Determination of Palladium by Means of J
Potassium Iodide F. E. BE-IMISH
4SD
J. DALE, University of Toronto, Ontario, Canada
A
A 1.2 per cent by weight nitric acid solution of ruthenium tetrachloride and iridium tetrachloride yielded no precipitate on warming and boiling.
LTHOUGII potassium iodide has long been used as a
reagent for the determination of palladium, very little data have been published concerning suitable conditions for precipitation. Scott ( 2 ) and Bugbee (1) have stated that excess potassium iodide must be avoided, and i t has been generally accepted in practice that the solution must not be boiled during precipitation. Winkler (3) has discussed the use of palladium salts for the determination of iodide in the presence of other halides. The data recorded below indicate desirable conditions for the complete precipitation of palladium iodide as well as the influence of associated precious metals on this precipitation.
Summary Palladium can be determined on the macro and micro scale by direct weighing as palladium iodide. The palladium iodide can be safely boiled in the presence of an acid concentration up to about 0.8 N. TABLE I. EFFECTOF
CONDITIONS O S PRECIPITATION O F PAL-
LADIUM BY
POTASSIUM IODIDE
Effect of Excess Potassium Iodide A standard palladium nitrate solution was prepared by dissolving 2.000 grams of pure palladium sponge in aqua regia and evaporating several times with nitric acid and sodium nitrate. The final residue was dissolved in 200 ml. of water and 15 ml. of nitric acid, then filtered and diluted to 1 liter. Nitric acid was used because it is associated with palladium in the wet treatment of the assay prill and regulus. When treated with dimethylglyoxime and burned as metal, 25.0 ml. of this solution yielded 50.0 mg. of palladium. A second standard solution similarly prepared yielded 25.1 mg. of palladium in 25.0 cc. of solution. 4 third solution yielded 25.2 mg. of palladium in 25.0 cc. of solution.
Sample NO.
1 2 3 4
5
6
Table I describes the results obtained on treating the standard solutions with potassium iodide under varied conditions. 7 8
Separation of Palladium and Gold Because gold iodide redissolves in excess potassium iodide and palladium iodide only in a n extremely large excess, i t was decided to determine whether or not these metals might thus be separated.
9 10
Each sample of 150-ml. volume contained 25.3 mg. of palladium, 25.0 mg. of gold, and 3 ml. of concentrated nitric acid. Preliminary experiments indicated that 40 ml. of 1 per cent potassium iodide would redissolve the gold iodide from 25.0 mg. of gold. Sample 1 n.as treated with 75 ml. of 1 per cent potassium
11
iodide solution and coagulated. The precipitate was allowed to stand with occasional stirring for 30 minutes and then washed with 600 ml. of cold water. The metal recovered weighed 25.5 mg. Sample 2 was treated vitli 60 ml. of the iodide solution. The crystals n-ere washed s-ith 0.5 per cent potassium iodide solution and then with 600 ml. of cold water. The metal recovered weighed 25.7 mg. Sample 3 n.as treated with 80 ml. of the iodide solution and the crystals \yere washed with GOO ml. of cold water. The metal recovered weighed 25.5 mg. Another sample similarly treated, except that the crystals were nashed with GOO ml. of hot water, yielded 25.3 mg. of metal. Sample 4 vas treated by pouring the precious metal solution into 80 ml. of hot 1 per cent potassium iodide solution. The crystals were washed with 600 ml. of hot n-ater and the metal recovered weighed 26.3 mg.
Conditione of Precipitation
-Palladium--Recovered with potassium .4dded iodide hfQ.
MQ.
Volume made up to 125 ml. 20 ml. of 1.0% K I and 2 ml. of concd. " 0 s added. Temperature lust below boiling for 20 min. Volume made up t o 125 ml. 15 ml. of 1 0 7 KI. Mixture left s t d d i n g about 15 hours Volume made up t o 125 ml. 50 ml. of 10% KI. Mixture kept just below boiling for 30 min.
50.0 50.0 50.0 50.0
50.0 50.0 50.1 50.0
25.0
25.0
25.0
23.7
Volume made up t o 125 ml. 5 ml. of concd. HNOr and 20 ml. of 1% K I added. .Mixture kept almost boiling for 20 min. Treated like 7 and 8, except that mivture was allowed to stand for 24 houra and then boiled for 5 min.
25.0 25.0
25.1 25.1
23.0 26.0
25.0 24.9
25.0
25.0 25.2; 85.3 25.2, 85.4" 50.0, 168,lG
Treated like 7 and 8, except that boiling was continued for 30 min.
12
25.2
13
25.2
14
50.0
5,Ol 5.00 Each sample of spectro6.27 6.28 graphically pure metal 4.67 4.66 dissolved in aqua regia and evaporated three times with HCI in presIodide ence of NaC1. precipitated by 0.8 ml. of 1 % K I and HNOa adjusted to about 0.8 N . Crystals filtered by 3-,ml. grade A2 filtering crucible and weighed as PdI? 11.35 11.36 Treated like 15 t o 17, but 18 mixture allowed t o stand for 24 hours 5.43 5,39 Treated like 15 to 17, but 19 8 times required amount of K I used 6.60 6.61 Treated like 15 to 17, but 20 14 times required amount of K I used a \$-eight of palladium iodide recovered.
15 16 17
A large number of determinations with this separation of gold from palladium produced results which were generally high. Because a reprecipitation would be necessary, the authors prefer to use existing methods for the determination of gold in the presence of palladium.
Platinum Metals with Potassium Iodide A solution of platinic chloride and sodium rhodium chloride made up to contain 1.2 per cent of nitric acid yielded a precipitate on warming with 1 per cent of potassium iodide solut,ion. Precipitation was not' complete.
Notea Filtrates from 1 t o 5 yielded no palladium dimethylglyoxime
Filtrate treated with d/methylglyoxime yielded 1.4 mg. of palladium, a total of 25.1 mg.
........... ..........
Same volume of concd. HzQ01 and HC1 used and similarly treated without loss of palladium
...............
Crystals of PdIr filtered b porousbottorne3 porcelain filtering crucible, A2 grade. dried a t 110' C. for a n hour, and weighed as PdIr. PdI. canbe burned L O the metal, reduced in hydroKen, and cooled i n carbon dioxide with much lesa danger of loss than with palladium dimethylglyosime
.............. .............. ...............
...............
............... ...............
d
697
698
IA DUSTRIAL AND ENGISEEKI\G CHE\IIYrlII
At least 10 times the calculated amount of potassium iodide may be added without danger of palladium loss. Palladium iodide is much more easily burned without loss than is palladium dimethylglyoxime. Palladium may be separated from gold by means of potassium iodide, but the results are -generally- high. -
\ 01,. 10. TO. 12
Literature Cited (1) Bugbee, “ T e x t b o o k of Fire Assaying,” p. 258, N e w York, J o h n K i l e v & Sons 1938. (2) Scott, “ S t a n d a r d M e t h o d s of Chemical Analysis,” p. 334, New York, D. Van N o s t r a n d Co., 1918. (3) Winkler, L. Ifr., 2. anal. Chein., 31, 102 (1918); 60,422 (19?1)
RECEIVED June 13. IS38
Determination of Free Sodium Cyanide and Ammonia in Brass Plating Solutions SAMUEL HEIMAN AND WALLACE RI. RIcXABB, Department of Chemistry and Chemical Engineering. University of Pennsylvania, Philadelphia, Pa.
T
HE free or uncombined cyanide content of a brass
plating solution refers t o the sodium cyanide in excess of that required to form the complex copper and zinc cyanides ( 2 ) . The amount of free cyanide in the plating bath has a marked effect on the appearance and composition of the brass deposit, the cathode and anode efficiencies, the anode polarization, and the conductivity of the solution (15, 16, 19). In general, the free cyanide in any cyanide plating solution is determined by titration with silver nitrate, using potassium iodide as indicator (2, 20). Inconsistent results are obtained, however, when this method is applied to brass plating solutions. The removal of the sodium carbonate before titration
!I C
FIGURE 1. DISTILLATIOS APPARATUS A . 300-ml. short-necked Kjeldahi flask B . Connecting bulb, Southern Oil Co. C.
D. E.
tvith silver nitrate is said to overcome most of the difficulty (3,8 ) , although the interference of sodium carbonate may be avoided by suitably diluting the sample to be titrated (16). Pan (16) has shown that ammonium hydroxide and potassium iodide influence the value of the free cyanide, and recommends that the titration be carried out in the presence of 0.37 N potassium iodide indicator in order to give accurate results. However, he neglected to consider the effect of free alkali or pH, which Blum and Hogaboom (3) have pointed out makes the determination and calculation of free cyanide uncertain. I n contrast nith this lack of agreement regarding the direct titration of free cyanide in Grass plating solutions, the deterinination of cyanide in copper and zinc plating baths has been satisfactorily 11orked out. Thompson (21) has shown that when a copper cyanide solution is titrated with silver nitrate, a reproducible value of free c y a n i d e t h a t is, the cyanide in excess of that required to form the compound Sa2Cu(CS)3-is obtained when 0.5 to 1.0 gram of potassium iodide per 100 ml. of solution is used as an indicator. If the same method of titration for free cyanide is applied to a zinc cyanide solution, inconsistent results are obtained, owing to the effect of variations in total alkalinity or p H on the equilibrium betn een sodium zincate and sodium zinc cyanide. However, an accurate and reproducible value of the total cyanide mag be obtained by adding an excess of sodium hydroxide to the solution and then titrating with silver nitrate (4). When this procedure for the determination of the total cyanide in a zinc plating solution is applied to a brass plating solution, the cyanide in the zinc complex plus the free cyanide should be obtained. One object of thls investigation was to evaluate this method.
Evolution ;\lethod for Total Cyanide and Ammonia Coat,es (7) has recommended an evolution met,hod for determining the total cyanide in a brass plating solution, by means of wliicli the uncombined cyanide can readily be calculated. Kick (22) has determined the total cyanide in a cyanide sih-er plating solution by dist,illing with sulfuric acid. It, v a s thought desirable to investigate this evolution method for the determination of total cyanide, n i t h the idea of making it as simple as possible in operation and detail, so that it could lie used as a routine test. Page1 and Carlson (14) have recently decomposed sodium cyanide quantitatively by the distillation of hydrocyanic acid from sulfuric acid solutions, and AIorris and Lilly (13) 11ave made further studies of this method.
