ion in 50 ml. of acetate buffer nere reduced with zinc amalgam. The liberated zinc ion was titrated in the usual manner. I n the absence of lead ion a blank of 0.08 ml. of 0.10M E D T A was found. For reductions performed in the presence of lead ion the quantity of zinc titrated corresponded not only to the number of millimoles of lead reduced, but also to a small increment due to the blank. This increment (blank) is only about one fourth a s large as that obtained in the absence of lead ion. The blank was found to be essentially constant within the limits of titration error and is independent of the quantity of lead ion within the range studied. Similar behavior can be expected for the reduction of other metal ions in the same solvent. This effect is less pronounced with amalgams less active than zinc. Blanks in these cases may be neglected. Other Parameters. Other factors which may effect the blank are tempeiature and violence of shaking. -411 reductions were performed a t room temperature (28’ C.), An increase in temperature will bring about a decrease in the time required to achieve quantitative reduction by increasing the diffusion coefficient of the substance reduced and decreasing the viscosity of the amalgam and solvent. It may also increase the blank by facilitating the reduction of hydrogen ion. Possibly these two opposing effects will compensate each other and reductions a t elevated temperatures may or may not be as feasible. The violence of shaking affects not only the magnitude of the blank but also the time required for quantitative reduction. I n this study, blanks obtained manually were erratic. This adverse effect vias minimized by employing a Burrell wrist-action shaker.
The position giving the most violent shaking was used, and blanks obtained in this way were reproducible. EXTENSIONS OF METHOD
As indicated by the blank study, the method may be extended to the determination of oxygen in water. A modification of the bubbling procedure and sampling techniques is all that is required. By a consideration of polarographic data, masking agents, choice of amalgams, use of other chelating agents, and pH effects, various schemes for the analysis of other metal ion mixtures can be undoubtedly devised. ACKNOWLEDGMENT
One of the authors (W. G. S ) gratefully acknowledges the support of an R. J. Reynolds Fellowship for the academic year 1956-57.
“Applied Inorganic Analysis,” 2nd ed., p. 479, Wiley, Xew York, 1953. (12) Kolthoff, I. M., Furman, N. H., “Volumetric Analysis,” Vol. 11, p. 322, Wiley, Sew York, 1929. (13) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Interscience, Yew York, 1952. (14) Korhl, J., Pfihil, R., Chemist Analyst 45, 30 (1956). (15) Korhl, J., PIibil, R., Chem. Listy 51, 106 (1957). (16) Kovama, K., Michelson, C., ANAL. CHEM:29, 1115 (1957). (17) Latimer, W,M.,“Oxidation Potentials,” 2nd ed., Prentice-Hall, Yew York, 1952. (18) Lobunetz, AI. M., C‘niu. etat Kiev, B d l . Sei., Rec. chim. 2, No. 2, 81 (1936). (191 Lobunetz. M ,AI., Per’e. 11.I.. Zbid.. ’ fro. 2, 2, 69 (1936j. (20) Meites, L., - 4 s ~C~m.x . 27, 1116 (1955). (21) Meites, L., “Polarographic Techniques,” Interscience, New York, 1955. (22) Sightengale, E., Anal. Chim. Acta 16. 493 11957). - , (23) ’Per’e, M. I., Lobunetz, 11. )I., Chem. Zentr. 111, 1187 (1940). (24) Per’e, &I. I., Lobunetz, 11. hl., Unit.. Btat Kiev. B d l . sei.. Rec. Chim. 2. Xo. 2. 45 11936). 125i. \ -
LITERATURE CITED
11) Biedermann, W,,Schwarzenbach, G., Chimia (Switt.) 2, 56 (1948). (2) Bjerrum, J., Schwarzenbach, G., Sillen, L., “Stability Constants, Part I: Organic Ligands.” The Chemical Sociexy, London, 1957. (3) Booth, H. E., “Inorganic SyntheEes,” Vol. I, p. 13, RlcGraw-Hill, Yew York, 14-24 -V””.
(4) Brenneke, E., T e u r e massanalytische Methoden.” a. 180. Ferdinand Enke. Stuttgart, ’1951 (5) Budesinsky, B., Chem. ListzJ 50, 1931 (1956). (6) Flaschka, H., Chemist dnalyst 42, 56 (1953). (7) Flaschka, H., Mikrochemie aer. Mikrochim. Acta 39. 315 (1952). (8) Flaschka, H.’, Z . anal. Chem. 139, 332 (19,53\. \ - - - - / .
(9) Flaschka, H., ,4bdine, H., Chemist 4nalyst 45,2 (1956). (10) Zbid., p. 58. (11) Hillebrand, W. F., Lundell, G. E. F., Bright, H. 4., Hoffmann, J. I.,
rius, R., Anal. Chim. Acta 10, (29) Someya, K., Z. anory. 21. allgem. Chem. 138, 291 (1924). (30) Suk, V., llalat, M., Chemist Analyst 45, 30 (1956). (31) Tananaev, I., Davitashvili, E., Bull. acnd. sei. C.R.S.S. Ser. chzm. 1937,
1397.
(32) Tananaev, I., Davitashvili, E., Z . anal Chzm. 107, 175 (1936). (33) Wehber, P., Mikrochim. Acta 1955, 911. RECEIVED for review January 17, 1958. Accepted May 23, 1958. Division of A4nalytical Chemistry, 133rd meeting, ACS, San Francisco, Calif., April 1958. Research supported by the U. S. Air Force through the Air Force Office of Scientific Research of the *Iir Research and Development Command under contrnct S o . ;iF 18 (600)-1160.
Ion Exchange Resins as Indicators WALTER E. MILLER Department o f Chemisfry, The City College, New York 37, N. Y.
b Acid-base indicator forms of ion exchange resins will act as indicators and may be used in conventional titrations. Such indicators are in a phase separate from the solution whose p H they indicate. Data on acid-base titrations with thymol blue, bromocresol green, and phenolphthalein resin indicators manifest an accuracy equal to that of conventional indicators.
A
indicator forms of ion eschange resins may be made by treating an appropriate resin with a CID-BASE
1462
ANALYTICAL CHEMISTRY
solution of an indicator (e). Acid-base indicators are themselves weak acids or bases. A weak acid indicator may h a r e its anion adsorbed by an anionic ion eychange resin. For a weak base indicator a cationic ion exchange resin may he employed. Indicators such as thymol blue (thyniolsulfonep hthalein) , bromocresol green (tetrabromo - m - cresolsulfonephthalein) . and phenolphthalein are weak acids. The ionization of a weak acid indicator may be represented as HIn H + In-
+
d i e r e H I n represents the un-ionized indicator molecule and In- represents the anion produced by ionization. Either or both may be colored. If the I n - ion is colored, its adsorption by an anionic ion eschange resin will impart the color of the ion to the resin, which is generally in the form of beads. This resin bead color, like that of the indicator itself. will then be dependent on the pH of the solution in which the resin is placed. The strong base anion exchangers, Rohm &- Haas IRA-IO0 and Salcite S I R , both 20-50 mesh, n-ere used. As
these resins. are almost colorless, the appearance of color at a titration end point is readily apparent. ,40.1% solution of the indicator in ethyl alcohol (0.1 gram of indicator in 100 ml. of 95% ethyl alcohol) is shaken with dry ion exchange resin beads to convert i t to the indicator form-Le., adsorption of the indicator anion by the resin. T o speed u p the coni-ersion the mixture may be warmed, though this is unnecessary, as conversion is rapid a t room temperature, or a higher concentration of indicator solution may be used. Because only a few resin beads are required n-hen used as an indicator, a 1-gram (dry basis) mass of resin shaken with 10 nil. of indieator solution will make a large quantity of resin indicator. Commercial ion exchange resin beads are generally supplied in the chloride form, so that no change will be apparent when thymol blue and phenolphthalein are added to the beads; the latter retain their pale yellon- color, but broniocresol green turns the resin beads green and then dark blue as conversion to the indicator form progresses. If the resin beads are initiallj- in the hydroxyl form they will! upon conversion, take on the color charact.eristic of a basic solution. T o be used a s an indicator, a few resin bcads (20 t o 30. dry weight circa 20 mg.) are added t o the titration vessel. Titration is done in the normal nianner and the end point is indicated by a color change in the resin beads. As with ordinary titrations. it may be necessary to run a blank on the indicator beads alone. The color changes of the indicators used and their p H intervals are given below ( I ) . Presumably the resin bead indicators h a ~ the e same cha,ract'eristics.
Thymol blue
Transformation Interval, PH 8 0-9 6
Color Change, AcidBase Yellom--
(2nd
the
3 8-5 4
Colorlessred TelloTv-
range) Phenolphthalriii S 0-9 8 Bromocresol green
blue EXAMPLES
Hydrochloric acid (0.1541S) \!as titrated \yith 0.1364.Y sodiuni hydroxide. A blank titration was run by adding the sodium hydroxide solution to indicator beads in a volume of water equal to that oi the final yolunie of solution after titration.
HCI, RI1.
lo.w 10 00
5.77 8 36
HC1, hI1. 10.00 10.00 7.12 6.28
B. Indicator. Salcite SAR (thymol blue) End point. Appearance of a green hue in resin beads NaOH SaOH NaOH NaOH, for Blank, Corrected, Theoretical, hI1. MI, hI1. hI1. 11.51 0.06 11.45 11.30 11.37 0.06 11.31 11.30 8.08 0.06 8.02 8.05 7.20 0.06 i ,14 7.10
The green color produced a t the end point disappears upon standing, but reappears if the titration vessel is shaken. Hydrochloric acid (0.154lS) was titrated with 0.1144-V ammonia. A blank titration was run with ammonia as described above.
HCl, MI. 10.00 10.00 7.46 6.72
ficiency." As little as one resin bead may be used as indicator. Indicator beads may be dried and stored for future use. DISCUSSION
The thymol blue and bromocresol
C. Indicator. Kalcite SAR (bromocresol green) End point. Appearance of a blue color in resin beads SH8 XH3 SHg "3, for Blank, Corrected. Theoretical, n11. 111. 111. h31. 13.53 0.12 13.41 13.47 13.58 0.12 13.46 13.47 10.16 0.12 10.04 10.05 9.23 0.12 9.11 9.05
The resin beads, initially yellon- in acid, pass through successively darker shades of green much before the end point. The end point is a distinct blue m-ith no residual green tinge. Though 3 transition from green to blue is difficult to judge, coniparison with a previously prepared standard-e.g., the blankmakes the end point unmistakable. The progressive color change of the beads n ith addition of ammonia gives notice of the approach of the end point, The color change of the various indicator beads, in going from acid to base. takes place instantaneously, whereas conversion to the acid form takes place relatively slowly. Titrations are therefore best carried out by addition of base to acid. I n the neighborhood of the end point, however, the color changes are readily reversible. and back-titrations can be donc. The basic fornis of all the indicator resins will maintain their basic color n-hen washed in distilled water; the acid forms of the phenolphthalein and thymol blue resins when washed in nater, do not change color, but the bromocresol green resin turns from yellon- to green. The indicator resin beads, once used. may be filtered off, washed in water, and re-used. A single sample of r e m beads was used in each set of titrations above merely by decanting the supernatant solution and washing the beads in the titration flask. Indicator beads have been used through a t least a dozen acid-base cycles without loss in "ef-
-i. Indicator. IRh-400 (phenolphthalein) End point. Appearance of a pink hue in resin beads SaOH XaOH NaOH NaOH, for Blank, Corrected, Theoretical. 111. 111. 1x1. 111. 11.40 0 12 11.28 11.30 11.41 0.14 11.27 11.30 6 49 0.06 6.43 6.52 9.53 0.12 9.41 9.4*5
green molecules are probably linked to the exchange sites of the ion exchange resin through their sulfone groups and the phenolphthalein molecule through its carboxyl group ( I ) . These linkages for the acid forms of the indicator resins are:
O-SO2-O-Resin /
Thymol Blue
Bromocresol Green
0-1-0-Resin
Phenolphthalein The term ,'resin" represents an exchange site in the ion exchange resin matrix to which the indicator molecules VOL. 30, NO. 9, SEPTEMBER 1958
1463
I
are joined. The phenolic groups of the indicators are free to react with acid or base, and in strong base it is possible to produce the symmetrically resonating forms which give rise to the characteristic basic colors of the indicators. The transition between the acid and basic forms of phenolphthalein upon addition of acid and base is:
from alcoholic solution. If cationic ion exchange resins are used, the indicators are apparently adsorbed by the resins, in that the beads present their basic color upon addition of alkzli but both the color and the indicator are rapidly and completely desorbed from the resin even in water. Microscopic examination of phenolphthalein resin beads un-
0
HO
Na +-0
-
red have also been prepared in the manner described, but their color changes were not so distinct. SUMMARY
The prime adrantage of the technique described is the preparation of an indicator in a phase separate from the solution whose p H it indicates. The ability of an ion exchange resin to load up to the limit of its exchange capacity with the indicator effectively results in a highly concentrated “point” indicator.
NaOH
c
d
c
HCI
Similar transitions would occur for thymol blue and bromocresol green resins. Further evidence of chemical linkage of the indicator molecule to the ion exchange resin rather than mere physical adsorption is that the indicator is eluted from the resin by ethyl alcohol to only a slight extent, and not at all by n-ater, whereas the indicator-resin readily forms
der 100 power shows them to be uniformly colored throughout their bodies after immersion in 0.1N sodium hydroxide. I n subsequent conversion to the colorless form upon addition of acid, the radial disappearance of the pink color can be observed as the hydronium ions penetrate the beads. Anionic ion exchange resin forms of methyl orange, methyl red, and Congo
ACKNOWLEDGMENT
The author wishes to express his gratitude to Refining Unincorporated, New York, K. Y., for permission to publish this article. H e is also grateful to Saul Soloway of The City College for his helpful discussions during preparation of this article. LITERATURE CITED
( 1 ) Kolthoff, I. PIT.,
“Acid-Base Indicators,” Macmillan, Xew York, 1937. ( 2 ) Miller, W. E., ANAL. CHEW 29, 1891-3 (1957).
RECEIVEDfor review June 5, 1957. Accepted April 21, 1958.
Separation of Rhodium and Iridium from Base Metals by Ion Exchange ALICE G. MARKS’ and F. E. BEAMISH University o f Toronto, Toronto 5, Canada The fire assay with lead as the collector is the one method universally used for determining the platinum metals in ores and concentrates. Generally, the lead button thus obtained is cupeled to form a silver alloy. The more insoluble platinum metals fail to alloy with silver and, to a degree, with lead, and result in an easily removed encrustment on the bead. No modification of the fire assay has been recorded to eliminate this mechanical loss. With a view to the production of a collecting alloy which would allow dissolution of all the platinum metals, a fire assay was devised, which permitted use of the siderophilic metals as collectors for platinum and palladium. However, the problem of collection and determination of rhodium and iridium remained. Now, by ion exchange processes, microgram amounts of rhodium and iridium can b e isolated from very large proportions of the associated base metals, iron, nickel, and copper.
1464
ANALYTICAL CHEMISTRY
T
extremely low content of precious metals in ores necessitates some method of concentration prior to determining precious metal values. The only practical procedure has been collection of the precious metals in molten lead, followed b y further concentration b y a cupellation process which removes lead. This method is not satisfactory for some of the insoluble platinum metals, as they are often found at the outer surfaces of the lead buttons and otherwise mechanically mixed. Losses are thus possible during subsequent cupellation or other operations. Rhodium, iridium, etc., may often be detected as a n excrescence at the surface of the silver bead resulting from cupellation. Because of the siderophilic nature of the platinum group of metals and the recognized ability of iron and nickel to form platinum metals alloys, it seemed probable that the platinum metals could be collected in molten iron or nickel. HE
Attempts were made to h d a satisfactory method of analyzing ferronickel assay buttons. As cupellation is impossible with iron, a wet chemical method was necessary for final concentration of the platinum metals. Coburn, Beamish, and Lexvis (4) developed a method for separating platinum and palladium from the base metals by cation exchange and supplied it to the analysis of ferronickel assay buttons. It is believed that these buttons will collect iridium more efficiently than the classical lead alloy in which iridium is relatively insoluble. Furthermore, ruthenium and osmium may be thus more efficiently collected from ores, etc. The behavior of these tn7o metals during fire assay, particularly osmium, is little understood, but ample data show that such assay with subsequent cupellation is an inadmissible process for the determination of osmium. The following report shows the 1 Present address, Canadian Industries, Ltd., Millhaven, Ontario, Canada.
