AN EXACT ELEXX‘ROI,YTIC METHOD FOR DETERMINING METALS. BY W. L. PERDUE AND G . A. HULETT
The electric current is our most effective means for obtaining metals from their compounds and the fact that metals may be separated from their solutions and each other at ordinary temperatures by this agent is the reason that SO much attention has been given to electroanalysis. From a quantitative standpoint it is essential that the metal be deposited in a pure state and with a surface such that it may be readily freed from $he electrolyte and brought into a condition to weigh without loss or oxidation. The purity of the metallic deposit is affected by the presence of other metals in the solution. An electrolytic deposit undoubtedly contains all the metals which are in the electrolyte, but the relative amounts vary widely as they depend upon the decomposition potential of each ion upon the particular metallic surface presented to it and upon their concentration at this surface. The solubility and affinity relations of the metals for each other are also factors in determining the makeup of the deposit. It is possible however to make many excellent separations, especially where the decomposition potential of one metal is below that of hydrogen and the other above,’ so that the hydrogen ion may act as a “safety valve.” Or where it is possible to hold the voltage below the decomposition voltages of the metals which are not wanted in the deposit.’ Aside from the metallic impurities in electrolytic deposits there is the inclusion of the electrolyte in the deposited metal t o be considered. There is very little information on this point and hardly any that is reliable. Considerable attention has been paid to the purity of the silver deposited in the silver coulometer. Analyses of these deposits for Bancroft: International Congress, 4, 703 (1904.) Sand: Jour. Chem. SOC.,91,373 (1907.)
148
W . L. Perdue an,d G. A . Hulett
total silver have been made by Rayleigh and Sedgwick' and later by Richards, Collins, and Heimrod.2 The results however were not concordant, so the authors finally resorted t o the method of reheating the deposited silver in the dish in which it was determined and observing any change in weight. This method has been employed in all investigations on the silver coulometer, but the results have been contradictory and were not capable of interpretation as there was no information as to the nature of the impurity. A new method was devised by Hulett and Duschaks in 1908. The silver was detached from the platinum dish, brought into a vacuum where it could be heated to the melting point and the gases evolved were examined as to their nature and amount. A continuation of this work, which will soon be published, is giving good quantitative results and indicates that these inclusions are not of the same composition as the electrolyte and are large enough to be taken into account in accurate work. That all metals deposited in the solid state contain inclusions can hardly be questioned. Even with a satisfactory method for determining the nature and amount of these inclusions it is still desirable t o avoid them entirely. If the metal is deposited in the liquid state this is accomplished or if it is deposited in mercury and forms an amalgam there are no inclusions. The surface of a mercury or amalgam cathode is always a good one so that greater variations of current density and conditions in the electrolyte may be employed, also the metal is more easily and completely removed from the electrolyte with a mercury cathode. Mercury has been used as cathode for a long time in electro-analysis,' but not with the idea of avoiding inclusions, It has always been a difficult matter to get the amalgam into a weighable condition without a loss or Phil. Trans., 175, 411. Proc. Am. Acad., 35, 1 2 3 . Trans. Am. Electrochem. SOC.,12, 255. W. Gibbs: Am. Chem. Jour., 13, 5 7 1 ; Luckow: Zeit. anal. Chem., 26, 1 1 3 ; Vortmann: Ber. chem. Ges. Berlin, 24, 2749.
Exact Electrolytic Method for Determining Metals
I 49
oxidation of the metal deposited in it. The method described by Prof. E. F. Smith’ is a great improvement on previous ones and is convenient for analytical pruposes, but washing the amalgam with water and other liquids w’fiich contain dissolved oxygen is bound to occasion some solu/tion of mercury and the metal, also the necessary agitation of1 the amalgam causes some of it to divide into very fine globples which are easily washed away and escape detection.’ I The fact that mercury does not wet its container, lbut allows a film of liquid to get beneath the mercury, isithe chief source of difficulty encountered in removing the $lectrolyte and drying the amalgam, so our first attention was given to this point. Mercury does not ordinarily wet latinum but it is possible to amalgamate the inside of a cru ible or cup so that mercury wets it completely and then the $lectrolyte never gets beneath the mercury. With this adyantage only the upper surface of the amalgam need be coqsidered in removing the electrolyte after the metal has yeen deposited. The electrolyte. may pf course be removed~by displacement with water and the amalgam washed +ith water without any loss of mercury or metal by solutioq or oxidation erovided we keep the amalgam cathode during this process. Furthermore, in the absence of a film of liquid between the amalgam and its container, this amalgam need not be disturbed or agitated so there is no tendency to form little globules which may be lost. The water is finally removed with a pipette and the drop or two of water which remains on the amalgam is allowed to evaporate in a vacuum desiccator over calcium chloride. One may readily observe the disappearance of the last trace of water and the dish and amalgam are ready to weigh in a few minutes. From the standpoint of accurate weighing, metallic surfaces are exceedingly well defined since there are no troublesome moisture films to contend against. Our desiccator contains
F
Electro-Analysis, p. 58. H. J. Sand: Trans. Chem. SOC., 91, 373 (1907); also T. Slater Price: Trans. Faraday SOC.,3, 94-7 (1907).
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W . L. Perdue and G. A . Hulett
a dish of mercury which maintains a vapor of mercury in the desiccator qnd so prevents a possible loss of mercury b y evaporation. This vacuum method of drying mercury and amalgams will not do when they are in glass vessels or containers which they do not wet as the film of liquid beneath the mercury or amalgam evaporates irregularly and explosively, throwing mercury out of the container in a quite uncontrollable manner but when the mercury wets the container perfectly there is no trouble on this score. We have made many blank experiments to test all these points. After a determination has been made and the weight of the crucible with the amalgam known, we have returned the exhausted electrolyte to the amalgam and turned on the current. After repeating all the manipulations the crucible and amalgam have been reweighed, this gives the errors of the method. ~-
Weight of t h e cup and amalgam I O I ,4909 109.6732 132,3499 98,0254
1 I
I
Weight after t h e blank
101.4908 109.6733 132 '3498 98 '02.54
These are typical trials and as we are determining two t o three grams of cadmium in each case it is seen that the errors are less than I part in 20,000. The vacuum correction is easily applied, and we are therefore in a position to determine the mass of metal in a given solution in a satisfactory manner and to know that nothing but the metal has been weighed. The solubility of platinum in mercury is very small, so that crucibles which have been amalgamated and used for this purpose may be readily cleaned with acids at any time. The loss of platinum is small and the crucibles are not injured. Our method of amalgamating the inside of a crucible is to plate it electrolytically from a mercury cyanide solution ; when washed out and rinsed with mercury it should
Exact Electrolytic Method for Determining Metals
I5I
show a contirwous film of mercury to within a centimeter of the top. It is very difficult to amalgamate some crucibles satisfactorily by this method but Mr. J. S. Laird of this laboratory has found that this may be readily done in all cases by filling the crucible with mercury to the desired height and then heating it nearly to the boiling point of mercury. After plating with mercury the crucible is washed with water, filled with the necessary amount of mercury and dried in the vacuum desiccator. Fig. I shows the arrangement we employ in carrying out a determination. The funnel ( f ) has a wide stem (7 mm),
+
while the rim fits nicely into the cup and thus prevents any loss of the electrolyte. The anode is a flat spiral made from platinum wire with the stem fused into a glass tube which is held by the clamp ( a ) and is independent of the funnel. The arrangement is such that the funnel may be raised clear of the cup without disturbing the anode. Towards the end of the electrolysis the current is increased to insure the complete removal of the metal. This is favored by the stirring of the electrolyte due to the increased generation of gas. The spraying thoroughly washes down the funnel and sides
W . L. Perdwe and G. A . Hulett
1.52
of the cup so that it is seldom necessary to wash these through the top of the funnel. After the metal has been completely deposited, the funnel is raised clear of the cup and the electrolyte removed with a large pipette while water is simultaneously run in. The stem of the pipette is drawn down to a thin tube and cut off a t a convenient length while a rubber tube is attached to the upper end so that the pipette may be conveniently handled. A reservoir of distilled water is provided with an outlet tube, nozzle, and pinchcock so that water may be run into the cup at any desired rate and thus the electrolyte is displaced with distilled water without interrupting the current, and when the ammeter shows '' zero " reading the displacement is complete and the amalgam throughly washed, but the amalgam is still cathode by I O or 1 2 volts and under these conditions there can be no loss of mercury or metal by oxidation or solution. Since the amalgam is not disturbed or agitated there is no tendency to form fine globules. The water is now removed with the pipette, the anode raised, and after attention has been given to the outside of the crucible it is put into the vacuum desiccator and is ready for weighing in a short time. Analysis OJ Cadmium SuZphate.-CdSO,.8/3H2O is an exceptionally well defined and stable salt which may be obtained in a high state of purity as will be shown in the following article. We have determined the percent of cadmium in this salt with the apparatus and methods just described and offer a few of the preliminary determinations to give an idea of the possibilities of the method: _ _ _ _ _ _ ~
I
I I 3* I
______
______._~~______
~
Mass CdS04.8/8H,0
8 ' 2375
9 2857 6.5312 '
1
Mass Cd
3.6076 4.0668
~
Percent Cd
43.795 43.798
A detailed study of the properties and composition of this compound will be given in the following article.
Exact Electrolytic Method for Determining Metals
I 53
Cadmium is one of the most soluble of the metals in mercury (5.6 percent), but some metals have only a very small solubility and in such cases a solid amalgam soon forms and floats on the surface of the mercury cathode. The metal soon grows up and away from the mercury and may include some of the electrolyte. By agitating the mercury during the deposition of the metal this difficulty is overcome, for any motion of the mercury not only prevented the “treeing” of the metal but kept the amalgam covered with a film of mercury which prevents inclusions. We have accomplished this result by rotating the cathode crucible during the electrolysis. The accompanying sketch is self-explanatory. The cup and shaft were turned from solid brass while the shaft works in a bronze socket which is fitted into a lead base. The platinum cathode cup or crucible fits exactly into the
+
Fig.
2
brass cup and must rotate without vibration. The platinum spiral which serves as anode is fused into a little funnel and so arranged that the spiral is only a few millimeters from the mercury cathode when the crucible is at rest and a t the
W . L. Perdue and G. A . Hulett
I54
same time the sides of the funnel nearly touched the top rim of the crucible. The funnel is conveniently held by an inverted funnel (the two are held together by a rubber band) and electrical connections are made as indicated in the cut. Rotating the cup not only keeps any solid amalgam covered with mercury but causes the mercury surface to assume the form of a parabola which gives a large surface for a small amount of mercury. The electrolyte may be small in volume and the conditions are such as to allow a large current density and rapid deposition of the metal. A few comparative determinations were made by determining the copper in equal portions of a copper sulphate solution and zinc in a zinc sulphate solution: ~~
Weight of zinc I 2
3
0,4793 0.4791 0,4794
I 2
3
A determination, weighings and all, took about 50 minutes and the average error seems to be about I in 4,000, and. while zinc is fairly soluble in mercury ( 2 . 2 percent), copper has a solubility of only I part in IOO,OOO.~Of course this method of rotating the cathode crucible offers many chances for accidental errors and cannot be compared in accuracy with the method previously described, but it has some advantages and is useful where great accuracy is not the main object. Princeton L'niversLty, November, 1910
Gouy: Jour. Phys., 4,
320.
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