Glass-Electrode Titration of indium in the Presence of Thiocyanate E. D. MOORHEAD’ and G. M. FRAME, 112 Department of Chemistry, Harvard University, Cambridge 38, Mass.
b The enhancing effect of a high concentration of alkali thiocyanate on the character of the pH inflections, coupled with titration a t elevaied temperature, has been employed in devising a simple procedure for the direct acidimetric determination of trivalent indium. An assessment of the procedure showed that, in the range of 2to lOmM, indium may b e determined with an error of less than 5 p.p.t.
E
OF THE literature pertaining to indium titrimetry revealed that no appreciable success has been reported thus far in devising a workable, precise acidimetric method for the direct determination of this element. This omission is presumed to be the result of severa; sources of error usually identified with the formation of basic salts (10, I+$), rronstoichiometric hydrous oxides of large surface area, or slow reaction in thi: vicinity of the equivalence point with attendant drift in pH. Methods that are most commonly recommended for the titrimetric determination of indium take advantage of either the complexLtion of In(II1) with EDTA, the change in potential of an indicating electrode on titration of the element with ferrocyanide in the presence of a small amount of ferricyanide, or the subsc>quent bromination of the dissolved compound formed between indium and 8-quinolinol (oxine) or a substituted derivative of thir organic precipitant. The details of these and other less common methods for the titrimetric d:termination of indium have been crii ically discussed in recent reviews (3, 9). Prior studies connected with the electrochemistry of gallium and indium (11, 16, 18, 19) revealed (17) that the acidimetric titration of the former element in the preaeni’e of very high concentrations (7 t o 1 O V ) of alkali thiocyanates resulted in greatly enhanced p H inflections near the equivalence points. Later it was found that XAMINATION
Present address, The School of Chemistry, Rutgers, The State University, n’ew Brunswick, N. J. * Present address, Duiister House-Cl5, Harvard University, Cambridge 38, Mass,
the titration of trivalent indium under like conditions yielded a neutralization curve which also exhibited a large and very well defined p H inflection in this region. Such behavior seemed to offer substantial promise as the basis for a straightforward determination of this element. This paper presents the results of our assessment of the titration of acidified trivalent indium over the concentration range 2 to 10mM In(II1) in the presence of concentrated alkali metal thiocyanate and reports also on the behavior of the titration in the presence of varying concentrations of alkali metal halides. -4procedure of titration is suggested which permitted the determination of indium to =t5 p.p.t. EXPERIMENTAL
The measurement of formal values of acidity was made with a Leeds and h’orthrup p H meter (Model 7664) and made use of an L and N “black-dot” glass electrode. When necessary, the meter and electrodes were calibrated with a saturated solution of potassium hydrogen tartrate (pH = 3.57 a t 25’ C.). Titrations were carried out with specially calibrated burets of SO-, 2 5 , and 10-ml. capacity and employed a commercial, variable speed magnetic stirrer with integral, thermostatically controlled heater. All other volumetric pieces employed in the investigation were individually calibrated, and the values of standard solutions were corrected for significant temperature variations. Reagents. Stock solutions of standard indium perchlorate were prepared by refluxing certified, spectroscopic grade indium metal (Johnson-llatthey, Ltd.) in a calculated excess of perchloric acid. Indium chloride solutions were obtained by quickly dissolving (with considerable evolution of heat) the requisite weight of spectroscopic grade, sublimed indium chloride (Johnson-Matthey) ; the salt was received in sealed tubes. These stock solutions of indium were acidified with perchloric acid to prevent hydrolysis. Sodium hydroxide solutions were carefully prepared by diluting “CO1free” base (obtained as S/1 and N/10 ampoules, British Drug House) with hot, carbon dioxide-free distilled water; Apparatus.
the base was cooled and stored under nitrogen, These stock base solutions were standardized subseauentlv against desiccatordried, reagent grade sulfamic acid (G. F. Smith Chemical Co.) using the glass electrode. Such standardizations were repeated from day t o day during a prolonged series of indium titrations, and each was the result of no fewer than three replicate determinations of satisfactory precision. Potassium thiocyanate (blallinckrodt, AR) was prepared as a 10M (very nearly saturated a t room temperature) stock solution by dissolving the calculated amount of (freshly-opened) undried reagent grade salt in warm (30’ to 40’ C.) distilled water (the resulting solution should be clear and colorless) ; the solution was then saturated with tank nitrogen and stored away from light. Sodium thiocyanate (Mallinckrodt, AR) was prepared in a similar fashion, but on titration with strong base this salt was found to contain approximately 1% of an unknown titratable weak acid which interfered seriously in the determination of indium. Satisfactory purity was achieved, as demonstrated by the disappearance of the acidity, on 3 X recrystallization of the salt from hot distilled water. All other chemicals were of reagent grade. Procedure of Titration. An indium solution containing sufficient perchloric acid (or other strong acid) to reverse completely the hydrolysis of the trivalent ion is adjusted to have a thiocyanate concentration of about 5 M and a volume of 25 t o 50 ml. The titrant sodium hydroxide is prepared carbonate-free and to the exclusion of concentrated thiocyanate, as moderate dilution of the titrand (test solution) thiocyanate during the course of a titration has little important effect on the ultimate shape of the curve and buret drainage difficulties are thereby eliminated. The titration of the excess strong acid is performed a t room temperature and the neutralization is carried just through the first p H inflection; the titration is then halted briefly while the temperature of the titrand is raised rapidly to 75-85’ C. [during this heating period, the formal p H of the solution drops to a lower value and the first “homogeneous” precipitation of indium (hydroxide) commences]. Titration of the hot solution is then resumed a t the lower p H level and carried on through the second p H inflection which is now VOL. 35, NO. 12, NOVEMBER 1963
1875
large in magnitude, sharp in appearance, and exhibits little or no downward pH drift in this region. RESULTS AND DISCUSSIONS
The direct titrimetric determination of the acidity inherent in highly charged, aquated metal cations proves difficult if not wholly impossible to achieve quantitatively under ordinary aqueous circumstances owing principally to errors which may arise from a number of sources associated with the premature precipitation of basic salts, the formation of hydrous oxides of very great surface area, and the slow neutralization with drifting pH in the vicinity of the equivalence point. Although trivalent cations of the Aluminum Group, especially aluminum and gallium, show these general characteristics to a classic degree, cationic acidity in this series decreases (except for thallium) somewhat with increasing ionic radius, with evidence ( I d ) that trivalent indium under some conditions may be precipitated with strong base as the stoichiometric hydrated hydroxide. The acidic character of freshly precipitated indium hydroxide is less than that of aluminum or gallium hydroxide. Indeed, Moeller (14) found evidence which suggested that the small "solubility" (indate formation) of indium hydroxide in excess base is the result of peptization of the solid hydroxide by excess hydroxyl ion. It would seem, however, that this conclusion is not fully substantiated by recent studies (1, 4, 6, 7 , 8). Titration of Acidified Indium Perchlorate. On titrating pure solutions of indium sulfate, nitrate, and chloride a t 25" C. Moeller (14) noted that precipitation, in the case of the sulfate and nitrate, began a t a hydroxide-to-indium mole ratio of 0.84 (pH range 3.41 to 3.43) whereas with the chloride salt it was 0.02. Flocculation commenced a t values of 2.G (chloride and nitrate) and 2.3 (sulfate) on the rising segment of the p H inflection; for each titration reported, the inflection of the final pH jump fell far short of the theoretical 3:1 stoichiometry (14). We found roughly similar precipitation behavior on titrating with standard sodium hydroxide a solution 3.3 X 10-sM in In(C104)s containing excess perchloric acid (Figure 1, Curve A ) , although our data approached more closely the 3: 1 ratio. No intermediate inflections corresponding to stepwise addition of hydroxyl ion were observed, and this is in agreement with estimates of the stepwise complexity constants (2, 6, IS, 16) which suggest that the aquo ion and the mono- and dihydroxo complexes exhibit nearly equal affinity for hydroxyl ion. On neutralization of the excess perchloric acid the 1876
ANALYTICAL CHEMISTRY
't
0
L L L L L L
0 Figure 1.
I 2 MOLE RATIO : OH/IN
3
4
Titration of Acidified In(lll) with Standard NaOH
Curves: A. 3.3rnM In(ClOd)a, O.OM KSCN (25' CJ; B. 2.0mM InCls, 3.OM KSCN (25' C.); C. 2.5mM In(ClO4)s, 7.5M KSCN (25' C.); D. 1 O.OmM InCls, 5.OM KSCN (25', 80' C.) Curves A, B, Cleft ordinate; curve D right ordinate.
curve passes through a very shallow inflection a t pH 3.2, a value almost identical to that (3.4) observed by Moeller for the pH of a pure, 5mM solution of indium chloride ( I S ) ; precipitation commenced soon after passing through the inflection. Our prior experience with the acidimetric titration of gallium (17) suggested that, with indium, the magnitude of the pH inflections and therefore the precision of the determination might be improved significantly should the titration of In(II1) be carried out in solutions of a weakly complexing ligand a t high concentration (and ionic strength), for example in the presence of concentrated thiocyanate. Titration of Acidified In(II1) in 7.5M Potassium Thiocyanate. Curve C of Figure 1 represents typical data obtained from the titration of acidified (HC10,) 2.5 X 10-3M In(ClOJ3 in 7.5M KSCN a t 25" C. Aside from the quite noticeable improvement in the quality of the first pH inflection, which now occurs at p H 4.3 indicating a decrease in the formal acidity of In(III), and the nearly stoichiometric results which were obtained, several other features of the titration are worth noting. (1) The test solution prior to the addition of titrant base showed a perfectly clear, very light pink coloration which on titration was rapidly discharged as the p H passed through the first inflection point (pH 4.3). This behavior, which was quite reversible, has been observed in previous studies (16-19) and might possibly
be due to the presence of a trace of Fe(II1) introduced with the thiocyanate. (2) The first observable turbidity occurred a t a hydroxide-to-indium mole ratio of 1.78, considerably later in the titration than has been observed previously (14). The onset of precipitation at a mole ratio slightly less than 2 would seem reasonable in an environment highly concentrated in thiocyanate, an ion which is complexing and not too prone to form basic salts, whereas precipitation a t a ratio of 0.84, as in the case of nitrate and sulfate, would seem to lend further substance to Moeller's suggestion that these precipitates are in fact basic compounds, possibly double salts. (3) Flocculation of the precipitate occurred a t a mole ratio of 2.73. (4) There occurred a serious downward drift of the pH readings in the region immediately following the second inflection point which resulted in a skewing of the curve in this region. I n these initial studies the sodium hydroxide titrant was prepared and used as a standard solution of the base in 7.5M KSCN so as not to effect dilution of the titrand; this technique was later discarded when buret drainage problems arose. T h e Titration of Indium in the Presence of Varying Concentrations of Alkali Metal Halides. A series of titrations of millimolar concentrations of acidified (HC10J trivalent indium was carried out (25' C.) in the presence of KSCh', NaSCN, KCl, KBr, and KI a t concentrations which
ranged upwards t o 5 M depending on the salt Some dec:ty of the magnitude and sharpness of the first inflection were noted a t the lower salt concentrations (1M) while the shape of the second was little altered from that observed in 7.5M KSCN. When 3 M KI was employed as a supporting salt the evident generation of appreciable triiollide ion which resulted from the itir oxidation of iodide had no observrtble effect on the general shape of the titration curve, but, compared with identical titrations of In(II1) in KBr or KCl, each of which showed random positive and negative titration errors ranging up to 270, the titration of indium in Ah’ iodide yielded a negative error a little greater than 12%. It is our belief that this large negative titration ernor probably occurred for the most p w t as a result of some consumption of hydrogen ion in the air oxidation of iodide during the course of the titration. On account of the fairly limited solubility of the alkali halides and the tested impurity of zvailable sodium thiocyanate (vide supra), potassium thiocyanate was chosen as the supporting salt, especially as it is convenient to prepare and use this salt as a 10M stock solution. From a series of 21 replicate determinations of acidified trivalent indium over a wide concentration range in 3M KSCN (Figure 1, Curve B ) it was found that titration of the ion a t room temperature yielded widely disparate results which were attributed, principally, to p H drift (thought to be due to some adsorption of excess OH-) in the region of the se:ond equivalence point. T o overcome .,his serious drawback and at the same time improve slightly the definition of the first p H inflection, a formal titration procedure was subsequently de vised which employed potassium thiocyanate a t an initial concentration of 51%’ with the titration of indium carried through the second equivalence point a t elevated temperature. Using this revised procedure the precipitate formed was not a t all gelatinous but was, indeed, dense and granular in nature arid settled out of solution rapidly when the magnetic stirrer was turned off [Milligan and Weiser ( l a ) using dif’ferent conditions also obtained a somewhat granular precipitate when preparing their samples for x-ray diffraction studies]. It is important when titrating low indium concentrations with dilute base, that dissolved carbon dioxide be rigorously excluded from the titrant. Results of a series of three replicate titrations (following the revised procedure described above but using 3M potassium thiocj anate) for each of seven indium concentrations yielded
entirely positive errors whose magnitude decreased to zero more or less continuously from a value of 10% for the lowest indium concentration. This behavior and the fact that the observed positive errors for any group of three replicate titrations (at a fixed indium concentration) increased in magnitude from the first to the third run were explained by the tendency of the freshly diluted base used to titrate the lower indium concentrations to absorb carbon dioxide, and, in a given series of three runs, the extended time which the diluted base (contained in a stoppered 100-ml. volumetric flask) had available to absorb carbon dioxide; these considerations were borne out by titrating aliquots of the diluted base with standard hydrochloric acid-definite pH inflections were noted for the presence of carbonate ion. Titration of the highest indium concentration, however, showed no progressive increase in positive titration error in a series of three runs; this lent support to the foregoing explanation, as the titrant base in this instance was taken directly from the protected stock bottle. Employing the precautions for the procedure described, a t least six replicate titrations were run a t each of three indium concentrations in 5M potassium thiocyanate (see Figure 1, Curve D). The results of these determinations are tabulated in Table I and indicate that, in the range from 2- to lOmM In(III), the technique is
Table I.
5
+
LITERATURE CITED (1) Akselrud, N. V., Spivakouskii, V. B., Zh. Neorgan. Khim. 4, 989 (1959); C.A. 54, 8392c (1961). (2) Biederman, G., Arkiv Kemi 9, 277 (1956); Rec. Trav. Chim. 75.716 (1956): 75,716 (1956). (3) Buse;, (3)’ Busev, A. I., “T? Analytical Chemistry of Indium, MacMillan, New Ynrk. 1962. (4) Deichman, E. N., Izv., Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 1957, 257; C.A. 52, 16108d (1959). (5) Deichman, E. N., Tananaev, I. V.. Kham. Redkikh Elementov, Akad. Nauk I
S.S.S.R..Inst. Obshch. i Neoraan. Khim.
N o . 3, 1957, 73; C.A. 52, 2 6 3 0 ~(1958). (6) Hattox, E. M., DeVries, T., J. Am. Chem. SOC.58, 2126 (1938). ( 7 ) Ivanov-Emin, B. .N., fib. Nauchn.
Tr., Mosk. Inst. Tsvetn. Metal. i Zolota,
No. 27. 1957. 7: C.A. 54. 7394h (1961). (8) Ivanov-Emin,’ B. N., Nisel’son,’L. A:, Greska, Yu., Zh. Neorgan. Khim. 5, 1996 (1960); C.A. 54, 7013c (1961).
Titration of Acidified In(lll) with N a O H in the Presence of 5M KSCN
Titration number 95 96 97 99 100 101 128 129 130 102 103 104 119 130 i3i 132 133 106 107 108 138 139
effective for the determination of indium to i 5 p.p.t. The determination of indium by this method was carried out in the presence of zinc ion up to about 2‘/1 times the extant indium concentration with no detectable change in the shape of the titration curve, although errors in the few titrations that were performed in the presence of zinc ran as large as 3%. No formal attempt was made to identify or assess the effects of interfering ions, although there is a good deal of certainty that the presence of other cations of high charge and roughly equivalent aciditye.g. , Cr (111), Fe (111), A1(111), Ga(II1) , etc.-would be expected to show serious interference.
Taken 68.98 58.98 58.98 59.02 59.02 59.01 54.77 54.81 54.81 29.52 29.52 29.52 29.55 29.55 27.43 27.43 27.43 11.83 11.83 11.83 10.98 10.98
Milligrams, In( 1II)a Found
Difference
58.75 58.96 59.00 58.95 59.00 58.95 54.53 54.66 54.78 29.54 29.65 29.54 29.65 29.63 27.49 27.40 27.44 11.81 11.78 1l.87 11.01 10.99
-0.23 -0.02 $0.02 -0.07 -0.02 -0.06 -0.24 -0.15 -0.03 +0.02 f0.13 0.02 +O. 10 +0.08 +0.06 -0.03 +0.01
+
-0.02
-0.05 + O 04 + O . 03 +0.01
Error (P.P.t.1 -3.9 -0.34 +0.34 -1.2 -0.34 -1.0 -4.4 -2.7 -0.55 $0.68 $4.4 +0.68 +3.4 +2.7 +2.2 -1.1 +0.36 -1.7 -4.2 +3.4 +2.T +0.91 u = f2.5
Milligrams of indium in a titration volume of 50 ml.
VOL. 35, NO. 12, NOVEMBER 1963
1877
(9) Kolthoff, I. M., Elving, P. f.,“Treatise on Analytical Chemistry,” Part 11, Vol. 2, Sect. A., Interscience, New York,
1962.
(I?) Kolthoff, I. M., Stenger, V. A., Volumetric Analysis,” Vol. 11, p. 181 et seq. Interscience, Kew York, 1947. (11) MacNevin, W. M., Moorhead, E. D., J . Am. Chem. SOC.82, 6283 (1959). (12) Pvlilligan, IT.O., Weiser, H. B., Ibid., 59, 1670 (1937).
(13) Moeller, T., Ibid., 63, 1206 (1941). RECEIVEDfor review May 29, 1963. (14) Ibid., p. 2625. Accepted August 7, 1963. One of the (15) Ibid., 64, 953 (1942). authors (G. M. F.) expresses special gratitude to the National Science Founda(16) Moorhead, E. D., Ph.D. dissertation, Ohio State University, 1959. tion for support he received in the form of (17) Moorhead, E. D., unpublished rean Undergraduate Summer Training Grant sults, Harvard University, 1961. while engaged in a portion of this study. (18) Moorhead, E. D., Furman, K . H., Gratitude is expressed also to the Harvard ANAL.CHEM.32, 1507 (1960). University Research Foundation for a (19) Moorhead, E. D., MacNevin, W. >I., grant of research funds part of which supported the present work. Ibid., 34, 269 (1962).
Polarimetric Investigation of d-Tartrate-Ort hotel Iurate Complex JOHN G. LANESEI and BRUNO JASELSKIS2 Department of Chemistry, The University o f Michigan, Ann Arbor,
b Orthotellurate and d-tartrate in aqueous solution a t 22” C. form a one to one complex of moderate stability. The nature of the complex was established by Job’s continuous variation method and the apparent stability constants of 45 2.5 a t p H 9.5 and 62.0 f 3.0 a t p H 1 2 were determined b y the Benesi-Hildebrand method using optical rotation measurements a t 31 2 mF. Stoichiometry of the complex was confirmed b y preparing the crystalline salt, lithium
*
d-tartrate-orthotellurate.
T
forms chelates not only with cations but also with oxyanions such as borate and tellurate. Oxyanion chelates with polyhydroxy alcohols have been extensively studied by Edwards and eo-workers (4-6, 11). Reactions of arsenate n-ith sorbitol and mannitol have been investigated by Englund (‘7, 8 ) . Recently, optical rotatory measurements of borate chelates with mannitol and d-tartrate have been used in the quantitative determination of borate (3, 9). Optical rotation as a technique for determination of stability constants and stoichiometry in reactions involving optically active compounds has been reviewed by Rossotti and Rossotti (10). Since the orthotelluratetartrate complex has no characteristic absorption peaks in the ultraviolet region, except a continuous absorption below 260 m l , optical rotatory measurements are very suitable for the study of this reaction. In this htudy a n attempt is niade to (,lucidate the naturc of thc orthotellurate rcltctioii with d-tartmte ant1 to estimttc ARTARIC ACID
Present adtlrcss, Union College, Schenectndy, N. Y. Present address, Department of Chemistry, Loyola University, Chicago 26, 111. 1878
ANALYTICAL CHEMISTRY
Mich.
the potential possibility in applying optical rotatory measurements to the determination of orthotellurate. EXPERIMENTAL
Apparatus. Optical rotation mas measured with a Rudolph Photoelectric Polarimeter using a mercury arc source and a 10-cm. cell with quartz windows. Solutions were adjusted t o approximate p H by means of a Beckman Model H pH meter. Final p H measurements were made with a Beckman G p H meter using a blue tip glass electrode. Materials. White label grade dtartaric and orthotelluric acids were obtained from the Eastman Kodak and Amend Drug Co. Other materials m r e reagent grade. Preparation of Lithium &TartrateOrthotellurate Salt. Orthotelluric and d-tartaric acids were weighed in a 1:2 mole ratio and were dissolved in a small amount of water. The resulting solution was adjusted to pH 9.5 with lithium hydroxide. Most of the solution was evaporated a t room teinperature under reduced pressure. illethanol \\as then added until a slight turbidity appeared. The mixture was transferred to a glass-stoppered Erlenmeyer flask. The flask was sealed and heated a t 70” C. for 4 days. White crystals were formed on standing. After filtration, they were washed with methanol-water mixture and were dried by air suction. The amount of tartrate and orthotellurate u as determined titrimetrically after passing the sample through an ion exchange column in hydrogen ion form. Tellurium was determined gravimetrically by sulfur dioxide and hydrazine reduction. In all preparations cryqtals contained from 3 to 10% of lithiiim tartratc as an impurity. Atteinpti to purii“> the I)rc>cipit:ttc by recry~tallizntionwcre ut~~iicccs~ful. Study of &Tartrate Reactions with Orthotellurate. Optimum conditions for thc orthotellurate ant1 d-tartratc reactions mcre detcrniined by nicasur-
ing the optical rotation as a function of the wavelength, pH, and concentration. Concentration effects were studied by mixing appropriate aliquots of standard stock solutions of dtartrate and orthotelluric acid. Hydrogen ion concentration was varied by the addition of 1.0M sodium hydroxide until the pH reached 9.5; beyond this pH, 0.1M sodium hydroxide was used. Final pH of the solution was determined after dilution and measurement of optical rotation. Optical rotation of d-tartrate-orthotellurate solutions as a function of dtartrate was studied at pH 9.5 and 12 by keeping the concentration of orthotellurate constant at 1.74 X 10-2M, and varying the amount of d-tartrate. Blank solutions, containing the same d-tartrate concentrations as the solutions of the complex, were measured. The difference in optical rotation between the blank and d-tartrate-orthotellurate complex was determined. Effect of time upon the optical rotation of d-tartrate and of d-tartrateorthotellurate complex at pH 12 was checked at various time intervale in a 24-hour period. The combining ratio of d-tartrate to orthotellurate was determined by the Job’s continuous variation method. RESULTS AND DISCUSSION
The optical rotation of d-tartrate alone increases when going to shorter wavelengths, while that of the dtartrate-orthotellurate complex remains relatively constant, as shown in Figure 1. The greatest separation between the optical rotation of d-tartrate alone and the complex occurs at 300 m l or below. However, orthotellurate, when present in high concentration, starts to a h o r b apprcci:rhly a t 300 i n l , ant1 thus the mercury liiie at 312 tnp may be used when orthotellurate concentration is less than 1.60 X 10 -zM. The effect of pH on the optical rotation of the d-tartrate-orthotellurate
