.2 0.1221 0.1211 Empirical results corrected to a 10070 basis. Table II.
~
286
~
~~~
~
~
ANALYTICAL CHEMISTRY
rination method) - ( % pyroester calculated as chloridate) Method for Pyrophosphorus Esters
as Impurity in Chloridates. TSTeigh 0.0800- t o 0.1200-gram samples in glass ampoules. P u t t h e ampoule into a Teflon or glass cup containing 2 ml. of 50% 2-propanol. With a long forceps lower t h e cup into t h e dry end of a tilted 500-ml. iodine flask containing 50 ml. of peroxide reagent. Stand t h e flask upright a n d with a glass rod break the ampoule and agitate the mixture in the cup. After 1 minute wash down the rod with water and swirl the flask to mix the contents of the cup thoroughly with the peroxide reagent. Continue swirling for 2 minutes and proceed as described in the wet method. Calculate per cent pyrophosphorus ester as chloridate and apply this correction to the fluorination method, This alternate method precludes the necessity for the volumetric dilution described in the wet method. It is most useful for the determination of pyrophosphonate as impurity (0 to 10% in the presence of larger quantities of chloridate) . Choice of Peroxide Reagent. The reagents tested at t h e time of this study were sodium perborate, sodiuni carbonate peroxide, urea peroxide, benzoyl peroxide, cumene hydroperouide, tert-butylhydroperoxide, and sodium pyrophosphate peroxide. Of all the peroxides tested, sodium pyrophosphate peroxide sho\Ted t h e best stability as a reagent under the optimum conditions of p H for quantitative analysis. At p H 10 it could be used for a t least 3 days. The hydroperoxides, although stable, were more difficult to determine on back-titration. Conditions for Reaction of Peroxide with Phosphonohalidates and Anhydrides. T h e most rapid, complete, and stable reaction of peroxide with halophosphorus compounds and anhydrides occurred a t a buffered pH between 9.5 and 10.2. Below pH 9.5 t h e reaction is slower and lower results are obtained, partially because of competition from hydrolysis. Above p H 10.5 the accuracy is lower as a result of rapid decomposition of the peroxide reagent. Reactions for all the compounds tested were completed within 1 to 2 minutes m-ith no further apparent reaction occurring over a period of 1 hour. The sample must be added with mixing to the peroxide buffer solution because it is necessary to maintain a strong excess of peroxide during the reaction. A reversal of the addition results in the localized depletion of peroxide with the loss of some compound to hydrolysis. Results as low as Soy0 of the theoretical are obtained depending on the rate of addition of peroxide t o the compound. Stability of Reagent. Tests made on three batches of Becco sodium
pyrophosphate peroxide in pH 10 borate buffer at 25' f 5' C. indicated t h a t t h e reagent solution could be used for 5 days with a n approximate peroxide decrease of 1% a day. Occasionally, t h e prepared reagent n-as discarded after 3 days' use because t h e rate of peroxide loss was greater. KO specific cause was found for this difference but it; is presumed that the peroxide may have decomposed cxtalyticallj. or autocatalyticnlly bec:iuse of the presence of trace impurities. The reagent can be used as long as an excess of 3.5 equivalerits of peroxide to each equivalent used in the reaction is n u intained, Analysis for Phosphonofluoridates a n d Pyrophosphonates. Khen an adequate excess of peroxide is present' in alkaline solution, the rate of peroxidation of fluoridate or pyrophosphonate is more rapid t h a n t h e rate of hydrolysis. T h e presence of alcohol, as in t h e joy0 solution cit'ed in t h e procedure further increases t h e stability t o hydrolysis. The w t procedure is recommended because this precludes interferences from chloridates, dichloridates, and difluoridates (when present 3s impurities) nhich hydrolyze rapidly in n-ater to produce acids which do not interfere with the peroxide method. Analysis for Phosphonochloridates. In general, t h e phosphoro- a n d phosphonochloridates are much less hydrolytically stable t h a n are their corresponding fluoridates and pyrophosphonates. T h e chloridates studied in these laboratories hydrolyze rapidly. Diisopropyi phosphorochloridate, ethyl methylphosphonochloridate, and isopropyl niethj-lphosphonochloridate hydrolyze almost insta.ntaneously. The wet method can be used to determine phosphoro- and phosphonofluoridates and pyrophosphonates quantitatively in the presence of chloridat'es that were used as intermediates in their synthesis. When det,ermining t:he chloridates as intermediates, the fluorination modification quantitatively produces the more stsble but highly toxic fluoridate which yields satisfactory analytical results. \Then this procedure is used, all analJ-SCS should be conducted in a n efficient 1nbor:itory hood. The chloridate result can be corrected for pyroester as impurity using the method outlined previously. An alternate method, although empirical and less accurate, is the dry procedure (noted under the wet mcthodj. K i t h this method, which iii\-olves less x-olatile and less toxic inaterials, consistent recoveries of i0 to 727, n-ere obtained in these laboratories on the chloridates mentioned later in this report. When the dry method is used, the empirical perosidation us. hydrolpis or esterificlation characteristics of the particular chloridate of inter-
Table 111.
Effect of Phosphonate Impurities on W e t Peroxide Method
ReSample, Gram covery, Added Recovered % 99.2 0.2513 Prlethylphosphonofluoridic acid Isopropyl methylphos- 0.2533 100.0 0.2489 phonofluoridate 0.2489 0.2116 99.5 Methylphosphonic acid (di0,2127 99.5 0.2174 hydroxymethylphosphonate) 0.2185 0.2294 99.3 Bis (isopropyl methyl- 0.2309 0,2004 99.3 phosphonic) anhy0.2017 dride 0,1943 99.4 hlethylphosptionic dichloridate 0.1955 0,2294 99.7 0.2301 0.2284 99.4 Isopropyl methylphos- 0.2298 99.8 0.2096 phonofluoridate 0.2100
Impurities Added as 20% by Jj'eight
Compound
Methylphosphonic difluoridate
0,2422 0.2476 Bis( methylphosphonic j 0.2064 anhydride 0.2386 Bis(isopropy1 methyl- 0.2124 phosphonic) anhydride 0,2144 Isopropyl niethylphoephonoIbopropyl methylphos- 0.1963 chloridate phonofluoridste 0.2038 0.2451 Isopropyl hydrogen methyl- Isopropyl methylphos- 0 1973 phosphonate phonofluoridate 0.2184 0 2235 Bis(isopropy1 methyl- 0 2107 phosphonic) anhy0,2118 dride Ethyl hydrogen 0,2443 methylphosphonate 0.2525 Isopropyl methylphos- 0.2389 phonofluoridate 0.2472 0.2357
est should be experimentally predetermined. Correction is then made for the difference between the analytical recovery on pure samples and that calculated for 100% recovery as found for the compounds mentioned in this report. Dichloridates interfere additively with t h e dry method. RESULTS
The tested phosphorus compounds were assayed on the basis of elemental and alkoxy group analysis. Fluoridates and chloridates were further analyzed on the basis of ionic halide 2's. phosphorus halidate. The compounds used for standardization studies were fractionated samples and crudes. The precision and accuracy of the methods were found t o be within approximately lC/o, except TI here stated othernise. Results obtained on some alkylphosphonofluoridates, pyrophosphonates, and alkyl phosphorofluoridates and pyrophosphates are shou-n in Table I ; results for some chloridates are shown in Table 11. Effect of Impurities on Method for Alkyl Phosphonofluoridates and Pyrophosphonates. Analysis of prepared organophosphorus compounds from various stages of purification s h o w d t h a t , depending on the synthetic method used, one or more of the type
0,2409 0,2456 0,2062 0,2372 0,2108 0.2137 0,1961 0,2024 0,2435
99.5 99.2 99.9 99.4 99.2 99.7
0,1965 0.2167 0,2272 0.2098 0.2113
99.6 99.2 99 5 99.5 99 8
0.2429 0.2516 0.2376 0.2459 0.2332
99.4 99 6 99 8 99.5 99.5
99.9 99 3 99.4
compounds shotr-n in Table 111 could be present as impurity. Mixtures containing approximately 20y0 of impurity were made with samples of isopropyl methylphosphonofluoridate arid diisopropyl dimethylpyrophosphonate [bis(isopropyl methylphosphonic) nnhydride] as a test of the wet method. OKing to the catalytic decomposition of certain of the phosphorus compounds in the presence of phosphorus acids, some of the mixtures were made immediately before analysis. Although not shown here, similar results were obtained when the fluorination procedure was used on phosphono- and phosphorochloridates. None of the impurities shown below exhibit interference in the wet or fluorination methods. Results obtained on tivo organophosphorus compounds are shown as examples in Table 111. LITERATURE CITED
J., Roscnblatt, D. H., Demek, AI. XI., J . Oro. Chem. 21, 790 (1956). ( 2 ) Gehauf, B., Epstein, J., TT'ilson, G. W.,Witten, B., Sass, S., Bauer, V. E., Rueggeberg, IT. H. C., , ~ N . I L . CHEX 29,278 (1957). (3) Gehauf, B., Goldenson, J., Ibid., 29, 276 (1957). (4) Marsh, D. J., Neale, E., Chern. & Ind. (London) 1956,494 RECEIVEDfor review July 16, 1059. Accepted November 23, 1959.
(1) Epstein,
VOL. 32, NO. 2, FEBRUARY 1960
287
