studied indicate that comparable results are obtained even when UOz(I1) and Sn(1V) are used. LITERATURE CITED
(1) Adams, R. N., “Progress in Polarography,” P. Zuman aiid I. M. Kolthoff, eds., chap. 23, Interscience, New York, 1962. -_
(2) Kelley, M. T., Miller, H. H., ANAL. CHEM. 24,1805 (1952).
(3) Kolthoff, I. ,,&I., Lingsne, J. J., “Polarography, 2nd ed., p. 24, Interscience, New York, 1952. (4) Ibid., p. 28. (5) Laitinen, H. A . , Rhodes, D. R,., 6. EEeclrochem. Soc. 109,413 (1962). (6) Miller. F. J.. ANAL. CHEM.35. 929 (1963). ’
.,
(7) ‘LPyrolytic Gryhite, Preliminary Engineering Data, Metallurgical Products Department, Specialty Alloys Section, General Electric Co., Detroit 32, Mich., 1062.
(8) “Pyrolytic Graphite Propert Data,”
High Temperature Msteriag, Inc., 130 Lincoln St., Brighton, Maw., Oct,
6, 1961. (9) Walker, P. L., Jr., Am. Scienfhl 50, 259 (1962). RECEIVEDfor review June 14, 1963. Accepted August 12, 1963. Division of
Analytical Chemistry, 144th Meeting, ACS, Loe Angeles, Calif., April 1963. ORNL is operated by Union Carbide Nuclear Company for the U. S. Atomic Energy Commission.
A Direct Argentometric Titration of Orthophosphate Application to Coulometric Titration GARY D. CHRISTIAN ‘and EDWARD C. KNOBLOCK Division o f Biochemistry, Walter Reed Army lnstifute of Research, Walter Reed Army Medical Cenfer, Washington 7 2, D. C.
WILLIAM C. PURDY Departmenf o f Chemisfry, University of Maryland, College Park,
b A direct titrimetric procedure for orthophosphate is described. The orthophosphate is titrated with silver ion, either from silver nitrate solution or coulometric generation from a silver anode, in an 80% ethonol0.1M sodium acetaie medium. The end point is detected either potentiometrically or amperometrically. The method is applicable to a minimum orthophosphate concentration of 2 X 1O-4M with potentiometric end point detection and 1.7 X 1O-3M with amperometric end point detection. A relative error of 0.9‘5 is obtainable. Halide ions interfere because of coprecipitation An ecual molar quantity of sulfate can b e tolerated bui calcium, aluminum, and ferric iron must b e absent.
M
VOLUMETF:IC methods for orthophosphate have been described. Some involve the precipitation of phosphomolybdate (6, 15) and others use either cerium (10) or bismuth (IS) solutions as the titrant. Calamari and Hubata (2) have described a volumetric method for phosphate based on a titration between two pH values. In addition, there are nimerous methods involving the titration of liberated acid after reaction of the mono- or dihydrogen orthophosphate with excess silver ion (I, 3,6,9,11, 12, 11:). Flatt and Brunisholz (4) developed a direct potentiometric titration with silver ion in a borate buffer solution of p H 9.0. Although these authors do not state the concentration range of their determination, they describe it as being “of limited accuracy.”
ANY
Md.
The present paper describes a direct argentometric titration of orthophosphate in an aqueous-ethanol medium. This medium renders the silver phosphate precipitate sufficiently insoluble to give an adequate end point. This method should prove useful as a rapid and convenient means of standardizing orthophosphate solutions used as standards for orthophosphate determinations in biological or other samples. It could be used t o determine orthophosphate levels in samples free from interfering substances. If the coulometric application is used, no standard solutions need be prepared. EXPERIMENTAL
Reagent grade chemicals were used without further purification. The standard silver nitrate solutions were prepared from primary standard silver nitrate. The normality of the silver nitrate solution was checked periodically by titration against a standard solution of sodium chloride; dichlorofluorescein was the indicator. Standard orthophosphate solutions were prepared by dissolving potassium dihydrogen phosphate in a suitable volume. The orthophosphate-ion concentration in these solutions was determined independently by the ammonium phosphomolybdate and magnesium pyrophosphate gravimetric methods. The bromide and iodide solutions were standardized by titration with the standard silver nitrate solution. I n all titrations of orthophosphate, the titration vessel was covered with aluminum foil to inhibit light-induced decomposition of the precipitate. All potentiometric measurements were made with a Beckman Model G p H meter using a silver wire indicating electrode and a saturated calomel refer-
ence electrode (S.C.E.). All amperometric measurements were made by manually recording the current on a Sargent Model XV polarograph, using a platinum-foil indicating electrode (0.9 sq. cm.), the potential of which was set a t zero volt us. the S.C.E. During the course of the titration, the solution was stirred with a magnetic stirring bar. The S.C.E. was separated from the test solution by a salt bridge containing 0.5M potassium nitrate and 3.5% agar-agar. Silver ions were generated with a Sargent Model IV coulometric source a t a coiled silver-foil anode (11 sq. cm.). The platinum-foil cathode (1 sq. cm.) was isolated from the solution by placing it in a glass tube, the end of which was closed with a sintered-glass frit; the catholyte was 2M sodium acetate. The coulometric vessel used was either a 250-ml. beaker or a 50-ml. weighing bottle. Volumetric Procedure. The sample in 100 ml. of supporting electrolyte containing 80% ethanol and 0.1M sodium acetate was titrated with a standard solution of silver nitrate. The end point was determined either potentiometrically or amperometrically. I n the former case, a silver wire served as the indicating electrode and the S.C.E. w~lsthe reference. In the latter case, the electrode system was a platinum foil-S.C.E. pair across which was impressed a potential of zero volt. For the amperometric end point, the supporting electrolyte contained 0.2% gelatin. Coulometric Procedure. The sample in either 30 or 100 ml. of supporting electrolyte containing 80% ethanol and 0.1M sodium acetate was titrated with silver ion. The silver ion was generated a t a silver-foil anode a t a rate of 19.3 ma. The platinum-foil cathode was isolated in a glass tube containing a 2 M sodium acetate VOL. 35, NO. 12, NOVEMBER 1963
0
1869
4001
5001 400
-
300 -
J.p
200-
v)
vi
’
2oo 100-
I60
1
2
3
4
5
6
7
8
rnl. AgNO,
Figure 1. Titration of 2.00 ml. of 0.1 017M potassium dihydrogen orthophosphate in 100 ml. of 80% ethanol., 0.1 M sodium acetate with 0.1 001M silver nitrate
ANALYTICAL CHEMISTRY
0
Figure 2.
I
2 3 4
5 6 7 8 9 1011 12
Titration of phosphate-halide mixtures
Curve 1. 5.00 ml. o f 0.104M KBr plus 2.00 ml. of 0.095M KHzP04 in 100 ml. of 80% ethanol, 0.1 M sodium acetate; titrant IS 0.1 001 M silver nitrate Curve 2. Curve obtained from litration of 5.00 ml. of 0.1 0 4 M KBr in the absence of orthophosphate Curve 3. 5.00 ml. of 0.1 OOM KI plus 2.00 ml. of 0.095M KHzP04 in 100 ml. of 80% ethanol, 0.1 M sodium ocetate; titrant i s 0.1 001 M silver nitrate Curve 4. Curve obtained from tiiration of 5.00 ml. of 0.1 OOM KI in the absence o f orthophosphate
DISCUSSION
1870
250
ml. AgNO,
catholyte and was connected to the test solution through a sintered-glass frit. The end point detection mas the same as in the volumetric procedure.
Ethanol water solutions were chosen for the solvent system to decrease the solubility of the silver phosphate; the solubility of silver phosphate is reported to be 0.7 X 10-4X (4). Tenth molar sodium acetate mas used as the electrolyte to maintain a pH a t which the precipitate would remain insoluble. In Figure 1 is shown a typical potentiometric titration curve of 2 x 10-3M orthophosphate in 80% ethanol, 0.1M sodium acetate; the titrant is 0.10013f silver nitrate. It is apparent from the end point that the precipitate is the Ag8P04species. The end point potential was determined to be 296 mv. us. S.C.E. by a second derivative plot of the titration curve. Good end point breaks were obtained with orthophosphate-ion concentrations as small as 2 X 10-4-U. A series of 10 titrations was made, either coulometrically or with standard silver nitrate, between these two quoted orthophosphate concentrations m-ith a relative error of 0.8%. The titration of 6 X 10-5M orthophosphate with 0.01M silver nitrate did not give a sufficiently reproducible potential break for end point detection. Flatt and Brunisholz (4) recommended that the pH of the medium for their direct potentiometric titration be maintained at a value of 9.0. Titrations were carried out in an 80% ethanolborate buffer solution. In this medium, considerable coating of the electrode surface was noted; the drifting of the potential made measurements difficult. However, the apparent end point potential break was about 50% greater than that for the titration carried out in the sodium acetate solution.
-200L-FKL--
ml. AgN03 Figure 3. A.
Time ( S e d
Typical amperometric titration curves of orthophosphate
Titration of 5.00 ml. of 0.1017M KHzPOI in 60 ml. of 80% ethanol, 0.1 M sodium acetate, 0.2% gelatin with 0.1 02M silver nitrate. Current corrected for volume changer Blank titraiion for generation of silver ion at 19.3 ma. in an elecirolyte consisting o f 80% ethanol, 6. 0.1M sodium acetate, 0.2% gelatin. Total volume i s 3 0 ml. 0 0.50 ml. of 0.1 06M KH2P04 in 3 0 ml. of 80% ethanol, 0,1 M sodium acetate, 0.2% gelatin. Silver ion was generated a t a current of 19.3 ma.
Titrations were attempted with mixtures of halide ions. In Figure 2 is bhown the titration curves of approximately cquul normnl i~oncrntrationiof ioditlr nntl bromide ion, atlmi\ed nith ortlioiilii~-l)hate 7’he prpciiioii of the hnliilp cnil puiiiti \\a\ not :ippre(+ildy Y I llin\rLer, t l i c b ort1iol)lii)sl’li:Lte end point5 occurred 23 and GO%, too soon, reyiectively, indicating con4 e r a b l c coprecipitation of the orthophohphate nith the silrer halide. Othrr poqsible interf 3rences 11ere also in\ c-tigated potentiorietricdly. Five milliliterb of 0.1.11 ortliophosphate mas titrated with 0 1JI s i l ~er nitrate in the preqence of an equal niolar quantity of the subdance being tested. The presence of sulfate lielded titration values : h u t 1 % greater than in it, absence. Khile this i. noticenbli., it appears that u p to equal molar q113 ntities of sulfate can be tolwatccl nithout great 10‘s of nccuraq . C‘alcium and aluminum each interfered niarkctllI\-. In t h ? case of calcium, the addition of a tno-fold excess of EDTd only partially eliminated the interference. Ferric iron also markedly affected the titration. The addition of a ten-fold excess of sodium fluoride again only partiallj eliminated the interference. Bniperonietric end point detection wab a1.o investigated. In this case, it n a b nece.sary to add gelatin to prevent depolarization of the ir dicator electrode by the sill cr phosphate precipitate. Silver (ahloride is known to depolarize a plntinuin electrode. Laitinen and co-
workers (7, 8 ) wed 0.1% gelatin to prevent depolarization by silver chloride. In the titration of orthophosphate with silver ion, the best amperometric end points mere obtained in the presence of 0.2yogelatin. Even in the presence of gelatin, huwever, the silver phosphate precipitate depolarized the electrode somewhat, especially in the more dilute solutions. This is apparent in the amperometric titration curves to be discussed. Figure 3 shows typical amperometric and titration curve3 of 8.4 X 1.7 X 10-3X orthophosphate. The curve in 3 A was obtained by titration with 0.102M silver nitrate solution. The current readings were corrected for volume changes. Curve 3 B was obtained by coulometric generation of silver ion at a generation rate of 19.3 ma. The appreciable current readings noted as soon as qilver ion was added indicated some depolarization of the electrode by the silver phosphate formed. Because of this appreciable depolarization, a special procedure had to be used in obtaining the end point. Instead of extrapolating the current to zero and finding the end point a t this value, the base line had to be drawn through the line of points immediately preceding the end point, as in Figure 3 A . In the case of the more dilute solutions of orthophosphate, there was noted a rounding of the current increase beyond the elid point. Therefore, the current had to be extrapolated in a curJed fashion, as shown in Fibure 3 b. This method was used with several titrations
and vias found to be reproducible. There was always an apparent current break between one third and one half of the end point value (see Figure 3 B ) . At phobphate concentrations less than 1.7 X 10-3.11, the amperometric end point was inadequate. I n a Yeries of coulometric and volumetric. titrations between the orthophosphate concentrations of 1.7 X 10-3 and 8.4 X l O - 3 J I , a relative error of 1% wa> obtained. LITERATURE CITED
(1) Brunisholz, G., Heln. Chim. Acta 30, 2028 (1947). ( 2 ) Calamari, J. .4.,Hubata, R., IXD. ESG.CHEAT., BSAL.ED. 14,55 (1942). (3) Cullum, D. C., Thomas, D. B., Anal. Chzm. ilcta 24. 205 11961). ( 4 ) Flatt, It., hrunishcilz,’ G., Ibitl., 1, 124 (1947). ( 5 ) Gerber, A. B., Miles, F. T., I r u . ENG. CHEM.,ANAL.ED.13,406 (1941). (6) Harrison, T. S., Parratt, T., J . SOC. Chem. Ind. iI,ondon) 64.218 (1935).
( 7 ) Laitinen, ‘ H. A.,’ Jennings, k. P., Parks, T. D., IND.ESG. CHEM.,AUL. ED. 18, 355 (1946). (8) Laitinen, H. A., Kolthoff, I. M., J . Phys. Chem. 45, 1079 (1941). (9) Moerk, F, Hughes, E., Am. J . Pharm. 94, 6 ~ (1922). 0 (10) Rancke-Madsen, E., Kjaergard, T., Acta Chem. Scand. 7 , 735 (19,53). (11) Sanfourche, iz., Focet, B., Bull. SOC. Chim. France 53,963 (1933). (12) Simmick, H., 2. dngew. Chem. 48, 566 (1935). (13) Thistlethwaith, W. P., Analyst 77, 48 11952). (14) Toller; W.,Mztt. Ver. Grosskesselbesztzer 1946, 88. (15) Wilson, H. X., Analyst 76, 65 (1951).
RECEIVEDfor review May 24, 1963. Accepted August 9, 1963.
Polarography of Nitroferrocene A. M. HARTLEY and R. E. VISCO1 Department of Chemistry and Chemical Engineering, University o f Illinois, Urbana, 111.
b The polarographic reduction of nitroferrocene in newiral or alkaline buffers is found to b e a diffusion controlled, concentraiion dependent, six-electron process. The half wave potential of the reduction is observed to shift 58 mv. per p H unit. In situ electrochemical reduction of nitroferrocene within an electroil spin resonance (e.s.r.1 spectrometer produces an unstable radical with a nitrogen coupling constant of 15.1 gauss. In aqueous buffers, nitroferrocene undergoes a photuchemical decomposition with 1 nitrocyclopentadieneide as one product.
-
s
THE DISCOVERY of the socalled L‘sandwich” compound, dicyclopentadienyl iron or “ferrocene,” numeroils investigations have been
INCC
concerned with the aromatic character of the parent compound and its derivatives. These studies have been reviewed by several workers (14, 18, 19). Recently, Rausch, Fischer, and Grubert (17) have compared the reactivities of ferrocene to the corresponding ruthenium and osmium compounds. It has been s h o m that ferrocene can undergo many of the reactions typical of aromatic benzenoid compounds such as Friedel-Crafts acylation and alkylation, sulfonation, arylation, and metallation with butyllithium. On the other hand, the compound cannot be considered completely an aromatic substance since even the mildest of nitration conditions usually employed with aromatic compounds leads to destruction of ferrocene. Nitroferrocene has been prepared by Helling and Shechter (6) and by Grubert and Rinehart (4)using a method
which is unusual for aromatic nitrations. Ferrocenyllithium was treated with n-propyl nitrate a t -70” C. leading to the displacement of lithium by nitrate. The structure has been confirmed by elemental analysis and conversion to As an illustrative aminoferrocene. point concerning the quasiaromatic character of ferrocene, the nitroferrocene once prepared shows an infrared absorption a t 1507 cm.-l typical of aromatic nitrogroups. Ferrocene can be oxidized to the ferrocinium ion electrochemically in water (db), in 90% ethanol ( I S ) , and in acetonitrile (10). The electrochemical process has been shon-n to be reversible Present address, Electrochemical Research & Development Department, Bell Telephone Laboratories, Inc., Murray Hill, K. J. VOL. 35, NO. 12, NOVEMBER 1963
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