POTENTIOMETRIC TITRATIONS N. HOWELL FURMAN Princeton University, Princeton, N . J .
T
tion was found to be most effective in detecting gelatin. He likewise found ( 8 ) that urine lowers the jump in potential during the titration of lead with ferrocyanide ion. Wenger and associates (88) found that 0.001 N silver ion could be titrated with a precision of 0.5% with 1-nitroso-2-naphthol at pH 8. In more concentrated solutions the p H must be below 8 for 1.5 X lo-* AVand below 7 for 1.5 X lo-’ N solut,ion. Silver and the reagent combine in 1 to 1 molar ratio. Usatenko and Datsenko (84 ) proposed the addition of standard .silver solution prior to titration of copper with standard cyanide solution in anunoniacal solutions of nonferrous alloys. T h e difference between this titration and one of a like amount of the silver solution gives a measure of the amount, of copper. Seizoldi ( 4 7 ) has affirmed t,he usefulness of the potentiometric cyanide method for nickel as well as the reduction of chromate with ferrous solution in routine steel analyses. Hindnian et d.(SO) have used the silversilver chloride electrode in the estimation of the “chlorinity” of sea water. Other Processes. The titration of chloride ion by standard iiiercuric nitrate has been reinvestigated by Vyakhirev and Guglina ( 8 6 ) , using a platinum wire indicator elect,rode us. a salt bridge and either a silver-silver chloridr or a quinhydrone electrode. An extensive study of the change in pH during precipitation of barium sulfate has been made hy .kites ( 4 ) . The precipitation of barium ion with aulfatert of lithium, sodium, potassium, rubidium, cesium, magnesium, nickel, and zinc was studied, as was the influence of potassium, chloride, lithium chloride, and colloids. Because of hydrolytic adsorption, there is in general a pH change which can be followed with a glass or an antimony indicator electrode. In simple situations the error does not exceed 0.5%. Heavy metal sulfates are decomposed by sodium carbonate and the sulfate solution after filtration is adjusted t o pH 5 to 8 before titration. The titration of zinc ion by ferrocyanide, using the platinuncalomel electrode system, has been recommended by Pollak and 8hemyakin (65)in connection with the analysis of magnesium alloys. Magnesium, manganese, and aluminum do not interfere. Iron, if present, must he complexed by oxalate prior to the titration. Talipov and Teodorovich (79) found that a ratio of ferric ion to ferrous of 0.033/0.002 was desirable in the reagent t,hat is used for the estimation of fluoride. Chirkov (12) applied the fluoride complex reaction to the rapid estimation of aluminum in ores after fusion with sodium hydroxide, acidifying with hydrochloric acid and buffering with sodium acetate. The standard solution contained sodium fluoride. -1 potentiometric and spectrophotometric study by Ahrland ( 3 )has given evidence of the following complexes of uranyl ion in monochloroacetic acid solutions: PO?CH?CICO?+!UO,(CH,CICOI.h, and I:O,(CH,ClCO?k.
HIS review is in the main confined to potentiometric titration methods as applied in chemical analysis. Brief mention, with literature references, is made to the newer techniques of amperometric titration and coulometric titration. I n the year subsequent to a previous review (17 ) by the writer there has been a steady flow of papers on detailed applications of potentiometric and related methods. The writer (19) has published a briei general review of electrochemical methods. The use of electrometric methods in the determination of functional groups of organic compounds has been reviewed by Lykken ( 4 3 ) . The application of physical methods in the analysis of beryllium-copper alloys has been reviewed by Seguin (66). One of the novel developments during the year has been the application of potentiometric methods to reactions with lithium aluminum hydride for the estimation of organic functional groups. This drvrlopnient is ieviened under olidation-reduction methods. APPARATUS
:iutoniatic recording equipment for p H measuremelit or titrations, developed by Kraus et a l . (S) uses ,the vibrating reed condenser in attaining balance; a high degree of precision and accuracy is attained. h stabilized circuit for use in detecting end points by a tuning tube (“magic eye”) has been developed hy Iiieselbach ( 3 2 ) . The apparatus is adapted to the Karl Fischer method for estimat.ion of water. A short summary of potentiometric micromethods and apparatus is to be found in the recent treatise on ultramicroanalysis by Kirk ( 5 3 ) . The following sections list applications by fields of volumetric analysis. hi1
PRECIPITATIOY AND COhIPLEX FOR%lATION
Processes InvoIving Silver Ion. Gauguin (21) has shown that platinum electrodes that are used during titrations with silver may indicate either silver ion concentration or the slower change that is present, during thiocyanate tit,rations: 2HCS (CNS)2 2 e . Silver follow the silver-ion concentration in it. indication, but, as has long been known, many titrations involving silver ion may he followed potentiometrically with a platinum indicat,or electrode. In a further paper Gauguin ( 2 2 ) has presented data on the reducing action of cyanide and thiocyanatc ions. Shchigol ( 6 7 ) has shown that silver ion may be titrated successfully in ammoniacal solutions that contain bromate, iodate, phosphate, oxalate, nitrate, or chromate. The technique has also been applied to collargol and protargol preparations. A standard solution of either ammonium thiocyanate or potassium iodide wa.4 used. The analysis of mixtures of dichromate and chromate was investigated by Shchigol and Birnbaum (69). One portion of the solution was titrated with alkali, using a platinum-platinum black electrode system. I n another portion of the solution chromate was titrated with silver in ammoniacal solution, using a silver wire as indicator electrode. I n the titrat,ion of a mixture of chloride and chromate with silver in ain~noniacalsolution, these investigators (68) obtained a good first break in e.ni.f. corresponding to the reaction of the chloride. In case of chromate there are jumps in potential corresponding to the formation of Ag(NH3)2(CrO,)Z---, L4g(NH3)2Cr01-, and [.kg(KH3)z]zCr04,respectively, the last being useful in the 0.1 dVammonia solution. Achiwa ( 1 ) has noted the effect of gelatin on the following titrations: silver with chloride, cupric with cyanide ion, lead with ferrocyanide ion, and mercuric with iodide ion. The lead titra-
+
iCID-ALKALI PROCESSES
Important work by Schwarzenbach and Biedermann [6’.;, 6 6 ) has made possible the determination of various metallic ions by titrating the acid that is liberated when the metallic ions are complexed by various agents. Salts of nitrilotriacetic acid may be used in two ways: In one the dipotassium salt, L H X (where X represents nitrilotriacetate), which reacts neutral is added to the solution of metallic ion. Bivalent ions liberate one hydrogen ion: l f + & HX-- = M X - H + , whereas trivalent
+
21
+
22
ANALYTICAL CHEMISTRY
ions such as Ce+++, La-'+, or F e + + + liberate two hydrogeri ions: C e + + + 2HX-- = CeX22H+. I n either case the hydrogen ion that is liberated may be titrated with alkali. Good results were obtained for cadmium, cobalt, copper, iron (II), mercury, manganese, lead, nickel, zinc, cerium, and lanthanum; presumably other rare earths may be estimated. Beryllium, chromium(III), and iron(II1) were not determined satisfactorily. The second procedure involves direct titration with a solution of the salt KIX. The p H of the solution increases only after the end of the process of complexing the metallic ion. These authors (65) studied in similar fashion the use of sodium salts of ethylenediamine tetraacetic acid. The salts XaZHBY (where Y represents ethylenediamine tetraacetate), known in this country as Versene, and Na3HY or NaPYmay be used in titrations. The first two salts liberate hydrogen ion, which may be titrated:
+
+
I n titrations with Na4Y, acid is bound and the pH rises only after the complex has been formed. Good results were obtained with bivalent metals, cerium and lanthanum. Aluminum, chromium, and ferric ions form hydroxy complexes, (AIY0H)--, etc. The formation of these complexes causes the potentiometric titration curve to have two jumps, one after one equivalent and the second after three equivalents. I n case of iron, only the change after three equivalents is marked. The modern method for hardness of water is based on titration with Na2H2Y, with eriochrome black T added for indication. Magnesium forms a colored complex with the indicator dye, and as soon as both calcium and magnesium are complexed by Versene there is a sharp color change. An antimony electrode was used by Guntz (B6) to follow precipitations of calcium, barium, etc., ions with standard sodium palmitate solution. Blaedel and Panos ( 6 ) found that comple\ing of aluminum with o ~ a l a t ewas the most satisfactory procedure for estimating free nitric acid down to 0.001 molar in 2 JI aluminum nitrate solutions. The effects of phosphate, fluoride, acetate, and ferric ion on this process were studied. Katchalsk) and Gillis (51)studied the potentiometric titration of polyacrylic and methylacrylic acids and developed equations to account for the difference in the titration curves of these acids as compared with simpler acids. The copolymers of vinyl acetate and crotonic acid were also studied. Roe and Swern ( 6 1 )found that longchain hydroxamic acidq may be estimated by hydrolyzing with mineral acid :
0
11 11 RC-K-OH
+ HY + 13.~0
RCOOH
+ HOXH2HS
niter which the mixture is titrated potentiometrically to pH 4. dhcliigol and Birnbaum (69) have recently utilized the titration of mixtures of dichromate and chromate with alkali in conjunction with argentimetric methods. Ryabchikov ( 6 3 ) found that electrometric t,itration with acid reveals differences between cis and trans isomers of complex ammines-for example, cisPt( XH3)z(OH)~ has two inflections in the potentiometric titration curve, whereas the trans compound has one. hfichel ( 4 6 ) found a correlation between enzyme activity and pH as measured by pH change in an electrometric method for determination of red blood cell and cholinesterase activity. Chaplin ( 10) has reviewed the various factors in the industrial control of p H during various types of processes. Leonis and van Nechel ( 4 0 ) have demonstrated the utility of titrations of two-component weak acid mixtures in nonaqueous media.
SyrokomskiI and Melamed ( 7 7 ) have found that the platinumperiodate-iodate electrode gives a break of 0.17 volt between p H 9.2 and 9.5 and have proposed it as an indicator in acid-alkali titrations. Tuddenham and Anderson (85)have found that a calcium chloride-calcium hydroxide buffer is suitable for the range pH 11 to 12.65, a region in which there have hitherto been few, if any, effective buffer mixtures. OXIDATIOY-REDUCTION PROCESSES
Determinations with Ceric Sulfate or Potassium Permanganate. Gladushko (24) has utilized the familiar process of titrating manganese(I1) to manganese(II1) with potassium permanganate in a solution 2 N in hydrochloric acid and sodium fluoride. After the solution is made ammoniacal, cobalt is determined by the ferricyanide oxidation process. Pribil (64) has reinvestigated the process originally published by the writer (28) for the successive potentiometric titration of antimony(II1) and arsenic(II1) in the same solution with ceric sulfate. After the antimony has been titrated a t proper acidity, iodine monochloride catalyst is added and the arsenic is titrated. Pribil found that lead, tin, copper, cadmium, and zinc do not interfere. Pribil and Lfalicky ( 5 5 ) titrated small amounts of cobalt ( 5 to 30 mg.) in presence of 0.5% Versene and with calcium acetate added, using standard ceric sulfate. Nickel and manganese interfere, but many other metals no not. Stehlik ( 7 2 )found that molybdenum blue that was produced by reduction of molybdate by zinc, stannous, lead, or titanous chloride may be titrated with ceric sulfate or with permanganate in presence of manganous sulfate if air is excluded: Mo020H
+ C e f 4 + H20 = 11004-- + Cef3 + 3 H +
Hypobromite. According to Reichmann (58) the potentiometric titration of thiocyanate with hypobromite proceeds best in a solution that contains 10% of sodium hydroxide a t the end point. Zethelius ( 9 1 ) found that phenol, thymol, etc., could be oxidized most effectively by hypobromite by adding an escess of reagent followed by back-titration. Iodate, according to Gauguin ( 2 1 ) , oxidizes thiocyanate by an over-all process 2SCN-
+ 3103- + 2 H + + C1-
=
2S04--
+ 2ICN + IC1 + H20
In this process SCN-, HCN, and I - react successively. Reference should also be made to his studies of the reducing action of SCN- and C N - (28). Bromate and Bromide. Srinivasan (70) used the polarized platinum electrode system in the bromometric titration of procaine, sulfanilimide, and related compounds. A 0.001 N potassium bromate solution was used. The dead-stop method has also been employed by Braae ( 7 ) to determine the end point of the mercury-catalyzed addition of bromine to double bonds. Oxygen. Tobias and Retondo (80) devised a hypodermic syringe cell with platinum and calomel electrodes for measuring the oxygen tension of fluids without gain from or loss to the oxygen of the air while taking specimens. Formal Potentials. Syrokomskii and associates (76) have made new measurements of the titanium(II1)-titanium( IV)potential extending to 6 N sulfuric acid (0.246 volt) or 10 N hydrochloric acid (0.279 volt). They ( 7 8 ) found 0.359 volt for the vnnadium(I1)-vanadium(II1) system in dilute sulfuric acid. IODINE-KARL FISCHER METHOD
Two different approaches have been proposed for the estimation of small amounts of water in oils, greases, etc. Roberts and Levin
V O L U M E 2 3 , NO. 1, J A N U A R Y 1 9 5 1 (60) collected the water froni oils and greases by distillation, with dry benzene for samples other than greases, where pyridine was used. The distillate, protected a t all times from atmospheric moisture. was titrated a i t h Fischer reagent. Hanna and Johnson ( 2 7 ) concentrated the small amount of water in benzene, decane, or petroleum paint thinner by evtraction n ith dry ethylene glycol. Over 90% of the water was extracted in a single treatment and three extractions gave practically quantitative recovery. Hydrogen sulfide, mercaptans (thiols), and ketones interfere by reaction with the Fischer reagent during the titration. FERRICYANIDE PROCESSES
In the preceding review ( 1 7 , page 38) numerous references were given t o such processes as applied to estimation of cobalt. Further application has been made of the process t o estimations of cobalt or manganese. Yardley (90) protects the solution of cobalt in dilute ammonia with petroleum ether during addition of excess of ferricyanide to prevent access of air. The back-titration is made with standard cobalt solution. Chepik ( 1 1 ) employed the Muller system of two platinum electrodes, one enclosed in a capillary during the titration of about 0.001% of cobalt in nickel by the ferricyanide procedure. Gladushko ( 2 4 ) used the back-titration method with standard cobaltous solution after adding a measured excessive amount of ferricyanide to the ammoniacal solution. This procedure mas applied after the manganese had been determined in the same sample by oxidation with permanganate in the presence of fluoride. Pribil and Simon (56) determined manganese by potentiometric titration with ferricyanide under benzene to exclude air. Versene was added t o the acidic solution, which was boiled r i t h 0.1 ,V potassium dichromate before being made ammoniacal. Vanadate and copper interfere with the process and if both iron and cobalt are present a bicarbonate-sulfite separation treatment must be made. Iron alone, if present, is reduced with zinc. Tomicek, Sandl, and Simon ( 8 2 )found the direct titration of manganese in ammoniacal solution to be satisfactory if oxygen was excluded. Cobalt could be determined satisfactorily only if an excess of ferricyanide was added, followed by back-titration with standard cobaltous solution, Tomicek and Sandl (81j found that 8-hydroxyquinoline methiodide requires 2 equivalents of potassium ferricyanide per mole when titrated Tvith ferricyanide a t 50 O to 60’ C. in 2 t o 3 % sodium hydroxide solution. Reduction with Ferrous Salt. Chlorite ion may be titrated accurately in slightly alkaline solution with standard ferrous sulfate solution, according to Keiner ( 8 7 ) . Gale and Rlosher ( 2 0 ) applied the dead-stop end-point technique to the determination of niilligram quantities of vanadate in uranyl solutions. Xeizoldi ( 4 7 ) has confirmed the utility of the well knoivn titration of chroniium(V1) with ferrous solut.ion for use in routine steel analyses. Thiosulfate and Stannous Chloride. Neumann and Meyer ( 4 8 ) investigated the determination of iron, ferrous and ferric oxides, and ferrous and ferric salts when present simultaneously in mixtures. The original ferric salt content is found by adding an excess of standard thiosulfate solution, followed by back-titration with standard ferric chloride. The original soluble ferrous content is determined by bromate titration and the sum total of ferrous and ferric iron is determined potentiometrically. alternatively, solut,ion of the material in ammonium tartrate is followed by titration with stannous chloride for ferric salt content and the sum of ferrous and ferric salts is then obtained by permanganate titration. Reduction by Cuprous Chloride. Grinberg and Maksimuk ( 2 5 ) have found the method of E. Muller and K. Tanzler (1932) for reduction with cuprous solution to be useful in the titration of either platinum(IT’), iridium(IV), or their sum. Reduction with Lithium Aluminum Hydride. The recently
23 proposed applications of lithium aluminum hydride as a reagent for the quantitative estimation of functional groups (60) have been used potentiometrically by Higuchi, Lintner, and Schleif ( 2 9 ) in the determination of various functional groups in organic compounds-namely ,
H -OH,
-NH,,
I
=CO, -CO,
-COOR
standard solution of lithium aluminum hydride in dry tetrahydrofuran is added t o the organic compound in a system protected from air ( 2 8 ) and moisture, and after the reaction is complete, the excess of the hydride is titrated with a standard solution of ethyl or propyl alcohol in benzene. The electrode system is silver-solution-bridge of lithium bromide in tetrahydrofuransilver bromide-silver. I n a special paper on alcohol determination, Lintner et al. (41j obtained a very sharp break in the potential a t the end of the back-titration process. The chief reaction is: 4ROH
+ LiAIH4 = 4H2 i- LiOR + AI(OR)3
RECENT DEVELOPMENTS IN AMPEROMETRIC TITRATIONS AND COULO.METRIC TITRATIONS
Amperometric titrations range in details of technique from those in which manually operated polarographic equipment and the capillary dropping mercury electrode are necessary t o simple assemblies in which the electrodes may be connected t o a high resistance in series Tvith a galvanometer or a suitable micro-
Table I. hmperometric Titration Methods Substance Determined Reagent Reference Sodium fluoride Aluminum Ringbom and Wilkman (Fe-3 added as indi(59) cator) %Hydroxyquinoline Cadmium Stock (74) Sodium fluoride Calcium Ringbom and Wilkman Sodium oxalate
Chromate Copper
Quinaldinic acid %Quinoline carboxylic acid Cysteine, cystine Silver nitrate Lead nitrate Fluoride Thorium nitrate &Hydroxyquinoline potassium bromate Potassium ferrocyanide Indium Cupferron Iron Sodium fluoride Magnesium Mercaptans
Nickel Phenol
Sulfate Vanadium Zinc
(59)
Ringbom and Wilkman (59) Parks and Agazzi (51) Ferrous sulfate Cupferron Kolthoff and Liberti (36) 8-Hydroxyquinoline Stock (74) Potassium ferrocyanide Chovnyk and Klebs (13) Kue’mina and Chovnyk (38)
Stock (78) Stock (73) Kolthoff and Stricks (38) Petrow and Nash (66) Luzina (42) Duyckaerts (15)
Nimer et al. (49) Kolthoff and Liberti (96) Ringbom and Wilkman (69) Strafford et al. (75) Silver nitrate Kolthoff and Harris (34) Iodine Sulfhydril compounds Rosenberg et al. (62) silver nitrate Potassium ferrocyanide Kuz’mina and Chovnyk ($8) Duyckaerts (fb) Potassium bromate Vasil’ev and Getsova (86) Lead nitrate Parks and Agaeai (51) Ferrous sulfate Stock (74) %Hydroxyquinoline Potassium ferrocyanide Butenko and Rynskaya ( 9 ) ; Kus’mina and Chovnyk (38) ; Kirner et al. (49) Stock (72) Quinaldinic acid
ANALYTICAL CHEMISTRY
24 ammeter. Table I gives a r h m 6 of some recent applications of the method. Laitinen and Burdett (39) have developed a suitable cell for amperometric titrations using the dropping mercury electrode. hlakar’eva et al. (44)estimate chlorine in a gas stream continually using silver vs. cadmium amalgam as the electrode system. Coulometric Processes. I n the preceding review (17) mention wm made of the work of Swift and associates in developing methods for coulometric titrations with electrolytically generated reagents. Their subsequent papers have dealt with the applications of the following electrolytically generated reagents: bromine (8, 89), chlorine ( 1 6 ) , iodine (57), and cuprous ion (4.5). I n each case they employed a n amperometric indication of the end of the coulometric reaction. Cooke and Furman ( 1 4 ) have used potentiometric indication in the coulometric titration of chromium(V1) or cerium( IV) with electrolytically generated ferrous ion. The potentiometric indication is useful, especially where macrodeterminations are contemplated. Austin, Turner, a n d Percy ( 5 ) have developed an instrument for continuous automatic titration of sulfur compounds with electrolytically generated bromine. T h e gas or other flowing sample is mixed with reagent in k e d proportion and continually titrated to a fixed potential by current. T h e automatic adjustment is triggered by potentiometric indication from an indicator and a reference electrode. The amount of current used is a direct function of the concentration of the substance that is being titrated. LITERATURE CITED
Achiwa, S.,J. Electrochem. SOC.Japan, 17,267 (1949). Ibid., p, 276. Ahrland, S., Acta chem. Scand., 3,783 (1949). Artes, 0.C., Anales univ. Murcia (Spain), 1948-49, 288-355. Austin, R. R.. Turner, G. K.. and Percy. L. E., Instruments. 22, 588 (1949). Blaedel. W. J., and Panos, J. J., ANAL.CHEM.,22,910 (1950). Braae, B., Ibid., 21, 1461 (1949). Brown, R. A,, and Swift, E. H., J. A m . Chem. SOC.,71, 2717 (1949). Butenko, G. A., and Rynskaya, E. S., Zhur. Anal. Khim., 5, 145 (1950). Chaplin, A. L., Instruments, 22,579 (1949). Chepik, M. N., Zavodskaya Lab., 15,1470 (1949). Chfrkov, S.K., Ibid., 14,783 (1948). Chovnyk, N. G., and Klebs, G. A., Zhur. Anal. Khim., 3, 303 (1948). Cooke, W. D., and Furman, N. H., ANAL.CHEM.,22,896(1950). Duyckaerts, G.,Bull. SOC. roy. sci. Lidge, 18,152 (1949). Farrington, P. S.,and Swift, E. H., ANAL.CHEM.,22,889 (1950). Furman, N.H., Ibid., 22,33 (1950). Furman, N.H.,J . Am. Chem. SOC.,54,4235 (1932). Furman, N. H., Record Chern. Progress, 11, 33 (1950). Gale, R. H., and Mosher, E., ANAL.CHEM.,22,942 (1950). Gauguin, R.,Anal. chim. Acta, 3,272 (1949). Ibid., p. 370. Gauguin, R., Ann. chim., (12)4,832 (1949). Gladushko, V. I., Zauodskaya Lab., 13,1014 (1947). Grinberg, A. A., and Maksimuk, E. A., Ann. secteur platins Inst. chem. om., 20, 149 (1947). Guntz, A. A , Proc. Intern. Cong. Pure and Appl. Chem. (London), 11, 135 (1947). Hanna. W. S., and Johnson, A. B., ANAL.CHEM.,22,555(1950). Higuchi, T., Ibid., 22,955 (1950). Higuchi, T., Lintner, C. J., and Schleif, R. H., Science, 111, 63 (1950). Hindman, J. C., Anderson, L. J., and Moberg, E. G., J . Marine Reaearch (Sear8 Foundation), 8,30 (1949). Katchalsky, A., and Gillis, J., Rec. trau. chim., 68, 879 (1949). Kieselbach, R.,ANAL.CREM..21, 1578 (1949). Kirk, P. L., “Quantitative Ultramicro Analysis,” pp. 47-51, New York, John Wiley & Sons, 1950. Kolthoff. I. M.. and Harris. W. E.. ANAL.CHEM.,21,963 (1949). Kolthoff, I. M., and Liberti, A,, Analyst, 74,635 (1949). (36) Kolthoff, I. M.,and Stricks, W., J . A m . C h m . Soc., 72, 1952 (1950).
(37) Kraus, K. A., Holmberg, R. W., and Barkowski, C. J., ANAL CHEM.,22, 341 (1950). (38) Kuz’mina, N. N.,and Chovnyk, N. G., Zhur Anal. Khim., 4,96 (1949). (39) Laitinen, H. B., and Burdett, L. W., ANAL. CHEM.,22, 833 (1950). (40) Leonis, J., and van Nechel, R., Bull. SOC. chim. belo., 58, 266 (1949). (41) Lintner, C. J., Schleif, R. H., and Higuchi, T., ANAL.CHBM.,22, 534 (1950). (42) Luzina, G. S.,Zavodskaya Lab., 15,1412 (1949). (43) Lykken, L., ANAL.CHEM.,22,396 (1950). (44) Makar’eva, S. P., Bezzubik, 2. G., and Proskurnin, M. A., Zavodskaya Lab., 13, 1347 (1947). (45) Meier, D.J., Myers, R. J., and Swift, E. H., J. A m . Chsm. SOC., 71,2340 (1949). (46) Michel, H. O.,J . Lab. Clin. Med., 34, 1564 (1949). (47) Neizoldi, O.,Arch. Melallkunde, 3,309 (1949). (48) Neumann, B., and Meyer, G., 2.anal. Chem., 129,229 (1949). (49) Nimer, E. L., Hamm, R. E., and Lee, G. L., ANAL.CHEM.,22, 790 (1950). (50) Nystrom, R. F., and Brown, W. G., J . A m . Chem. Soc., 69,1697 (1947). (51) Parks, T. D.,and Agazzi, E. J., ANAL.CHEM.,22, 1179 (1950). (52) Petrow, H. G.,and Nash, L. K., Ibid., 22, 1274 (1950). (53) Pollak, L. Y.,and Shemyakin, F. M., Zavodskaya Lab., 16, 24 (1950). (54) Pribil, R., Chem. Listy, 37,205,227 (1943). (55) Pribil, R., and Malicky, V., Collection Czechosloa. Chem. Communs., 14, 413 (1949). (56) Pribil, R., and Simon, V., Ibid., 14,454 (1949). (57) Ramsey, W. J., Farrington, P. S., and Swift, E. H., ANAL. CHEM.,22,332 (1950). (58) Reichmann, H., 2. anal. Chem., 130, 39 (1949). (59) Ringbom, A.,and Wilkman, B., Acta Chem. Scand., 3,22 (1949). (60) Roberts, F.M., and Levin, H., ANAL.CHEM.,21,1553 (1949). (61) Roe, E.T., and Swern, D., Ibid., 22, 1160 (1950). (62) Rosenberg, S.,Perrone, J. C., and Kirk, P. L., Ibid., 22, 1186 (1950). (63) Ryabchikov, D.I., A n n . sectmr platine, Inst. chim. gen., 20, 139 (1947). (64)Schwarzenbach, G., and Biedermann, W.,Hela. Chim. Acta, 31, 331 (1948). (65)Ibid., p. 456. (66) Seguin, M., and Gramme, L., Bull. AOC. chim. France, 1950,384. (67) Shchigol, M. B., Zavodskaya Lab., 15, 1420 (1949). (68) Shchigol, M. B., and Birnbaum, 5. M., Ibid., 15, 1027 (1949). (69) Ibid., 16, 150 (1950). (70) Srinivasan, K. R., Analyst, 75,76 (1950). (71) Stehlik, B., Chem. Listy, 38, 1 (1944). (72) Stock. J. T.. J . Chem. SOC..1949. 1793. (73j Ibia.,’p. 2470. (74) Stock, J. T.. Metalluruia, 40, 179,229 (1949). (75)Strafford, N.,Cropper, F. R., and Hamer, A., Analyst, 75, 55 (1950). (76) Syrokomskii, V. S.,and Avilov, V. B., Zavodskaya Lab.,15,769 (1949). (77) Syrokomskil, V. S.,and Melamed, 5. I., Ibid., 16,131 (1950). (78) Syrokomskii, V. S.,Silaeva, E. V., and Avilov, V. B., Ibid., 15, 896 (1949). (79) Taliwv, S. T..and Teodorwich. I. L., Ibid., 15, 1031 (1949). (80) Tobias, J. M., and Retondo, N., Reu. Sci. Instrummls, 20, 579 (1949). (81) Tomicek, O.,and Sandl, Z., Chem. Ldsty, 40,219 (1946). (82) Tomicek, O.,Sandl, Z., and Simon, L. V., Collection Cmchoakw. Chem. Commun., 14,20 (1949). (83) Tuddenham. W. M.. and Anderson, D. H.. ANAL.C H ~ M 22. .. 1146 (1950). (84) Uaatenko, Y.I., and Datsenko, 0. V., Zavodskaya Lab., 13, 1009 (1947). (85) Vasil’ev, K. A., and Getsova, 9.Y.,Ibid., 15,1414 (1949). (86) Vyakhirev, D.A., and Guglina, S. A., Ibid., 15,1426 (1949). (87)Weiner, R., 2.Elactrochm., 52,234 (1948). (88) Wenger, P. E., Monnier, D., and Jaccard, F., Helu. Chim. A c b , 33, 1154 (1950). (89) Wooster, W. S., Farrington, P. S., and Swift. E. H., ANAL. CHEM.,21, 1457 (1949). (90) Yardley, J. T.,Analyst, 75,156 (1950). (SI) Zetheliua, S.,Rm. columbium p i m . , 3, 1 (1949). ,
R
I
~CCIIVED October
31, 1950.
