Amperometric Titrations

rate of development. Emphasis on determination of organic compounds has decreased, while the application of organic reagents in the determination ...
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L REVIEW OF FUNDAMENTAL DEVELOPMENTS I N ANALYSIS

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Amperometric Titrations

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H. A. LAITINEN

Noyes Chemical laboratory, University of Illinois, Urbana, 111.

by the number of papers appearing in the period covered by this review (October 1, 1955, to October 1, 1057), the aniperometric method has settled down to a steady rate of development. Emphasis on determination of organic compounds has decreased, while the application of organic reagcnts in the determination of metals has been active. As in the last review (GO), the deadstop method is classified as an amperometric method with two indicator electrodes. RIethods based on potentiometry a t zero current or a t constant current are not covered. Coulometric titrations are included only if an amperometric end point detection method is involved. The “differential polarographic titration” of Klilopin and coworkers (50) involves the application of a potential difference through a high resistance across a pair of platinum electrodes. An opposed potential source is adjusted to give zero current after each increment of reagent. This method is a modification of a potentiometric titration a t constant current, and, therefore, should not be classified strictly as an aniperometric method. The method of Juliax (46) called by him an “amperometric titration with alternating current,” actually involves measurenient of an alternating current potential a t constant current, and consequently is an alternating current analog of a potentiometric titration a t constant current. On the other hand, the square-wave titrimetry method of Laitinen and Hall (61) involves the measurement of current and is properly classified ab an amperometric method. It differs froin the “polarization titration” of Pranck (89)in using a n applied square-n‘ave, rather than sine-wave, signal, and thereby avoiding the current due to charging the double layer. The theory of titrations with two indicator electrodes has been discussed by Dubois and Walisch (18); similar considerations for acid-base titrations with antimony (23) or quinhydrone (73) have bcen presented. A mathematical discussion of dead-stop precipitation titrations involving ferrocyanide in the presence of ferricyanide as depolarizer has been given (68). Samson UDGISG

(84)has considered a similar situation involving two idcntical second-order electrodes-e.g., silver,.silver chloride. Grunwald (S4) has considered the calculation of end points in conductometric and photometric titrations, and similar considerations could be applied readily to amprometric end points. Reviews of electrochemical methods (6), analytical applications of rotating platinum electrodes (65), and coulometry (99)contain material on aniperometric titrations. AIore specific rev i e w are those on ferrocyanide and ferricyanide titrations (51), dead-stop titrimetry (95), aniperometric titrations (58), and application of amperometric methods to the analysis of metals (7). APPARATUS AND METHODOLOGY

Berg (5) devised two cells for amperometric titrations using relatively small volumes of solution. In the first, for volunies not esceeding 2 ml., a fritted glass diaphragm was used to make connection with a calomel reference electrode. I n the second, designed for volumes of 2 to 15 ml., an interchangeable partition was used. A cell assembly involving an external calomel reference electrode with an intermediate compartment has also been described by Human and Leach

(59). A reference electrode based on the illn02-ilIn+f couple has been suggested (41) for titrations run a t relatively high positive potentials, thus avoiding the necessity of applying an external e.m.f. Various internal reference electrodes for voltnmnietry and amperometric titrations were compared by Jensovsky (42). Johannessoii (44) prepared a rotating platinum indicator electrode of relatively large area by using a glass bulb coated with a thin film of platinum. Electrical contact was made by a platinum wire sealed through the glass. AIalinek and Rehak (72) used a rotating platinuni crucible as the indicator electrode. Baumann and Shain (4) found a rotating gold microelectrode to be advantagcous over platinum in the iron- dichromate titration. Johannesson (45) used a rotating aluminum electrode for fluoride titrations with

thorium nitrate. Sppcial forms of platinum electroces incluclc the vibrating platinuni electrode used for percuprimetric titrations (43) and the dipping platinum electrode (28, 7 1 ) used in molten salts. The rotated dropping mercury electrode was p *oposcd by Stricks and Kolthoff ($6; as coiisiderably more sensitive than the conventional dropping mercury electrode. ION COMBINATION REACTIONS

Deschamps (16) titrated chloride in very dilute solution using a bimetallic electrode conr;isting of a silver or gold electrode COL pled n ith an amalgam electrode of platinum, gold, or silver. Acetone (007& or more) was used to decrease the solubility of silver chloride, permitting the titration of solutions a s dilute as 10-jV. For mixtures of chloride and bromide, or chloride and iodide, the titration curves showed breaks corresponding to the titration of the less soluble halide. For the titration of iodide, alcohol rather than acetone v a s recommended. Samson (84) describej a titration of chloride based on two polarized silver-silver chloride electrodes. It was concluded that the a~nperometric method a t constant potlintial is superior to the potentionietric method a t constant current, whe i applied to small concentrations of chloride. The beliavinr of sjlver halide suspensions has been studicd by Kolthoff and Stock (57). Addition of gelatin in halide titrations prevented the formation of a f lni of silver halide, which was reducibk a t potentials depending upon the solubility of the halide. Songina (91) used the anodic oxidation wave of iodide a t platinum to indicate the 4md point of the titration of silver nitli iodide. The best conditions were a potential of +1.0 volt in a lill amn.oniuni nitrate medium. Silver was titrated in the presence of copper, lead, and iron in ammoniacal tartrate medium using iodide as the titrant (76). An amper3metric method using a dipping platinum microelectrode was used in the evaluation of the solubility products of silver chloride and bromide in molten nitrates (28). The preciiitation of copper as copVOL. 30, NO. 4, APRIL 1958

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per(1) thiocyanate has been studied by Vasil'ev and Marunina (104). Selenite was unsatisfactory as a reducing agent for copper(II), but sulfite gave good results. The subsequent determination of zinc with oxine after precipitation of the copper was also described. Ries slid Hien (52), in a mathematical discussion of dead-stop precipitation titrations with ferrocyanide, concluded that titration errors may be minimized by decreasing the solubility, working with small titration volumes, and adding a considerable concentration of ferricyanide. The latter may, of course, interfere in some precipitation reactions, thus requiring the use of a single indieator electrode. Khosla and Gaur (51) reviewed aniperometric methods involving the precipitation of ferro- and ferricyanide, and glave results of recent work for several metals. Rleibs (55) studied the titration of silver with ferrocyanide, and reported the formation of the normal salt until, in the presence of excess titrant, the double salt IIAg3Fe(CN)s was formed. I n the reverse direction, the double salt was first formed, and then transformed to the normal salt. Gallium(II1) has been titrated to the normal ferrocyanide using two platinum indicator electrodes a t 50" C. (26). Even a t the elevated temperature, precipitxtioii was not instantaneous. Molybdenum and manganese have been determined by Degterev (14) in ferroniolybdenuin and ferroniaiiganese by ferrocyanide titration. Tuiigsteii was determined in ferrotungsteii by the same author (16). Vanadyl ion was used as a reagent for ferrocyanide and for phosphate (114). Lead ion has been used as a precipitant for sulfate in the determination of sulfur trioxide in aluniina (I), using acetone-water as the titration medium. The precipitation titration of molybdate with lead has been utilized to determine molybdenum in coordination compounds after destruction of the organic matter with nitric acid (3). Phosphate, chromate, arsenate, and sulfate interfered. The influence of dissolved oxygen in the titration of certain anions with lead has been discussed (47). Molybdate and tungsten have been titrated with mercury(1) using platinum microelectrodes (92). Silver ion was unsatisfactory because it catalyzed the reduction of molybdate and tungstate, forming mixed deposits of silver with molybdenum or tungsten. Pindeis and DeYries (27) precipitated potassium as the tetraphenylborate, dissolved the precipitate in acetonitrilewater mixtures, and titrated the tetraphenylborate ion with silver nitrate using a dropping mercury electrode as the indicator electrode. The titration of fluoride with thorium 658

ANALYTICAL CHEMISTRY

ner, and Temple (81) to the titration nitrate has been carried out dowii to of the mixtures bismuth-lend-calcium, 5 y in 5 nil. of s:llution ( I d ) , and down iron-manganese, and copper(I1)-calcium. to 1 to 10 y in 10 ml. (45). The anodic depolarization wave of Bismuth nitrate has been used as a EDTA was found advantageous in precipitant for ?)hosphate, using the titrations of substances which are not rotating platinL in microelectrode as readily reduced polarographically. The the indicator elrctrode (94). Ender feasibility of titrating 15 common metal (2.2) precipitated trimetaphosphate in ions n-ith EDTA was indicated. Nithe presence of other phosphates by kelly and Coolie (78) used mercury ineans of barium perchlorate, and after pool indicator electrodes for the EDTA hydrolysis titratcd the resulting orthotitration of micromolar solutions of phosphate with 13iOC104. Sodium tricadmium, copper, lead. nickel, and zinc. phosphate n as pi,oposed by Kobayashi I n favorable cases, titrations w r e suc(64) for the titrs tion of nickel and lead carried out a t concentrations to give complexeti of the form ~ L I P ~ O ~ O -cessfully ~. approaching l0-71lf. Tanaka, Oiwa, Such9 (97) jwxipitated cadmium, and IZodama (101) found correct recobalt, nickel, and zinc with ammonium sults for cadmium oiily in the presence thiocyanate in tE u presence of pyridine. of gelatin when the dropping mercury The reaction of copper proved to be electrode was used as the indicator unsatisfactory. Ho~vvever, Munemori electrode. Copper(I1) and lead(I1) (76) based a det2rmination of pyridine be titrated with EDTA in an on the prccipitai ion of C L I ( ~ J T ) ~ ( S C N could )~ acetate buffer of pII 4.2, using applied by titrating witP copper sulfate in the potentials of -0.12 and -0.60 volt, presence of a larg: excess of thiocyanate. respectively (100). Vladimirova (106) Zinc and cobalt(I1) were successfully deterniined zirconium by titration with titrated with IOHg(SCN),, using the EDTA in acid solution to an excess of diffusion current of mercury a t -0.8 reagent, which was back-titrated with t o -0.9 volt t o iiidicate the end point. bismuth. Divalent ions did not interAt more positiw potentials, a polarofere, but some interference was obgraphic maximum obscured the diffuserved with iron(II1) and aluminum. sion current plz teau. hIanganese(I1) and iron(I1) intwfered; the titration unsatisfactory. of copper(I1) mi8 OXIDATION-REDUCTION REACTIONS Vitkina (105) used copper(I1) as an aniperometric indicator for the titraMethods Based on Iron. Baumanii tion of calcium with oxalate in the and Shain (4) suggested gold as an presence of a sliglit excess of ammonia. indicator electrode for the iron-diCareful adji1stml:nt of conditioiis apchromate titration based on the cathodic peared necessary to avoid an indicator current of dichromate a t +0.4 volt, error. which can be observed n ithout interA reasonably t pecific, nlthough not ference from dissolved oxygen. Ishiparticularly seiis Live, method for cadbashi and coworkers (41) used elecmium was based on the titration with trodes of high positive potential, particiodide to form Ctl14-- in the presence ularly MnO&n++, as reference elecof an excrss of pyrsniidoii as precipitant trodes to avoid applying an e.m.f. in (48). The inteiiereiice of lead was observing the anodic diffusion current overcome by precipitation TT ith sodium of ferrous iron. sulfate; that of -iisniuth, by piecipitnVanadium(V) was titrated with irontioii with amnionia. hlercurg was (11), observing the anodic diffusion precipitated as copper(l1) ethylenecurrent of iron(I1) (83). diamine iodomci~curate(I1) [Cu(en),To determine chromium and vanaHgI,] by titration in the presence of dium in silica-alumina catalysts, IIiett iodide with ethy1:nediamine copper(I1) and ICobetz (35) digested the samples ion (79). The scllubility of the precipwith hydrofluoric and sulfuric acids to itate is a limiting factor; the principal remove silica, oxidized the chromium use is for determjnation of mercury in and vanadium with perchloric acid, the presence of excess halidf. and titrated the mixture with iron(I1). Gold has beel titrated in strongly To determine vanadium in the presence alkaline solution with thiosulfate or of chromium, the latter was selectively The mercaptobenzoth ,izole (25). reduced with sodium azidr, and vanamethod is recommended for the deterdium(V) was titrated with iron(I1). mination of gold in anode slimes a i d Alimarin and Terin ( 2 ) carried out a cyanide solutions titration of iron in the presence of Mann (73) dt lected acid-base end vanadium with cerium(1V) or permanpoints in aqueot j and alcoholic soluganate, using a rotating platinum electioiis by using two quinhydrone electrode a t a potential of +0.9 volt. trodes as amperclinetric indicator elecAfter the iron(I1) had been titrated, trodes. A simikr system, but based the potential n-as adjusted to 4-0.5 on two antimonj, electrodes, has been volt, and vanadinm(1V) mas titrated. proposed by Eno1.i and h1irisaka (23). Electrolytically generated cerium(1V) was used by Dilts and Furmaii (17) (Ethylenediniti.ilo) tetraacetic acid in determining titanium and niixtures (EDTA) was applied by Reilley, Scrib-

of titanium and iron. It was found best to generate about 90% of the required ceriuin(1V) before adding the reduced titanium froni a Jones reductor. Vanadyl sulfate was proposed (40) for the coulometric titration of iron(I1) with electrolytically generated vaiiadium(V), especially to aroid interference from high concentrations of sodium ion. Uraniuni(V) n-as used as an electrolytically generated titrant for iron(II1) (19); titanium(III), as titrant for iron(III), cerium(IS'), and vanadium(V) (70). Iron(I1) mas used as a titrant for nitrate nitrogen in explosives, with a dead-stop end point in the presence of relatively high concentrations of sulfuric acid (86). Ceriuni(II1) 11-as titrated to ccrium(ITT) with ferricyanide in concentrated solutions of potassiuni carbonate (64). Large amounts of ecrium(1V) did not interfere. Methods Based on Halogens. Bradbury (8) concluded that the aniperometric end point for iodine is more sensitive than the starch end point. In a critical study of tlie performance of the bromine-bromide electrode a t extrenie dilutions, Purdy, Burns, and Rogers (80) concluded that the amperometric method is somewhat more sensitive than tlie potentiometric method. This would presumably be true also for the iodine electrode, as compared mith potentiometry a t constant current (98). Lingane and Anson (68) have considered the current-determiiiiiig species present in systems containing copper(II), copper(I), bromine, and bromide with two indicator electrodes. Iodoinetric deterniinations of chlorine (37) and ozone (107) were based on the titration of excess thiosulfate n-ith standard iodate, using platinum indicator electrodes. Electrolytically generated iodine has bceii used to titrate antimony(II1) in tartrate solution a t p H 7 to 0 (69). To determine thallium, tliallium(1) was oxidized in the absence of chloride by means of perniaiiganate or peroxydisulfate, and, after eliniination of excess oxidant, titrated with standard bromide (93). The reduced current of thallium(II1) was observed a t a rotating platinum electrode. Liberti and Lazzari (67) generated hypobromite from a bicarbonate buffer, or hypoiodite from a sodium hydroxide solution, t o perform coulometric titrations with amperometric end points. Other Redox Titrations. Kolthoff and Jacobsen (56) took advantage of the formation of manganese(II1)-pyrophosphate coniplex to perform titrations of manganese(I1) vith permanganate or the reverse, and manganese(II1) with iron(I1). Procedures mere de-

veloped to eliminate the interference of cerium, chroniiunl, and vanadium. Jensovsky (43) based percuprimetric titrations using K4H3C~(103)2.H20 in 5M potassiuni hydroxide on the eathodic wave of copper(II1). A vibrating platinum electrode was used. The titration of gold with hydroquinone in 2.Y sulfuric acid a t BO" C. has been followed by means of a rotating platinum electrode (82). TITRATIONS INVOLVING ORGANIC REAGENTS

Zhdanov and coworkers (110) titrated bismuth with potassium iodide in the presence of excess 8-quiiiolinol (oxine) to form the sparingly soluble quinolinol ioclobismuthate. Cadmium, copper, and lead interfered. Oxine was used to determine zinc after 'the precipitation of copper as copper thiocyanate (104). lT7ilson and Rhodes (108) used 1nitroso-2-naphthol as a titrant for zirconium, measuring the diffu,'sion current of the reagent a t -0.4 volt. Small amounts of fluoride did not interfere nor did larger quantities of various metal ions. Cupferron has received considerable attention as a titrant. Elving and Olson (20) determined zirconium in magnesium alloys by titration in acid solution a t an applied potential of about -0.9 volt. The same authors (21) applied the method to samples containing uranium and niobium. Usateiiko and Bekleshova (102) determined titanium by adding excess cupferron and back-titrating, using a rotating platinum electrode to detect an anodic diffusion current. The procedure was also applied to ferrotitaniuni (103). Graham and VanDalen (52) used i,i-nitrophenylarsonic acid as a reagent for titanium a t p H 1.2. The precipitate normally is a monoarsonate, but with a large excess of reagent a diarsonate is formed. ITubicki and Cienciala (38) precipitated cadmium with 2-nitro-1hydroxy-4-benzene arsonic acid in acetic acid a t pH 3 and 70" C. illercaptobenzothiazole dissolved in ethyl alcohol or aqueous alkali has been used for several metals (9, 11). Lead can be titrated a t p H 3 to 7 using an applied potential beyond -0.5 volt or in alkaline solutions a t -0.9 volt. Cadmium was titrated in an acetate buffer; copper and cadmium, in cyanide medium. Optimum pI-1 ranges for the titrations of silver, mercury(I), mercury(II), and bismuth have been reported (10). Silver was used as a reagent for mercaptobenzothiazole (10). Gold can be titrated in alkaline solutions using the rotating platinum electrode (25). Silver was titrated with mercaptobenzothiazole or with mercaptophenylthiothiadiazolone (72). Rubeanic acid was found by Zhdaiiov

and coworkers (111) to be a suitable reagent for copper in sulfate, nitrate, or acetate solutions. Low results were obtained in the presence of chloride. This rcagent has been applied to the determination of copper in zinc, nickel, and cadmium plating baths (66), and for copper and nickel in steels (66). The latter cleterniination was based on the lower solubility of the copper salt, and was relatively poor in accuracy. Anthranilic acid has been used as a reagent for copper, zinc, nickel, and cobalt (112 118). Best results n-ere obtained a t concentrations of 50 mg. in 50 ml. cf t w t solution, containing 15 to 20% (:thy1 alcohol, a t pH 4.5 to 5.5. Using sodium anthranilate, Kostromin and Aparsheva (59) titrated copper in thi: presence of cadmium in an acetate buff1:r of pH 3.72, followed by the titration of cadmium a t p H 5.57. Lead anthranilate has sufficient solubility so that lead can first be titrated with sulfate, followed by cadmium with anthranilate Such: (97) found anthranilates and bromoanthr anilates suitable reagents for cadmiuni, cobalt, nickel, and zinc. Menis anll coworkers (74) titrated mercury(I1) in 0.4M nitric acid with tetraphenylarsoiiium chloride t o form a 1 to 1 compound, (C6H5)4A~HgC12K03. Shinagawa :tnd Matsuo (88) precipitated bismuth in the presence of iodide as ( C G H ~ ) ~ S Eusing B ~ I ~(C6Hj)3FjeClas the reagent. The precipitate fornied slowly; reac'ings were taken 10 niinutes after cach addition of reagent. Analogous reactions of bismuth(II1) and antimony(II1) were carried out by the samv authors (89), using dodecyltrimeth,d ammonium chloride as reagent. Dean and Bryan ( I S ) described the titration of magnesium with the dye Pontachrome Violet S W (Color Index 169), a t a FH of 11 and an applied potential of -0.6 volt. Ions which precipitate at p H 11 interfere by adsorption of the dye; several ions interfere by being reducible a t tlie working potential. Potassium xanthate has becn used as a reagent for copper ( S I ) . The copper solution is added to freshly prepared standard xanthate, and current readings are made a t -0.4 volt. Analogous rimtions for nickel and cobalt failed because of slow precipitation. Wilson and Wilson (109) described the titration of palladiuni(I1) with 1,2,3-benzotriazole in an acetate buffer. Several met2 1s interfered, but were easily separatxl from palladium. Tannic acid has been used as a reagent for germanium (77). The unk n o m soluticn was added to an acidified tannic acid solution, and current readings wer3 taken at -0.6 volt. Phosphoric avid has been precipitated VOL. 30, NO. 4, APRIL 1958

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as the o-oxyquinoline compound of phosphomolybdic acid (87). The use of hydroquinone as a reducing reagent for gold (82) has been mentioned. The reduction of iron(II1) with ascorbic acid (24) has been followed amperometrically, with strong oxidants found to interfere. DETERMINATION

OF ORGANIC COMPOUNDS

The amperometric nicthod has been used to study the sorption of iodine on starch (62) and the interaction between serum albumin TI ith the ions of mercury and zinc (86). Singh and coworkers (90) used a n amperomctric method involving a bimetallic platinum-tungsteii electrode system to folloir bromination reactions using bromate-bromide as a titrant. Aniline, o-aminobenzoic acid, phenol, p-aminobenzenesulfonic acid, o-cresol, 1-naphthol, and cinnamic acid were successfully titrated. Coulometric titrations using electrolytically generated bromine and a n amperoinetric end point have been used for the determination of trace unsaturation (63). Hypobromite and hypochlorite, generated coulometrically, were used for the titration of alanine, benzene sulfonates, and aminobutyrie acid (G?). Freedman (SO) titrated compounds containing positive chlorine or bromine n-ith sulfur dioxide in pyridine, using a dead-stop elid point. Various ATchloro and N-bromo compounds gave excellent results. Grebenovsky and Rezac (33) titrated formaldehyde in alkaline solution with hydroxylamine hydrochloride, using a pair of polarized platinum electrodes. Hinsvark and Stone (96) described the titration of oxalic acid with ammonium hesanitratocerate [(”& Ce(NO&] in glacial acetic acid. It was necessary t o add perchloric acid t o make the reaction reasonably rapid. The titrations of pyridine with copper(I1) in the presence of thiocyanate (76) and of mercaptobenzothiazole with silver (11) have been mentioned. B l BLlO GRAPHY

Alelrsandrov, S. N., Alekseev, S. A., Ir‘him. i Tekhnol. Topliva 1956, hTo.12, 65-7. Alimarin, I. P., Terin, S. I., Zavodskaya Lab. 21,777 (1955). Aylward, G. H., Anal. Chiin. Acta 14,386 (1956). Baumann, F., Shain, I., ANAL. CHEX.29, 303 (1957). Berg, H., Clumt. Tech. (Berlin) 7, 679 (1955). Bezier, D., Chiin. anal. 38, 273 (1956). Bozsai, I., Acta Chim. Acad. Xci. Hung. 9, 195 (1956). Bradbury, J. H., Chenz. &: Ind. (London) 1956, 922. Cihalik, J., ICudrnovska-Pavlikova, E., Chem. listy 49, 1640 (1955). Ibid., 51, 76 (1957). 660

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(11) Cihalik, J., 1i.udrnovslia-Pavlikova, E., Collection Czech Chem. Comniuns. 21, “18 (1956). (12) D’Amore, G., Faraone, G., Ann. china. (Rorie) 47, 142 (1957). (13) Dean, J. A. Bryan, H. A., Anal. China. Acttc 16, 180 (1957). (14) Degterev, N. AI., Zavodskaya Lab. 21, 917 (1C155). (15) Ibid., 22, 16;‘ (1956). (16) Deschamps, 1’. hi., Bull. soc. chim. France 19:;G, 126. (17) Dilts, R. ’t., Furman, N. H., ANAL.CHUL27, 1596 (1955). (18) Dubois, J. El., Walisch, W.,Conapt. rend. 242, 1161 (1956). (19) Edwards, I(. W.,Kern, D. M., ANAL.CHLU.28, 1876 (1956). (20) Elving, P. .,- Olson, E. C., Ibid., 28, 251 (l!M). (21) Ibid., p. 338. (22) Ender, G., 2:. anal. Chem. 138, 401 119531 (23) (24) (25)

(26) Fetter,.”. K.,Swinehart, D. F., ANAL.Cmni. 28, 122 (1956). (27) Findeis, A. P., DeVries, T., Ibid., 28, 1899 :1956). (28) Flengas, S. N., J . Chem. SOC.1956, 624

Fri&k, U. F., 2. Elektrochent. 58, 348 (1954). Freedman, R. W.,ANAL. CHEnr. 28, 247 (1956). Gagliardo, E., ICorner, hI., A t t i accad. nad. Lincei. Rend., Classe sci.fis., 7 n 7 t . enat. 14, 77 (1953). Graham, I:. P., VanDalen, E., Can. J . Clem. 35,418 (1957). Grebenovsky, E., Rezac, Z., Chent. listy 49, I 185 (1955). Grunmald, E., AXAL. CHEX 28, 1112 (1956). Hiett, T. A, ICobeta, P., Ibid., 28, 1495 (1956). Hinsvarlr, 0. H., Stone, IC. G., Ibid., 28, 334 (1956). Holluta. J.. Meissner. H.. 2. anal. Chem: 15:!, 112 (1956). ’ Hubicki, W..Cienciala. R.. Ann. Univ.. ill&iae Curie-Sklodowska, Lublin-Polonia, Sect. A A , 8 , 77 (1953). (39) Human, J. P. E., Leach, S. J., Chem. d * I n d . (London) 1956, 149.

Iinuma, H. Yoshimori, T., Besearch Xepts. I’m. Eng., Gifu Univ. No. 6 , 8 8 (1956). Ishibashi, LL, Fujinaga, T., Shinozulia, F.,Bull. Inst. Chem. Research, Kyoto Univ. 33, 229 (1955). Jensovsky, L., Chem. lis& 48, 1690 (1954); X. anal. Chein. 148, 119 (1955). Jensovsky, L., Chenz. listy 50, 1313 (1956). Johannesson, J. IC.. Chent. & Ind. (London) i956, 1141. Ibid., 1957, 480. Juhasz, E. Acta Chim. h a d . Sci. Hung. 9, 145 (1956). IChadeev, ‘ 1 . A., Trudy Sredneaziat. Gosudaral. Univ. irn. V . I . Lenina 5 5 , KO. :’, 25 (1954). IChadeev, V. A. , Makritskaya, E. IC., P’modskaya Lab. 22, 1286 (1956). (49) Iihadeev, V.A., Mirbadaleva, A. I.,

Zhur. Anal. Khim 11, 710 (1956). Iihlopm, N. Y., Gein, L. G., Bakhareva, A. A., Zavodskaya Lab. 21, 135 (1955). IChosla, 13. D., Gaur, H. C., Agra Univ. J . Research 4, 289 (1955). Icies, H. L., Hien, T. S., 2. anal. Chem. 148,91 (1955). Iileibs, G. A., Zhur. Anal. Xhim. 10, 244 (1955). Kobayashi, M., Nippon Iiagaku Zasshi 76,793, 796, 799 (1955). Iiolthoff, I. M., J . Assoc. Ogic. Agr. Chemists 39, 47 (1956). Iiolthoff, I. Af., Jaeobsen, E., Microchem. J . 1, 3 (1957). IColthoff, I. M., Stock, J. T., Analyst 80, 860,,(1955). Iionopik, N., Usterr. ChenzikerZtg. 57, 181 (1956). ICostromin, A. I., Aparsheva, hi. I., Zavodskaya Lab. 22, 544 (1956). Laitinen, H. A., ARAL. CHEM. 28, 666 (1956). Laitinen, H. A., Hall, L. C., Ibid., 29, 1390 (1957). Larson, B. L., Gilles, IC. A., Jenness, R., Ibid., 25,802 (1953). Leisey, F. A,, Grutsch, J. F., Ibid., 28, 1553 (1956). Leonard, G. W., ICeily, H. J., Hume, D. H., Anal. Chim. Acta 16, 185 (1957). Levitman, I