Amperometric titrations - Analytical Chemistry (ACS Publications)

Apr 1, 1974 - John T. Stock. Anal. Chem. , 1974, 46 (5), pp 1–8. DOI: 10.1021/ac60341a008. Publication Date: April 1974. ACS Legacy Archive. Note: I...
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Amperometric Titrations John 1.Stock University of Connecticut, Storrs, Conn. 06268

The period covered by this survey is from the previous review (261) through October 1973. Unless otherwise specified, potentials are with respect to the saturated calomel electrode (SCE). Reviews of amperometric titrimetry include general aspects (103, 104), the determination of small amounts of zinc ( 7 5 ) and of other metals (124), the use of chelating agents as titrants or masking agents (71), the assay of (227), or titration with (255), organic sulfur-containing compounds, and the titration of nonaqueous solutions (144). A book devoted to biamperometric titration has appeared (296). Amperometric titrimetry is referred to in general reviews of electroanalytical techniques (13, 20, 72, 77, 78, 99, 271) and in studies of methods for the determination of water (94), of the analytical uses of organic reagents (285), of the problems encountered in the high-accuracy analysis of plutonium (35), and of the application of catalyzed reactions for end-point indication (298). Theoretical studies include equivalence-point location from the coordinates of the minimum on the titration curve (157), the factors influencing the accuracy of linear end-point extrapolations (133), error propagation in linear titration methods (50), and a statistical model for endpoint prediction and confidence-limit estimation (193).

APPARATUS AND METHODOLOGY Amperometric apparatus based on a Wheatstone bridge (155), for the study of processes in solution (285), and for coulometric titrimetry (66, 67) have been described. Automation has received considerable attention. Titrators with a delay unit to allow complete mixing (73), with titrant generation controlled by the detection system (44)and with a memory block that automates sample addition and exchange of electrolyte (295) have been patented. Automatic titrators with transistorized circuitry (27), constructed by altering a potentiometric titrator (106), featuring increased sensitivity (116), or incorporating a filter to reduce current fluctuations (60, 61) have been reported. Automatic correction for dilution by the titrant is available (61). Related developments include a signal converter for the automation of amperometric and other titrations (134) and the use of a compensated recorder to increase the sensitivity (141). Titrators constructed from commercial electronic modules have been described (59, 115). A new coulometric amperometric titrator for chloride has provision for coupling to a computer (142). A variable-sensitivity actuator for vibrating electrodes (237), a universal modular cell (93), and a cell that provides reproducible flow past the electrodes (87, 226) have been reported. Reference electrodes for titrations run without external source of emf have been further studied (256).

ACID-BASE REACTIONS Sodium barbiturate and salts of other weak acids have been amperometrically titrated with HCl (278). The biamperometric titration of submillimolar concentrations of acids and bases at platinum or glassy carbon electrodes is reported to be more sensitive than potentiometric titration (225). Copper electrodes have been used in the biamperometric titration of amines in AcOH or iso-PrOH with HC1 (1357, and of alkylchlorosilanes and Sic14 in isoPrOH with KOH (42). Biamperometric indication a t bismuth electrodes has been used in the titration of tertiary amines and salts of organic acids in AczO-AcOH, either coulometrically (81) or with HC104 (117), of carboxylic acids in Me2CO-MeOH with NaOH or KOH (117), and of NH2OH salts, either coulometrically or with NaOH (120).

PRECIPITATION AND COMPLEXING REACTIONS Methods Involving Silver. Mixtures of cyanide and acetone cyanhydrin have been titrated with AgNO3 at a rotating platinum electrode (RPE) that is short-circuited to a Hg-HgI2 electrode (-0.23 volt us. SCE). Free cyanide is titrated a t p H 9.07, then NH3 is added and cyanide released from the cyanhydrin is titrated (107). conversion to thiocyanate and titration of this with AgN03 has been used to determine free sulfur in sulfur dyes (112). Selenocyanate in the range 10-5M to 10-2M has been titrated a t an RPE of zero potential. The differential titration of various anions has been surveyed (105). Titration of a mixture of cyanide and selenocyanate gives the sum, but such mixtures can be analyzed by titrating a second aliquot after masking the cyanide with formalin (111). Selenocyanate in 0.01M NH40H-0.1M KN03 can be titrated at an RPE that is short-circuited to a Hg-HgI2 electrode. Addition of H N 0 3 and change of reference electrode to SCE then sets the conditions for titration of thiocyanate, so that mixtures of this ion and selenocyanate can be analyzed (109). Milligram amounts of hexacyanoferrate( 11) and of hexacyanoferrate(II1) have been titrated at a dropping mercury electrode (DME) with electrogenerated silver ion (263). Chloride in serum (142) and in solutions made from petroleum or platforming catalysts (267) has been amperometrically titrated with electrogenerated silver ion. Milligram amounts of NH20H-HCl have been biamperometrically titrated a t bismuth electrodes (119). A precision of the order of 0.002570is obtainable in the biamperometric titration, a t silver electrodes, of chloride with electrogenerated silver ion. Standard reference KC1 and NaCl have been assayed by this technique (152). Silver in mixtures with arsenic and zinc has been titrated with KI a t a gold anode (123). Addition of AgN03 and back titration with KI has been used to determine sulfhydryl groups in muscle tissue (97). Argentometric amperometric titration at a silicone-rubber based electrode is reported to be applicable to iodide in concentrations down to 10-5M (250). Comparative automatic titrations with AgNO3 have shown that halides can be titrated more rapidly, precisely, and sensitively by RPE amperometry than by potentiometry (106). Successful redoxokinetic titration of halides at silver electrodes has been achieved by increasing the voltage to at least 300 mV ( 2 0 6 ) . Sulfur and mercaptans in crude oil products have been biamperometrically titrated at Ag/AgZS electrodes with electrogenerated silver ion (268). Addition of phenylmercuric acetate (168), AgN03 or N-ethylmaleimide (96, 9R), then of glutathione, which is then back-titrated with 0.001M AgN03, has been used to determine sulfhydryl groups in muscle tissue and hemoglobin (96), milk (98), and in proteins and enzymes (168).Biamperometric indication was used in the titration, with electrogenerated silver ion, of microgram quantities of reduced glutathione, cysteine, and thiourea (138). The titration with AgN03 of thiols that contain a NH2 or imidazole group is reported to show considerable positive errors (300). Dimercaptothiopyrones have been studied as amperometric titrants for silver ( 1 5 ) . Titration at a graphite electrode with the 3-methyl-5-phenyl derivative has been used to determine silver in alloys (16). Submilligram quantities of silver have been titrated a t an RPE with 2,j-dimercapto-1,3,4-thiodiazole (92). Amperometry has shown that silver and dithioloterephthalic acid form a sparingly-soluble 2:l complex (172). Millimolar concentrations of thioacetamide have been determined by addition of KH3, (NH4)2C03, and gelatin, then short-circuit titration with AgN03 by means of an RPE-Hg-HgI2 cell system (108). ANALYTICAL CHEMISTRY, VOL. 46, NO. 5, APRIL 1974

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The same medium, heated to 50-60 "C, was used for the titration of micro amounts of thiourea in NH4SCN (113). Methylthiourea has been used to titrate silver in solutions made from sediments and from silver-copper alloys (127). In the titration of silver with thiosemicarbazide (T), the Ag:T ratio is reported to be 1:l in 0.1N H2SO4, but 2:l in 0.005N K2SO4 (191). The RPE-SCE system has been used for the short-circuit titration of tetraphenylborate with AgN03, and for the indirect determination of potassium. Potassium in sea water has been determined by titrating excess tetraphenylborate plus all halide ions, then titrating halide ions only in another aliquot (110). Methods Involving Mercury. Dissolution in HN03, elimination of selenite by hydrazine sulfate, and DME titration with Hg2(N03)2, has been used to determine chloride in cadmium selenide (205). DME titration of chloride in 0.05M HCOONa in HCOOH with electrogenerated mercury(1) was used in a study of the calomel (anhydrous HCOOH) electrode (19). Amperometric titrimetry has been used to study the compositions of mercury(1) polyvanadates (233). Microgram amounts of mercury(1) have been titrated with thiopiperidone (272). Anodic titration with 5-bromo-8-mercaptoquinolinehas been used to analyze mixtures of mercury(1) and mercury(I1) ions. The current remains constant as mercury(II) reacts, then decreases as mercury(1) is titrated (31). Biamperometric titration with Hg(N03)2 at mercury electrodes has been used to determine submillimolar concentrations of sulfide in 2M KOH (203). Conversion to K2S by fusion with potassium, treatment with HCl to yield H2S, absorption of this in a pH 9.4 NH3 buffer, and automatic amperometric titration with Hg(0Ac)z at a gold amalgam electrode, have been used for the microdetermination of sulfur in organic substances (79). Mercury in "03 solutions of pharmaceutical preparations has been titrated with KI ( 3 ) . Micromolar concentrations of mercury(I1) have been titrated at a rotating gold electrode with EDTA and related compounds (288). The end point in the titration with EDTA has also been located by thionine photoredox indication (242). Although less selective than K3 Fe(CN)s, Hg(0Ac)l has been used to titrate Dpenicillamine hydrochloride (290) and in the titration of mercury(II) at pH 1 with thiopiperidone (272). Mercury and bismuth in solutions of pH 4.27 made from amalgams, etc., have been titrated at a graphite electrode with diethyldimercaptothiopyrone. Two inflections occur in the titration of mercury; the third inflection corresponds to the titration of bismuth (10). Mercury(I1) in the presence of much silver has been determined by titration in highly acid media with 8-mercaptoquinoline (266). Amperometric titrations of mercury(II) with thiosemicarbazide (191), N-bis(8-aminoethy1)dithiocarbamateacid (70), and potassium N,N-bis(2-hydroxyethyl)dithiocarbamate(284) have been reported. Cystine and cysteine in dyed wool have been determined by titration with ethylmercury chloride (12).

Methods Involving Lead. Biamperometric titration at bismuth electrodes with mixed Ba( N03)2-Pb(N03)2 has been used to determine sulfate (118) and hydrazine sulfate (119). Sulfate and cadmium in cyanide-containing plating electrolyte have been determined by repeated evaporation with HN03, dissolution, addition of KN03, EtOH, and Pb(N03)2, then RPE titration with Na2S203. Excess lead is titrated first; a second end point giving cadmium can then be obtained (136). In a study of the reduction of molybdenum(VI) in molten LiC1-KC1 eutectic, total molybdenum was obtained by amperometric titration with lead ion (208). Titration with Pb(N03)2, has been used to determine molybdenum in alloys with nickel (22). Sodium tartrate in 30% ethanolic 0.1M NaC104 has been titrated with lead ion (43). Submicromolar concentrations of organic ligands in sea water have been titrated with lead by use of a stripping-analysis technique (58). Lead has been titrated with EDTA at a platinum anode that is coated with T1203, Ni203, or Co203 (122). Titration with EDTA at a rotating tantalum anode (RTA) has been used to analyze mixtures of indium and lead. Indium is titrated first at pH 1.5 and at a potential of +1.2 volt. Titration is continued after adjusting the pH to 4.5-4.8 and the potential to +1.1 volt (307). The sparingly-soluble 1:1 complex formed by lead and dithioloterephthalic acid 2R

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A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 5, APRIL 1974

has been investigated by amperometric titrimetry (172). The titration of lead in neutral or faintly acid solution with unithiol has been performed at a graphite anode

(68). Titrations Involving Hexacyanoferrate. A gold anode

has been used for the amperometric 'titration of zinc with K4Fe(CN)s (123). This titration has also been run by short-circuit amperometry against a permanganate or vanadium reference system (256). Biamperometric indication was used in the K4Fe(CN)6 titration of nanogram amounts of zinc (202), zinc in the presence of 1,2-diaminocyclohexanetetraacetic acid (DCTA), when the presence of ions such as Fe2+, Cd2+, and Cu2+ can be tolerated (241), and also of cadmium in fusible alloys, after removal of tin, antimony, and copper (218). In the attempted K3Fe(CN)6 titration of scandium and other rare earths in 6Wo ethanolic 1M KNO3, the RPE slowly became passivated (279). &Fe(CN)6 has been used as titrant for D-penicillamine hydrochloride (290) and has been coulometrically titrated with silver ion (263). Methods Involving Molybdenum and Tungsten. Phosphate in fertilizers has been determined by addition of molybdate, the excess being back titrated with EDTA at an RTA (223). This electrode was used in the titration of molybdenum-zinc mixtures with EDTA (308). After addition of DCTA, molybdenum can be selectively titrated with 8-quinolinol; copper, iron(III), and tungsten do not interfere (126). Molybdenum in steel and in ferromolybdenum has been determined by formation of molybdosilicate and its amperometric titration with diantipyrylmethane. Tungsten is titrated along with the molybdenum (147). The titration of molybdenum in steel has also been carried out with 5-bromo-8-mercaptoquinolineor 4methyl-8-mercaptoquinoline.Phosphoric acid is added if tungsten is present (30). Titration as tungstophosphoric acid with diantipyrylmethane has been used to determine tungsten in ferrotungsten (148). Methods Involving EDTA or Analogous Reagents. The EDTA titration of copper and various other metal ions has been performed at a platinum cathode and an oxide-coated platinum anode. Suitable oxides are PbO2, MnO2, Bi205, PtOx (121), T1203, Ni203, or Co203 (122). Biamperometric indication has been used in the titration of various metals in pharmaceutical products (258). Various metals have been titrated by observation of the tensammetric wave of an indicator such as 8-quinolinol or alizarin complexone. This wave disappears on complexformation, but reappears when the indicator is displaced by EDTA (184). Catalytic indicator reactions have been used in the automated biamperometric titration of copper with EDTA (299). The reliability of the amperometric titration of copper has been demonstrated by comparison with pyridylazonaphthol indicator (63). EDTA biamperometric titrimetry at amalgamated silver electrodes has been used to determine the calcium salt of codecarboxylase (302). Amperometric titration at a rapidly rotating graphite anode has been used to study the compositions and instability constants of metal complexes such as of zinc with EDTA (89). Submicrogram quantities of zinc have been coulometrically titrated with EDTA to a biamperometric end point (128). The titration of zinc, calcium, or mercury(I1) has been run with the aid of thionine as an amperometric photoreduction indicator (242), and the technique has been extended to the determination of anions such as HP04*-, S 0 4 2 - , Cr042-, and Fe(CN)s4-. A metal ion is added as precipitant, then the excess is back titrated with EDTA (245). Titration with EDTA at an RTA has been used to determine lanthanum and the composition of certain lanthanum-containing mixtures (156), and to analyze mixtures of zinc with zirconium and molybdenum. Zirconium only is titrated at pH 0.3-0.5 and at a potential of +1.2 volt. The titration is continued after raising the pH to 4.5-5.0, when the sum of zinc plus molybdenum is obtained. Zirconium is titrated in another aliquot, then molybdenum is masked with ammonium citrate, the pH is raised, and the potential is changed to +1.1 volt. Continued titration then gives zinc only (308). Mixtures of indium with zinc or with lead (307), or of thorium and bismuth (312), were analyzed by a somewhat similar method. The high precision obtainable in the amperometric ti-

John 1. Stock is professor of chemistry at the University of Connecticut. He was born in England and received the PhD and DSc degrees from the University of London. After extensive industrial and academic experience, he joined the faculty at the University of Connecticut in 1956. His publications reflect his interest in electroanalytical chemistry, microchemical techniques, history of chemistry, and in the design of apparatus and equipment for the teaching of chemistry.

tration of EDTA with electrogenerated zinc ion was illustrated in a study of the stoichiometry of gallium arsenide. Gallium was determined by addition of EDTA and its back titration with zinc at a gold amalgam cathode. For single-crystal material, the Ga/As mole ratio is reported to be 0.999994 f 0.000065 (152). The lower limits for the RTA titration with EDTA of praseodymium, neodymium, and europium are reported to be 1, 0.25, and 0.35 pg/ml, respectively (153). RTA-EDTA titrations using changes of pH and of potential have been used to analyze mixtures of yttrium with magnesium, scandium, or thorium (311). A biamperometric (platinum) version of this type of method has been used to analyze mixtures of magnesium, scandium, and praseodymium (154). Iron in iron ores has been titrated biamperometrically (47). RTA-EDTA titrimetry has been used for molybdenum (223) and in the analysis of yttrium-iron (314) and magnesium-yttrium-iron (313) mixtures. Microgram amounts of thorium have been titrated with EDTA at a rotating mercury electrode (80). Manganese(I1) has been used as biamperometric titrant for EDTA in the presence of H202, which acts as indicator. The first excess of titrant catalvzes the decomDosition of Hz02 (297). The DME titration of antimony(II1) with EDTA or DCTA has been reported (222) and the DCTA titration has been used to determine antimony in gold-antimony plating baths (28). The lower concentration limits for mercury(II) titration at a rotating gold electrode are reported to be 1OW6M for EDTA or diethylenetriaminepentaacetic acid, and 10-5Mfor triethylenetetramine (288). Methods Involving Other Organic Compounds. Aldehydes in isopropanolic 0.1M pyridine has been biamperometrically titrated with NHZOH.HC1 (41). Borate in CaClz-Ca( 0 H ) z medium has been titrated at a DME with fructose. Boron in borosilicate glass has been determined by this method (114). The reactions between gallium or lanthanum and oxalic acid (167), and between gallium (90) or lead (43) and tartaric acid have been studied by amperometric titration. The RPE anodic titration of vanadium(II1) with P-resorcyclic acid can be carried out in the presence of vanadium(1V) (240). Another titrant is pyrogallolcarboxylic acid, but the end point is imprecise if the V(1V)-V(II1) ratio is greater than 6:l (239). Amidopyrine has been titrated at a DME with Cd(SCN)Z (189). Nickel in alloys with molybdenum has been determined by anodic titration with dimethylglyoxime (22). Diaminoglyoxime has been used to titrate copper at a DME (151). Titration with cupferron has been used to determine cerium in cast-iron, steels, and magnesium alloys (164). Titration at a graphite anode with N-(acetylsalicyloy1)-Nphenylhydroxylamine has been used to determine niobium in steels (82). Gold(II1) (289) and perrhenate ion (which is reduced to rhenium(V) and then complexed (275)), have been titrated with a-naphthylamine or the @-isomer.Copper in alloys containing metals such as iron, cobalt, and nickel has been determined by titration with poly(ethylenimine) (286). 1-Ethylquinolinium iodide and 1methylquinolinium chloride have been titrated in alkali metal thiocyanate melts with sulfide or cyanide ion (51). The DME titration of copper and nickel with sodium quinoxaline-2-carboxylate has been reported. Addition of excess reagent and back titration with the metal salt is more convenient for the determination of zinc and cobalt (74). Various azopyrazolone derivatives have been examined as titrants for zirconium (207). Diantipyrylmethane has

been used to titrate molybdosilicate ( 1 4 3 , molybdoarsenate (146), and tungstophosphate (148). These reactions have been used to determine silicon and molybdenum in steel (147), arsenic (146), and tungsten in ferrotungsten (148). Nitron has been used to titrate molybdophosphate (180) and molybdosilicate (179). The latter titration was used to determine silicon in aluminum alloys. Titanium(II1) and vanadium(II1) have been anodically titrated with gallocyanin (280). Uramil-N-N-diacetic acid has been used to determine beryllium in alloys containing aluminum, magnesium, and zinc (83). Palladium and gold have been successively titrated at pH 2-4 with l-p-sulfonatophenyl-3-phenyltriazene.Palladium is titrated first at a potential of +0.4 volt. The potential is then raised to +0.9 volt for the titration of gold(III), which is really a redox process (276). Cetyltrimethylammonium bromide has been titrated with bismuth(1II) solutions; the reverse titration is also possible (65). Dodecylamine hydrobromide has been titrated with NaAsOz (64). Copper(1) in acid medium has been titrated with thioacetamide without interference by zinc, thallium(I), lead, iron(II), cobalt, or nickel (213). Thiourea has been used to titrate palladium in industrial solutions (195) and has been titrated with AgN03 (113) or with electrolytically generated silver ion (138). Silver in alloys has been titrated with methylthiourea (127). Cadmium, lead, and bismuth have been titrated with unithiol at a graphite anode. The method was used to determine bismuth in low-melting alloys (68). Iridium in alloys with platinum has been determined by titration with diethyldithiocarbamate (234). The titration of mercury and palladium with N,N-bis(2-hydroxyethyl)dithiocarbamate has been reported (284). Mercury (70) and selenium in pyrites (54) have been determined by RPE titration with N-bis(B-aminoethy1)dithiocarbamic acid. The complexation by dithioloterephthalic acid of copper, silver, and lead (172), or of thallium(I), samarium, and praseodymium (171) has been studied by amperometric titration. 8-Mercaptoquinoline has been used as an amperometric titrant for copper, silver, zinc, lead, and molybdenum (29), mercury (266), and vanadium (283). The determination of vanadium in steels by this method is reported to be quite selective and rapid (283). The amperometric titration of thallium(III), indium, an& gallium with 8-mercaptoquinoline has been discussed (282, 304). Titration with 5-bromo-8-mercaptoquinoline has been used to determine copper in bronzes and molybdenum in steel (30) and mercury(1) and mercury(I1) when present together (31). Copper and molybdenum have also been titrated with 4-methyl-8mercaptoquinoline (29, 30). The interaction of 2-mercapto-4,6-dihydroxy-5(4methoxybenzy1)pyrimidine (HL) with gold(II1) to precipitate AuL3 has been utilized in the RPE titration of submilligram quantities of gold (232). Gold(II1) has been titrated a t a graphite electrode with thionalide (231). Thiopiperidone has been used as titrant for mercury(1) and mercury( 11) (272). Diethyldimercaptothiopyrone has been used for the analysis of mixtures of bismuth with mercury(I1) or thallium(II1) (10) or with copper ( 9 ) . Various dimercaptothiopyrones have been tried as amperometric titrants for metal ions (15, 18). The methyl derivative has been used to titrate palladium (17), while 3-methyl5-phenyl-2,6-dimercapto-4H-thiopyran-4-onehas been used to determine silver in alloys (16) and copper in nickel-plating baths (238). Palladium has also been titrated with benzo-2,1,3-thiodiazole and its selenium analog. Other platinum-group metals react very slowly with the titrant and do not interfere (281). 2,5-Dimercapto-1,3,4thiadiazole has been used to titrate silver (92), while the titration of gold(II1) with 5-mercapto-3-(2-naphthyl)1,3,4-thiadiazole-2-thione has been used for the analysis of gold-tellurium ores (49). A rotating copper or copperbased electrode has been used in the amperometric determination of methionine by copper coulometry (228). The amperometric titration of potassium with sodium tetraphenylborate (TPB) has been run at a silicone-rubber-based graphite anode (249, 251). TPB has also been used to titrate aminophenazone (57) and corypalline ( 262).

Miscellaneous Titrations. Potassium salts in isopropanolic solution have been biamperometrically titrated with A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 5. APRIL 1974

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H2S04 at copper electrodes. The method is reported to be more accurate than titration in aqueous media with NaBPh4 (252). Oxygen-flask combustion and DME titration of sulfate in CaC12 medium with barium ions has been used to determine sulfur in organic and organometallic compounds (277). The use of a glycine-containing buffer is reported to improve the reproducibility of the titration of phosphate with bismuth(II1) (223, 224). Mixtures of bismuth and antimony have been analyzed by titration with KI in the presence of isoquinoline. One aliquot is titrated to obtain bismuth plus antimony, then tartaric acid is added to another aliquot, which is titrated for bismuth only (215). The isoquinoline-KI method has been used to determine palladium. KSCN or NaN3 can be used as titrant in place of KI (216). A study has been made of the interferences in the titration of palladium to a PdYzXZ end point, where Y is pyridine or 2,2'-bipyridine and X is iodide, thiocyanate, or azide ion (217). Amperometry at a rotating aluminum electrode has shown that aluminum and fluoride ions form AlFZ+ in acetate-buffered alkali metal nitrate solution. Compounds containing more fluoride are formed in aqueous alcoholic media (56). The combination n-silicon semiconductor anode-stainless steel cathode has been used in the titration of H F with iron(II1) ion (165).

salt-HC1-oxalic acid medium has been used as a biamperometric titrant for titanium(II1) (169). Total iron in iron(I1)-iron(II1) electrolytes has been determined by oxidation with HzOz and titration at 50 "C with ascorbic acid (36). Submilligram amounts of K3Fe(CN)6 have been biamperometrically titrated with externally generated vanadium(1V) ion (6).The oxidation of N,N-his(@-aminoethy1)dithiocarbamic acid by alkaline hexacyanoferrate(II1) has been studied by addition of the oxidant, then amperometric back titration with arsenite (53). Microgram amounts of chromium(II1) in chromium(VI) compounds have been biamperometrically titrated in NaOH solution at 60-70 "C with electrogenerated hexacyanoferrate(II1) (158, 159). Micromolar concentrations of NazS in a pH 9.2 NH3 buffer have been biamperometrically titrated with K,Fe( (1).

Nonferrous Methods Involving Cerium, Titanium, Vanadium, Chromium, and Manganese. Cerium and vanadium in alloys containing nickel and aluminum have been determined by direct coulometric titrimetry-ie., without the generation of an auxiliary agent (187). RPE titration with HgN03 has been used to determine milligram amounts of cerium in orthite ( 5 ) . Platinum in catalysts or in nickel-platinum alloys has been determined by dissolution, addition of CuCl to reduce platinum to the + 2 state, and biamperometric titration with Ce(S04)z. OXIDATION-REDUCTION REACTIONS Excess copper(1) is titrated first; a second break in the curve occurs when platinum has been titrated (100). NiMethods Involving Iron. Cerium in cast iron, steel, trite, azide, NHzOH, and NZH4 have been determined by and magnesium alloys has been determined by selective oxidation and RPE titration with Fe(NH&( S 0 4 ) ~ addition of Ce(C104,)4 and its back titration with NazCz04. The direct titration of NHzOH and the analysis (Mohr's salt). The differential titration of cerium and vaof NzH4-NHzOH mixtures is also possible (76). Oxalic, nadium has been carried out in HzSO4 medium either malonic, and certain other aliphatic carboxylic acids have with Mohr's salt (37, 38) or with electrogenerated iron(I1) been amperometrically titrated with Ce(C104)d (101). N (188). The titration of vanadium(V) in H3P04 medium substituted phenothiazine compounds have been titrated with iron(I1) (260) and the various iron(I1) titration methwith electrogenerated cerium(1V) or manganese(II1) (197). ods for the determination of vanadium in steels (192) have Electrogenerated titanium(II1) has been used as titrant been studied. Biamperometric titration with NH4V03 has for copper(II), vanadium(V) (I32), and low concentrations been used to determine iron(I1) in yttrium iron garnet (48). Vanadate (243), dichromate (244), and iodate (244) of molybdenum(VI) (85). One of the techniques used to study the titanium(II1)-NHzOH reaction was amperomethave been coulometrically titrated with iron(I1) that is ric titration (201). generated in the presence of EDTA. RPE titration with The direct and reverse titrations of vanadium(I1) with Mohr's salt has been used to determine vanadium and ICN have been examined (196). RPE titrations with chromium in slags (291). Either electrolytic or chemical KzCrz07 of vanadium(II1) and vanadium(1V) have been preoxidation can be used in the differential coulometric titration of vanadium and chromium in iron alloys (139). studied. A satisfactory reaction rate for vanadium(1V) requires heating to 80 "C. At room temperature, vanadiMixtures such as of yttrium, chromium, and iron have um(II1) can be titrated in the presence of vanadium(1V) been analyzed by a combination of iron(I1) and EDTA ti(254). Vanadium(II1)-vanadium(1V) mixtures have been trimetry (314). Traces of vanadium and chromium in analyzed by use of the two sharp end points obtained in high-purity aluminum have been determined by titration of two aliquots of solution with electrogenerated iron(I1). the RPE titration with KMn04 (26). Submilligram amounts of pyrocatechol have been titrated with electroKMn04, urea, and NaNOz are added to one aliquot, generated vanadium(1V) (129). Vanadium(1V) (131), which is titrated for vanadium plus chromium. The other vanadium(V) and chromium(V1) (24) have been titrated aliquot is treated with NazS03 and bromine water, then with electrogenerated tin(I1). The conditions for the titratitrated for vanadium alone (173). In the coulometric detion of vanadium(IV) with electrogenerated mangantermination of cerium(1V) and dichromate, the latter ion ese(II1) (140) and for the reactions of vanadium(V) with was titrated with iron(I1) after reducing cerium(1V) with 2,4-dithioburet (265) and with thiourea (253) have been NaN3 ( I 70). A method for the determination of plutonium studied. without interference by cerium, chromium, or manganese Electrogenerated chromium(I1) has been used for the involves oxidation by Ago, selective reduction of the inbiamperometric titration of silver in photographic films terferences by osmium(VII1)-catalyzed arsenite, addition of iron(I1) and amperometric titration with KzCrz07 (95). (130),for the DME titration of titanium(1V) (236),and for the titration at a mercury-coated platinum electrode of Plutonium oxide samples have been assayed by Ago oxisubmilligram amounts of uranium(VI) (186).KzCrz07 has dation, destruction of excess oxidant with sulfamic acid, been used to titrate manganese(I1) in 12M H3P04 (247), and amperometric titration with iron(I1) (294).Chromium thiosulfate ion (309),uranium in the Purex process stream and manganese in organic compounds have been deter(293), and both uranium and plutonium in mixed-oxide mined by conversion to dichromate and permanganate, fuels ( 5 2 ) . which are amperometrically titrated with electrogenerated Titration in HzS04 medium with KMn04 has been iron(11) (32). used to determine copper(1) in the presence of a 1000-fold In the titration of iron(1II) and copper with HgN03, excess of copper(I1) (149). The titration of manganese(I1) iron can be masked with pyrophosphate or fluoride, so in pyrophosphate medium with permanganate has been that only copper is titrated; The method was used to destudied in a forced-flow cell (88). Biamperometric titratermine microgram amounts of iron and copper in ores, mineral water, and silicates (273).Iron(1II) (24, 131, 132) tion of excess H z 0 2 with KMn04 has been used to determine niobium in ferroniobium (220).The RPE titration of and hexacyanoferrate(II1) (24) have been titrated with KSCN with KMn04 is reported to be reproducible and electrogenerated tin(I1). Microgram amounts of iron in alaccurate (310).The electrochemical oxidation of reagents loys have been determined by this method (131). Condisuch as cupferron has been measured by titration of extions for the electrolytic generation of tin(I1) and titanicess reagent with KMn04 (235).Conversion to permangaum(II1) at a lead cathode have been studied and used to nate and RPE titration with HgN03 has been used to dedetermine traces of iron in zinc and of iron and copper in termine manganese in steel (274). cadmium (132). Iron(II1) electrogenerated in a Mohr's 4R

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Methods Involving Bromine. The ease with which bromine or hypobromite can be generated electrolytically and monitored amperometrically or biamperometrically is reflected in the diverse uses of this titration technique. Amylase in serum has been determined by incubating the sample with starch substrate to produce reducing sugars. These convert copper(I1) to copper(I), which is then titrated (177).Barbiturates in serum have been determined by a method that terminates with the addition and back titration of arsenic(II1) (175). Microgram quantities of water adsorbed on metals and glasses have been measured by heating in argon, so that the liberated water can react with NaNH2 to form “3. This is titrated with hypobromite. Hydrogen in beryllium has been determined by this method after oxidation to water (303). The hypobromite titration of submilligram amounts of :yanide has been shown to apply to coordinately-linked cyanides such as C ~ ( C N ) E (174). ~A new method for the determination of salicylates and urinary metabolites sidesteps the slowness of the salicylate-bromine reaction by use of a “residual” biamperometric titration with KSCN (190). Phosphatases have been determined by incubation with disodium phenyl phosphate and titration of the liberated phenol (45, 46). Beryllium in alloys and in ores has been determined by precipitation under appropriate masking conditions with 2-methyl-8-quinolino1, dissolution of the precipitate in H N 0 3 , and subsequent biamperometric titration (21). Rate constants for the bromination of phenol and of metal chelates of 5-chloro-8-quinolino1 have been determined by a technique that permits the bromination to take place in CHC13, although generation and monitoring are done in the aqueous phase. Difficulties due to insolubility or partial dissociation in water are therefore obviated (23). Titration with KBr03 has been used to determine thallium(1) (212, 248), thiosulfate ion (309), ascorbic acid (248), compounds such as resorcinol (4, 210), p-chloroaniline or acetanilide (210), phenolic steroids (270), Streptocide and Sulfacyl (2), thiourea (248), and sulfenamide compounds in technical products such as rubber (39, 40, 143, 145). Submilligram amounts of nitrite have been determined in the presence of nitrate by digestion with urea, followed by addition of NaBrO and back titration with NaAsO2 ( 5 5 ) . Potassium propyl xanthate has been titrated with BrCN. Addition of KI is necessary if SnC12, VC12 (196) or N z H ~ ~ H ~(200) S O is~ t o be titrated. N-Bromosuccinimide has been used to titrate submillimolar concentrations of arsenite (183) and of NzH4.HzS04 (182). Methods Involving Iodine. The RPE (7) or biamperometric (7, 214) titration of germanium(I1) with iodine is reported to be more sensitive and more reproducible than visual or potentiometric titration. Submicrogram quantities of mercury have been determined by its catalysis of the ligand exchange between hexacyanoferrate(I1) and ophenanthroline. The liberated cyanide is titrated with electrogenerated iodine (219). The titration of arsenic(II1) with iodine has been run a t a gold electrode (123) and a t a platinum-graphite electrode system (211). The latter system was also used in the titrations of tin(I1) and antimony(II1) (21I ) . The amperornetric and biamperometric titration of arsenic(II1) has been examined in a cell that also permits spectrophotometric titrations (86). Arsenic, antimony, and copper in organic compounds have been determined by decomposition with H2S04-HN03, oxidation of arsenic and antimony t o the +5 state, addition of KI and N a ~ S 2 0 3 ,and amperometric titration with electrogenerated iodine (33). Amphetamine sulfate in tablets has been determined by the addition of a slight excess of bromine, then of arsenite. The excess arsenite is titrated with iodine (292). A study has been made of the iodine-thiosulfate titration a t the platinum-graphite electrode system (125) and the procedure has been used for the indirect determination of certain metals (209). A sensitive automatic meter for the continuous sampling of atmospheric oxidants such as ozone utilizes iodine liberation and its biamperometric titration (259). Titration with electrogenerated iodine has been used to determine polythionates after conversion to thiosulfate (34), selenium in urine by a method that terminates with the back titration of NazSz03 (198), and maltose. This is incubated with maltase, when formation

of H202 oxidizes iodide to iodine, which is determined by back titration of an excess of Na2S203 (178). Silver and mercury have been determined by precipitation with thionalide, dissolution of the ppt in EtOH, and coulometric titration (199). Selenium plus tellurium in lead and zinc by-products has been determined by iodine liberation from acidified KI and titration with NazS203. A second aliquot is neutralized by NaHC03, when iodine equivalent to the selenium can be titrated with N2H4 (305). Mixtures containing both sulfite and thiosulfate have been analyzed by titration with iodine. One aliquot gives the sum of these two ions; a second, titrated after the addition of HCHO, gives thiosulfate only (8). Cumene hydroperoxide (73) and peroxide groups in caprolactam fibers (269) have been determined by indirect iodometric titrimetry. Procarbazine hydrochloride has been biamperometrically titrated with electrogenerated iodine (194). The RPE titration of KSCN with iodine has been described (310). Titration of KIO3 has been used to determine thallium(1) a t a platinum-graphite electrode system (212). thiosulfate (309), and submillimolar amounts of organic peroxides. These peroxides are caused to liberate iodine from a KI-citric acid-tert-BuOH medium, then excess NazSz03 is added and back titrated (176). The titration of KI with K I 0 3 has been used to check the performance of a new recording amperometric titrator (61). ICN has been used as titrant for SnC12, VC12, potassium propyl xanthate (196), N2H4-H2S04,thiourea, and potassium allyl xanthate (200). Amperometry has been used to study the kinetics of the reduction of iodate by iodide (150) and of the oxidation of maleic acid with cerium(1V) (102), as well as to determine microgram amounts of osmium(VII1) by its catalysis of the H202-KI reaction (11). Another well-known catalyst for this reaction is molybdenum(VI), nanogram amounts of which have been determined by biamperometric monitoring of the liberated iodine (221). Picogram amounts of phosphorus (229) and of arsenic (230) have been determined by conversion into the heteropoly molybdo-acid, extraction of this with BuOAc, liberation of molybdenum and its kinetic biamperometric measurement. A survey of methods for determination of water includes biamperometric titration with Karl Fischer reagent (94). In a critical evaluation of biamperometric and bipotentiometric titration with this reagent, sodium tartrate dihydrate was found to be unreliable as a standard because of water occluded inside the crystal structure (25). Some examples of biamperometric titration with Karl Fischer reagent are the determination of water in ethanol (27), acetonitrile and other organic solvents (181), silicone compounds (266), and resins (91, 302). Tertiary or 0-unsaturated alcohols have been determined through dehydration by boiling with p-toluenesulfonic acid suspended in toluene. The liberated water is then titrated (306).Tertiary alcohols have been determined in the presence of hydroperoxides by the differential Karl Fischer titration of two aliquots (246). Other Reactions. The biamperometric titration of copper with electrogenerated titanium(II1) (132) or tin(I1) (131, 132) has been studied and applied to the determination of copper in alloys. Small percentages of gold in alloys with copper, gallium, and indium have been determined by titration a t a graphite anode with 2-cyanoethyl benzenesulfonyldithiocarbamate (84). Tin(I1) in the presence of iron(I1) has been titrated with the species produced a t a mercury anode in 4M-tartaric acid-2N H2S04 (69). The RPE titration of thiosulfate with thallium(II1) has been reported (309). Lead(IV), which is relatively stable in H2S04/H3P04 medium, has been titrated at a DME with NaAsO2 (14). The following are some examples of amperometric or biamperometric titrations with NaN02. Urea- and carbamate-pesticides have been determined by hydrolysis to, and titration of, amino compounds (137).The titration of sulfanilic acid (116, 160) and of various other primary aromatic amines (160) has been examined. Benzenesulfinic acid has been titrated both amperometrically and biamperometricaliy (162). The titration of l-p-sulfophenyl-3methyl-2-pyrazolin-5-one(161) and the automatic titration of sulfamethazine (59), sulfadimidine ( 6 0 ) , and sulfanilamide pharmaceuticals (62) have been described. Diazepam and nitrazepam have been determined by hydrolA N A L Y T I C A L CHEMISTRY, VOL. 46,

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ysis to, and titration of, the aminobenzoquinones and aminoquinolines (163). Primary aromatic amines that are poorly soluble in water have been titrated in H2SO4AcOH-H2O medium with nitrosyl hydrogen sulfate (204). Tellurium in ferrous alloys and semiconductors has been determined by conversion to tellurium(1V) and anodic titration with 2,4-dithioburet, when elementary tellurium is precipitated (264). Although titration with 8-mercaptoquinoline reduces selenium(1V) to elementary seleni-

um over a wide range of concentrations, the analogous reduction of tellurium(1V) occurs only if the HzS04 concentration is a t least 4M. Reduction to tellurium(II), which combines with the titrant, occurs in less acid media. These reactions have been used for the successive determination of selenium and tellurium in technological samples (257). Oxygen in water and in gases has been biamperometrically titrated with electrogenerated viologen radical cation (287).

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Analytical Electrochemistry: Theory and Instrumentation of Dynamic Techniques David K. Roe D e p a r t m e n t of Chemistry, Portland State University, Portland, Ore. 97207

Two years ago, this review appeared under the title “Electrochemical Relaxation Techniques” ( 8 A ) ; the title originated by William Reinmuth in 1964. Since then, much of analytical electrochemistry has been concerned with the development and application of relaxation techniques. As a result, there has been an increasing amount of overlap with the “Polarographic Theory, Instrumentation, and Methodology review. To meet the understandable demands of the editor to keep these reviews succinct, Peter Kissinger and I decided to divide and review the territory according to the new titles. Thus, applications, noninstrumental details, and results will be found primarily in the accompanying review by Kissinger. A number of publications may be cited in both reviews, since many authors have adopted the following format for their articles: 8R

A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 5 , APRIL 1974

i) derive a few equations; ii) describe the new or modified instrumentation that permits the necessary measurements; iii) make a series of measurements on a model system (usually cadmium). Consequently, we must wait until we can read each other’s review to assess the degree to which we have saved space and the reader’s time. A change has also been made in the organization of this review. Instead of emphasizing the technique-potentiostatic, rotating ring-disk electrode, etc.-the general headings are phenomenological. Although this is contrary t o the way that many of us think about electrochemical methods, it does allow a rapid assessment of progress in an area having a common chemical or physical basis. And that is why reviews are written.