Study of the Iodide Titration for Determination of Palladium RICHARD N. RHODA and RALPH H. ATKINSON Research Laboratory, lnternational Nickel Ca., Bayonne,
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A systematic study has been made of a volumetric method for the determination of palladium in palladium-rich’alloys. The method is based on the precipitation of palladium as palladous iodide, using the precipitate as its own indicator. This cloud method has been found to have an accuracy comparable with the present gravimetric methods and is especially applicable as a “go-or-no-go” test.
THE
need for further systematic study of methods of separation and determination of the platinum metals, which was recognized a t the Sixth Annual Summer Symposium in 1953 (Q), is especially true in the case of palladium. The increasing use of this metal and its alloys in the arts and industry has created a demand for a rapid method for its determination. The most common determinations of palladium in alloys are the gravimetric methods in which the palladium is separated and weighed as the dimethylglyoximate complex (8), or in which the palladium is separated and precipitated as palladous iodide and converted to the metal before weighing (11). Recent publications by Atkinson and others (2, S) describe a direct volumetric method based on the titration of palladous chloride with potassium iodide using the precipitate as its own indicator. The usefulness of this method in laboratories with limited equipment and the possibility of its adoption as a control procedure in hallmarking were considered adequate justification for a systematic study of its limitations and accuracy. REAGEKTS
Standard Palladous Chloride (chloropalladous acid). Exactly 1 gram of palladium sponge was dissolved in 4.5 ml. of concentrated hydrochloric acid, 0.9 ml. of concentrated nitric acid, and 10 ml. of water in a covered, lipless beaker. The solution was evaporated almost to dryness, taken up with 10 ml. of 6 N hydrochloric acid, and evaporated again. After one additional evaporation the residue was taken up with 16.3 ml. of 6N hydrochloric acid and diluted to 1 liter. Ferrous Sulfate Solution. Twenty grams of ferrous sulfate crystals, FeSOc. 7H20 xyas dissolved in 100 ml. of distilled water with a few drops of concentrated sulfuric acid. Fresh solution was made every 2 or 3 days. Standard Potassium Iodide Solution, 0.01N. Pure potassium iodide (1.660 grams) was dissolved to make 1 liter in distilled water that had been deoxygenated by bubbling an inert gas (argon) through it for 2 hours. The solution was kept in a dark bottle with a tightly fitting stopper and was treated with inert gas a t the end of each day. The solution was standardized against pure palladium foil or sponge each day. EXPERIMENTAL
Analytical Method. A complete description of the analytical procedure is given by Atkinson ( 2 ) . For this study a sufficient quantity of standard chloropalladous acid solution was prepared and aliquot portions were used throughout. I n each run, 10 ml. of chloropalladous acid, 5 ml. of concentrated hydrochloric acid, and 15 ml. of 20% ferrous sulfate solution were mixed in a 250-ml. Erlenmeyer flask and titrated with 0.01N potassium iodide solution. About 0.5 ml. less than the ex ected amount oi iodide solution (18.74 ml.) was run in slowly w h l e the system was kept well stirred. The flask was stoppered and shaken vigorously for 1 minute, after which about 5 ml. of the solution was transferred t o a centrifuge tube and centrifuged a t or above 8500 ft. per min. for 15seconds. One drop of the iodide solution was added to the clear solution in the centrifuge tube and the intensity of the cloud of palladous iodide judged, using a diffuse light background. The contents of the tube were returned to the flask and the titration and testing continued until the addition of 1 test drop failed to give a recognizable cloud in 1 minute.
N. J. Sensitivity of End Point. Attempts were made to improve the sharpness of the end point by studying the coagulation of the main precipitate, the illumination of the cloud, and other operating factors. Coagulating agents, such as trisodium phosphate, albumen, and nitrobenzene, and one wetting agent, Duponol WE, were tried; but vigorous shaking followed by centrifuging, as described above, still gave the best clarification of the mother liquor. Artificial light was found t o be less satisfactory than diffuse daylight for illuminating the cloud. A “diffuse light cell” suitable for detecting the cloud under all conditions was made by supportixig the centrifuge tube in the center of a beaker which had onion skin paper taped to a portion of its inside surface as a background. The cloud could be seen best when the test drop of potassium iodide w-as allowed to fall directly on the surface of the liquid from a short distance, rather than run down the side of the tube. An induction period of about 15 seconds was needed for the precipitate t o form well enough t o be used as an indicator; development of the cloud for a period of longer than 1 minute did not improve the accuracy of the titrations. It was found that the preferred time for the cloud to form and the choice of the particular degree of diffuse illumination varied 7%-iththe individual analyst. The effect of such variables mas eliminated by having each operator do his own standartlieation. Verification of End Point. I n spite of the good agreement between the experimental and calculated end points, it was felt t h a t some other confirmation of the end point in addition to visual observation of the formation of the precipitate was desirable. Adsorption effects might be made apparent by comparing the usual end point with that obtained on reversing the order of titration. Although the end point of the reverse titration was not sharp because of insufficient contrast between the color of the added drop and that of the cloud, the titer indicated that there was no great error due to adsorption. Additional evidence for the absence of adsorption effects is found in a comparison of this method with the corresponding method for silver (6). I n t h e silver titration the solution is turbid a t the end point instead of clear, because of the peptizing action of iodide ions on silver iodide. No similar peptizing action has been observed in the numerous iodide titrations of palladous chloride made in this laboratory. Because peptizing action cannot take place without adsorption, the absence of any peptizing may well indicate that the end point of the palladium titration is not affected by adsorption. Incidentally, the adsorption indicators fluorescein and eosin did not appear to be applicable in this titration. A potentiometric titration, similar to that of RIuller and Stein (IO),was performed using an atmosphere of argon gas, a goldcalomel electrode system, and a sensitive direct current potentiometer. The titration of the mixture of chloropalladous acid and ferrous sulfate with potassium iodide a t room temperature and a t 50” C. gave meaningless results. A titration a t 50” C. with potassium iodide of chloropalladous acid without ferrous sulfate resulted in a potentiometric end point which agreed with the visual and calculated end points. It i8 thus apparent that this procedure for palladium, which depends upon the presence of ferrous sulfate, does not lend itself t o a potentiometric technique. Solubility of Palladous Iodide. A search of the literature revealed no data concerning the solubility of palladous iodide in water. A saturated solution of palladous iodide, with a portion of the standard chloropalladous acid diluted to a concentration of 1 y of palladium per 0.05 ml. of solution, was tested with stan-
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ANALYTICAL CHEMISTRY
nous chloride and with nickel dimethylglyoximnte according to Feigl ( 5 ) ; both tests were negative for tlie palladous iodide solution. If the limits of sensitivity of these spot tests for palladcus ion as reported by Feigl are correct, palladous iodide has an estimated solubility in water somewhat below 2 X gram yer liter. The maximum error introduced by this solubility is equivalent to 0.06 ml. of 0.01N potassium iodide in a titer of about 20 ml. Even this small error is avoided by direct standardization of reagents against pure palladium metal. The possibility that the solubility of palladous iodide in PXCCBO potassium iodide might affect the titration was examined. Such interference seemed improbable, however, in view of the findir g of Beamish and Dale (4) that up to ten times the calculated amount of potassium iodide could be added to a precipitate of palladous iodide without danger of palladium loss. The reaction PdIr 2III e IIsPdIc
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was studied by adding C S C ~ S Spotassium iodide to palladous iodid. until a visible red color formed and until a portion of the precipitate appeared to dissolve. Under these conditions more thaii six times the volume of potassium iodide needed for precipitation was necessary to shift the equilibrium to the right. Oxidation-Reduction Considerations. I n a titration involving iodide it is obviously important to avoid the presence of oxidizing agents such as residual aqua regia from the dissolving of the palladium alloys. On the other hand, the reducing agent used to destroy the residual aqua regia should not reduce or react with the palladous chloride. Hydroxylamine hydrochloride, which is used in some analytical procedures for decomposing residual aqua regia, appeared to cause some complexing or reduction of the palladium, because titers mere about 10% lorn when i t was used. Arsenious acid also lowered the iodide titcr. Stannous chloride dissolved the precipitate of palladous iodide, so was not considered further. Ferrous sulfate solution did not interfere with the titration and did effectively remove any ordinary excess of aqua regia used in dissolving the alloys, The ferrous ion, however, mould be expected to react with the palladous ion according to the reaction (7) 2Fe++ P d + + PdO 2Fe+++ This was found t o occur a t high concentrations of ferrous ion, at temperatures above room temperature, and a t low acid concentrations. Therefore the ferrous sulfate should not be added t o the palladium solution until after the hydrochloric acid has been added and the resulting mixture cooled to room temperature. Tlie concentration of the ferrous sulfate in the titration mixture should not exceed 100 grams per liter; in some cases, as when aqua regia is absent from the palladium solutions, this concentration should be lowered to 75 grams per liter. Effect of Other Metals. Tests were performed to determine the effect of interfering metals by introducing solutions of the n i e t d ions into the chloropalladous acid before titration and by actual analysis of palladium-rich alloys. A compilation of work done for this paper and of work reported elsewhere (2, 3) shows t h a t the following metals do not interfere with tlie accuracy of the titration when present up to 5% by weight of tlie palladium: bismuth, cobalt, copper, chromium, iridium, iron, manganese, molybdenum, nicltel, platinum, rhodium, ruthenium, tin, and tungsten. Gold ordinarily interferes with this titration by competing with the palladium for the iodide ions. It was found that, if the ferrous sulfate solution was added to the palladium-gold soIution and the mixture allowed to stand for 1 hour a t room temperature, the gold, accompanied by some palladium, was precipitated when the solution contained 3.6 grams of hydrogen chloride per liter, but that only the gold precipitated if the solution contained about 70 grams of hydrogen chloride per liter. Preventing the interference of gold is thus another good reason for the use of ferrous sulfate in the presence of a high concentration of hydrochloric acid.
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Silver also interferes with this determination. Such interference can be partly obviated by filtering off the insoluble silver chloride after dissolving the alloy in aqucz regia; some palladium, however, is occluded by the precipitate. It is preferable to determine the palladium by a modified technique without removing the silver (3). “Go-Or-No-Go” Method. The development of the cloud method in the analysis of palladium suggested its investigation as a test such as would be used in the hallmarking analyses of metals for jewelry. Runs were performed in which one drop less than the exact amount of 0.01N potassium iodide needed for complete precipitation was added to a 10-ml. portion of the standard chloropalladous acid solution (with the hydrochloric acid and ferrous sulfate). After centrifuging, one more drop of potassium iodide should give a cloud of precipitate, thereby indicating the presence of enough palladium t o pass the test. In another series of runs the simulated sample solutions contained not quite enough palladiu‘m to pass the test.
Table I. Deviation of Experimental Titers from Calculated Titer of Potassium Iodide Necessary to React with Stnndard Chloropalladous Acid Solution
Number of runs Runs within 0.05 point ( z t 2 . 5 thousand) Runs within 0.10 (+5.0 point thousand)
ml. of end p u t s per ml. of end parts per
Analytical
Method “Go-Or-No-Go”
41
69
Both 110
1 9 (70.7%)
65 (94.2%)
94 ( 8 5 . 5 % )
3G (87.8%)
ti7 (97.1%)
103 (93.6%)
ANALYSIS OF TITRATIOiV DATA
Analytical Data. The statistical analysis of a large number of determinations under optimum conditions for the ordinary volumetric method is given in Table I. The spread of values for the titrations corresponds roughly to that to be expected from probability considerations. For a given volume of the standard chloropalladous acid solution, the average volume of the potassium iodide a t the end point, the median value, and the theoretical end point coincide. The distribution of the end point values of the titrations appears to indicate identical tendencies tonrard high and low results. L‘Go-Or-No-Go” Data. Statistical analysis of a number of runs by this method is also given in Table I. The total number of runs, 69, represents 38 runs iii which an exact or excess amount of palladium was present and 31 runs in which an insufficient quantity of pdladium was present. The distribution of the results from these tests likewise resembles a probability curve, but in this case the skewness of the curve (1) is f0.002, indicating a slight tendency t o pass samples containing insuacient palladium. These analys2s are normally run in triplicate, however, and such a scheme would guard against an occasional erratic titer. Almost 9570 of the runs gave results viithin 2.5 parts per thousand of the correct vzlue, which is sitisfactory accuracy for platinum group metal analyses. DISCUSSION
The various aspects of this somewhat novel application of a cloud method have bcen investigated. It has been shown that the best operating conditions for achieving a sharp end point involve vigorous shaking after titration, adequate centrifuging, and a suitable light cell for observing the cloud. The method has been shown to have an end point corresponding to the stoichiometric value and free of any great interference from adsorption of solubility effects. The method is of especial application as a “go-or-no-go” test, and with care in operation has an accuracy of =k0.25%, which is comparable to the usual gravimetrid methods.
