I~ctcrminatian of phosphorus in organophosphorus compounds

Many mclhotls IIilVC bccti rcporlcd for the dctcrtniti;ttion of phosphorus in or~itt~ophosp,horlts compounds. IJsu;~lly the phosphorus is convcrtcd to ortho- phospha~c by one of the following tncthods : closed flask combustion. digestion with I-ICIO, I-1NO_3 or I-lzSO, I-INO_, tnixturcs. pcroxidc fusion. or pyrolysis in ;I silica tube. The ph<~sphittc formcd is then dctcrmincd by one of ;t nutnbcr of proccdurcs bitSCd on colorimclry. acid bilSC litritlions. volutnclric or coulotnctric prccipitittion ~ilrittions. indirect cotiiplcxinictric tilritlions. or gt-avimctric methods. The rcvicw articles of Ma’ should bc consulted for rcfcrcnccs to specific methods.

The potcntiomctric titration of phosphoric acid with st:ttlditrd bust using it glass pl1 clcctrodc its the indicatitlg clcctrodc is well-known’*-‘. Ncvcrthelcss. thcrc sccnis to bc no rcfcroncc in the litcrillurc to the ;tppliCiltiOtl of this mcthod to the

JctcrtnitlittiOtl Of phosphorus iii Orpiltlopl~ospll0rl~s compounds. In the tncthod prcscnlcd hcrc. rhc org~ttlopllosphorlts con~pound is burned by the Schiinigcr mclhodJ , atld the combustion products itrcitbs<>rbcd in itn itlk;tlinc bromine solution. The solution is acidified and boiled to rcmovc carbon dioxide itnd bromine. The resulting solution of hydrochloric itcid and orthophosphoric acid is titrated with clcctrogcnerittcd hydroxide. The ~1-1 is rccordcd us it function of titnc. Frotn the time bctwccn the l-1 zPO; ccluivitlcncc point itnd the H 1’04’ - cquivalcncc point and the V:~IUC of the clcctrolysis current. ~IIC phosphorus content is citlculitIcd.

rlpptr~*trrlrs. TIIC ~3~~km~tt~ Ek~ttoScitt~ 30 with potcntiostitt accessory furnished the constittlt current and wits also used to record the pH continuously with time. A Sitrgctlt-Welch combinittion pH clcctrodc NO. S-30072-15 W;IS used to ~OIIOW tllc titrution. A Deckman 39273 pliltitlum clectrodc and it larger platinum foil electrode of 6 ctn’ wcrc both used i\S 117~’ gctlcritt(>r clcctrodc. A Beckman 39017 platinum ituxiliitry clcctrodc wits bridged to 111~’ solution through 11 U-type SitIt bridge. For most titrations dcscribcd hcrc it current of IO.00 mA and ;I chart speed of 1.2 in. min.- ’ wcrc used.

Thotnris-Sch(inigcr 500-ml cotnbustion flasks were cmploycd. Although mos! of the combustions were pcrformcd with the conventional platinum mesh sample carrier used with thcsc flasks. the simplest combustion technique was to use a Thorna> 6514-F45. Ogg-type sample carrier hooked on a platinum wire extending from the

SHORT COMMUNICATION 149

stopper of the Schijniger flask. The sample was contained in a black wrapper and ignited by a Thomas 6516-SlO infrared igniter.

C’henliur1.s. The triphcnylphosphine, triphenylphosphine sulfide, and tri- butylphosphine were from Eastman Organic Chemicals. The 2-quinolylphosphonic acid, phenylphosphonic acid, and phcnyl-,+bromophenylphosphinic acid were supplied by Professor R. K. Ingham, and they had been prepared by Sanders” and Denhamh*‘.

Weigh 15-45 mg of the sample onto the filter paper. Add 20-25 ml of water. 0.5 ml of 1 M sodium hydroxide and l-2 ml of saturated bromine water to a 5OO-ml Scheniger flask. Burn the sample in the usual way. Allow 30 min for all vapors to be absorbed, acidify with I n/r hydrochloric acid, and boil to remove carbon dioxide and bromine. Cool the solution to room temperature, add 2 drops of methyl orange and adjust the acidity by adding 1 M sodium hydroxide and 1 M hydrochloric acid until the solution is just acid to methyl orange Boil again if much sodium hydroxide had to be added. Transfer to the titration cell containing a magnetic stirring bar. Insert the electrolysis and indicator electrodes. Adjust the pH rccordcr sensitivity to 7 pH units full scale. Start the magnetic stirrer and the chart drive. Turn on the dcsircd electrolysis current. Adjust the pH calibration control until the rccordcr pen is at the low pH end of the chart. When the HzPOi inflection point is passed, adjust the pH calibrate control to position the pen at the low pH end of the chart. Do not interrupt the chart drive while doing this. This prevents the pen from going off-scale at the HPO*‘- equivalence point. and although it makes a discontinuity in the titration curve. the distance between inflection points is thcquantity of intcrcst, not the absolute pH values. When the H P04’ - equivalence point is passed, turn off the electrolysis current and chart drive. Measure the time interval between the H,PO, and HPOh2- equivalence points in inches. Calculate the yd, P by eqn. (1).

‘X, p = LL;~,~;?_7,;,?!? (1)

where i= current (A), r=chart speed (set in. - ‘), S =distance between H,PO, and H POb2- cquivalencc points (in.), and u’= weight of the sample (g).

The method was first applied to the determination of phosphorus in tri- phenylphosphine and in triphenylphosphine sulfide. The absorption solution used initially contained no bromine and the phosphorus results were slightly low. Belcher and Macdonald* recommended the use of bromine to insure oxidation of phosphorus to phosphate. When bromine was added to the absorption solution, the results improved. The acidity of the solution was adjusted just before titration to a value

just on the acidic side of the H,PQ, equivalence point. This is desirable so that time is not wasted titrating a large excess of hydrochloric acid before the H,PO, break is reached. Methyl orange is a convenient indicator to use for this initial pH adjustment as it changes color slightly on the acidic side of the H,PO, equivalence point. The excess of bromine and any hypobromite formed by prior hydrolysis of bromine in basic medium must be removed by boiling before addition of methyl orange as they destroy the indicator.

/lncrl. Chirn. Acrcc. 61 (I 972)

IS0 SHORT COMMUNICATION

TABLE I

COULOMETRIC TITRATfON OF I’HOSI’HORUS . .._-_- ._-__- .._ _ . . ._ -. . __..... - _ ._____..__ -_..

~O??,~“JlIllt/ ‘I’hor. nrtr. ‘%, f’

‘i:, I’ .-_--_ _. _..-_..-_.._ . . _... - . . . . . _.

_ /imf . . _ ._. _ _. _

~friphcrrylph<lsphit,c tl.Ht Il.72 Tripticnylpliosphinc

SlllfldC 10.52 IO.57 I’h~nyl-rrr-hrctnro-

pllcnylph~spl~ini~ acid IO.42 IO.35 Tribu~ylpl~osptlinc” 15.31 15.02. 13.59 _._._ -_- . . . . --..-. _., _.. .._.. . ,_ ._.. -.. ._-... .--..

” I<csulW corrcelccl for I’ in ccllulosc ;iccI;lIc wpsolcs.

_..... _._. -- .__.. .._---_-_..--.- ..__ No. wplicurc Srd.

trrrcrl,vsr.~ der:. (‘y,) ._ _.__ __. _. __-.. . . . ..-._ _. ._.__. -_

11 0.15

H 0.22

7 0.27 2 -.-

.,. .__ __...,._ _.- __.....___ -- _-._ --

In the first attempts to pcrfotm the titration, the auxiliary electrode waz immcrscd in ;I potassium chloride solution in a straight isolation tube furnished as ar accessory with the Beckman Elcctroscan. High results for phosphorus were alway: obtained, and it was dccidcd that hydrogen ions fibcrated at the auxiliary electrode were carried into the titration solution by Icitkage from the isolation tube. The problem was eliminated by USC of a U-type salt bridge.

The results of a scrics of titrations of various organophosphorus compound: arc summarized in Tablo I.

The determination of phosphorus in organic liquids is complicated by the fact that the ccllulosc acctatc capsules used to weigh out liquids contain a significnn amount of phosphorus. An approximate correction for the phosphorus present il thccapsuleswasobtaincd by burning twoempty capsulesand titrating thecombustior products of the capsules. A phosphorus content of 0.738 and 0.690 mg was found it the two capsules burned. The average value of 0.714 mg P was subtracted from thl total phosphorus content calcutated from titration of the two tributylphosphinl samples.

Several attempts were made to determine phosphorus in 2-quinolylphosphoni acid by this method. The results were low and scattered over a wide range. It appeatec that the phosphonic acids did not burn completely in the combustion step. Th reason for this is not apparent, cspccially in view of the fact that no problems wer encountered in the combustion of phcnyl-nl-bromophenylphosphinic acid ant subsequent phosphorus determination.

The presence of sulfur and bromine in the compounds did not interfere in th analysis. There is no reason to expect any interference from chlorine in the molecul especially since chloride from hydrochloric acid was present in the solutions titrate<:

The method has the advantages usually associated with coulometric titra tions. No standard solution is required; and because the titration is followed b recording the pH at a known chart drive rate, little operator attention is require during the titration. Sulfur and halogens do not interfere as they do in many of th other methods for phosphorus. The accuracy is as good as or better than mo: present methods.

The author gratefully acknowledges the advice and assistance of H. I+ Benedict of the Diamond Shamrock Corporation.

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REFERENCES

1 T. S. Ma, Ar~crl. Clmr.. 42 ( 1970) 105R. 2 G. W. Ewing, I~zsrrun~errtol Methods oJC/ren~ical Amdysis. McGraw-Hill. New York. 3rd Ed.. 1969. p. 5X8. 3 H. H. Willard. L. L. Merritt ;md J. A. Dun. Insrrwmwttrl Methods o~:Amrlysis. D. Van Nostrand. Prince-

ton, 4th Ed., 1965. p. 575. 4 W. SchBnigcr. Mikrochirn. Acta. (1955) 123. 5 J. W. Sanders. M. S. Thesis, Ohio University, 1962. 6 J. M. Denham. M. S. Th4.s. Ohio University. 1956. 7 J. M. Dcnham and R. K. Inpham, J. Or!/. Chm.. 23 (1958) 1298. 8 R. Bclchcr and A. M. G. Macdomtld, 7’u/trr1rn. I (1958) 185.

And. Ckiul. Actu. 61 (1972)