Rinnmann's Green Test for Zinc A. A. BENEDETTI-PICHLER, Washington Square College, New York University, New York, N. Y.
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Rinnmann's green lest for zinc can be carried COrrect ratio zinc-cobalt would HE f o r m a t i o n of Rinnmann's green is generally out in a very simpleway by a drop of the finally prevail in some zone of recognized as one of the the strip, but this method occabest of co&ing the test solution on a piece O f filter paper impregsionallyfdled, presence of zinc, and the test is naled Previousb' with Potassium CobaltiWanide, Later Emich thought that if a compound c o n t a i n i n g t h e recommended by all of the drying, and ashing the paper; 0.0006 mg. of c o b a l t a n d t h e z i n c in the zinc still produces a disk of green ash on the o u t s t a n d i n g t e x t b o o k s on ~h proper ratio is first prepared, qualitative a n a l y s i s (8, 7 , 8). spot where the solution had been chance could no Ionger influence Up to the present, h o w e v e r , test is best carried out with a solution obtained by there has b e e n n o p e r f e c t l y the result. He also pointed satisfactory way of carrying out dissolving the zinc SUwde Precipitated in the out that zinc c o b a l t i c y a n i d e , course of a separation in nitric acid. Some of which can easily be obtained by the test. This can be easily understood from a discussion of the limit proportions for the presence of other precipitation w i t h potassium . the reactions involved. metals are given. The reasonsfor the reliability cobalticyanide f r o m a n acid solution, seems to possess the de*"Ording to Hedvall @)' qf the technic employed may be seen from the sirableproperties. the so-called Rinnmann's green discussion of the reactions involved. consists of mixed c r y s t a l s of The zinc cobalticyanide of a p p r o x i m a t e l y the formula zinc oxide and cobaltous oxide. The mixed crystals are obtained by heating the oxides to Zn3(CoCy&l resembles the corresponding ferricyanide very about 800" C. On heating for a long time at 1100" C., the closely with respect to crystal form and solubility. It forms crystals will be obtained as hexagonal plates, needles, and a white finely crystalline precipitate which is very difficultly soluble even in acid solutions. On ignition it gives zinc oxide pyramids of about 2 to 3 mm. in length. Owing to the limited solubility of cobaltous oxide in zinc and cobalt oxide in the ratio 3 to 2. Therefore, not all of the oxide, it is found, after ignition, that more than 1 part of cobaltous oxide formed can be dissolved by the zinc oxide. cobaltous oxide to about 10 parts of zinc oxide produces As a result of the ignition we may expect a mixture of mixed reddish brown crystals of cobaltous oxide in addition to the crystals and crystals of cobaltous oxide. I n fact, the Rinngreen mixed crystals. With a further increase in the amount mann's green obtained in this way exhibits a brownish hue. of cobaltous oxide present, i t finally becomes impossible to It was shown that it is possible to confirm the presence of confirm the presence of green mixed crystals. Furthermore, very small amounts of zinc by precipitating a droplet of the though the cobaltic oxide, CozO3,is not stable above 900" C., unknown solution on a narrow slide with potassium cobaltioxidation of the cobaltous oxide present in the mixed crystals cyanide, washing the precipitate with dilute nitric acid, may take place in the temperature range from 900" to about drying, and igniting on the slide. Even with a little less than mg. of zinc it was possible to identify the Rinnmann's 300" C. when the preparation cools to room temperature. The cobaltic oxide formed then gives a solid solution with the green formed under the microscope by its color and shape cobaltous oxide, and the color of the preparation changes to (pseudomorphic crystals showing the square-shaped outlines brown or black. of the former zinc cobalticyanide crystals, I ) . The properTherefore, in order to obtain a good Rinnmann's green ties of the zinc cobalticyanide and of the Rinnmann's green can be investigated in succession even with a single crystal test the following requirements must be fulfilled: of the former compound. 1. Zinc and cobalt must be used in the form of oxides or It is obvious that the reliability and sensitivity of this procompounds which are easily transformed to oxides on ignition cedure cannot be surpassed. It has only one disadvantage. (nitrates, carbonates). 2. Zinc and cobalt must be present in certain roportions. In carrying out the ignition on the slide, the slightest overEspecially must an excess of cobalt be ?voided. I n excess of heating causes complete oxidation of the small quantities of zinc oxide is by far less dangerous because it acts more or less as a Rinnmann's green and, therefore, the ignition must be colorless solvent. In the presence of not too large an excess of carried out in a current of inert gas (carbon dioxide). This, zinc the green color becomes paler but can be perceived. 3. While hot (below 900' C.) the preparation must be pro- of course, renders the procedure complicated and will cause tected against oxidation (presence of a reducing agent-e. g., many to refrain from using it. charcoal or filter paper as carrier of the test-or imbedding in an For the student's use in qualitative analysis the author inert slag), or it must be cooled rapidly through the dangerous has always recommended the old practice of soaking a piece temperature range. of filter paper in a solution containing zinc and cobaltous nitrate, drying, and ashing. Under these conditions failure Accordingly, aside from the special method of carrying out due to oxidation of the Rinnmann's green already formed never the test, it is usual to avoid an excess of cobalt by adding a results. This may be due to the protective action of the very dilute solution of the latter in small portions to an as- gaseous products formed by the burning paper, or to the sumed excess'of the zinc preparation, and to repeat the test rapid cooling of the small quantity of ash. The ash of the with increased quantities of cobaltous nitrate until a defi- paper may also have an effect. Of course, the test is often nitely green residue is obtained. spoiled by adding too much of the cobaltous nitrate solution. Emich (3) was the first to attempt an automatic adjustThe next step was obviously to try the precipitation of the ment of the zinc-cobalt ratio. He allowed a droplet of the zinc cobalticyanide in filter paper followed by ashing of the unknown solution and a 1 per cent cobaltous nitrate solution paper. This method, using the automatic adjustment of the to diffuse toward each other in a narrow strip of filter paper 1 Cy Cn. which was then dried and ignited. It was expected that the
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July 15, 1932
zinc-cobalt ratio and a carrier for the test which prevents oxidation during the ignition, appears to be satisfactory in every respect. It combines reliability with seneitivity and outstanding simplicity.
PROCEDURE According t o the technic involved, the test described may be classified as a spot test (6). The necessary reagent paper is prepared by soaking “ashfree” filter paper in a solution of 4 grams of potassium cobalticyanide and 1 gram of potassium chlorate in 100 cc. of water and drying a t room temperature or a t 100’ Ce2 When slightly heated, this paper bursts into flame and leaves a perfectly black ash consisting mainly of cobaltous oxide. For carrying out the test, two ways may be suggested, depending upon the quantity of zinc available.
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contain only a small amount of salts, the potassium cobalticyanide originally present being rinsed out by the liquid coming from zone a. In zone c the salt concentration becomes increased. When the paper is dried and ashed as already described, the Rinnmann’s green will appear in the center zone, a, and zone b will form the ring zone containing little ash. The final appearance of the test depends, of course, not only upon the absolute amount of zinc present, but also on the concentration (and volume) of the zinc solution, etc. It is obvious that the study of ash pictures obtained in similar experiments might offer a means for the investigation of diffusion and adsorption phenomena. Of course, a very small amount of zinc cannot form a disk of green ash. In this case only a small thread or a delicate network of green fibers will be found, but the green color can be distinctly observed under the microscope using a low-power objective and reflected light. Daylight illumination is, of course, most convenient for the inspection of colored objects. SENSITIVITY OF TEST The zinc solution used for the test should be acid. The sensitivity of the test was determined using solutions containing about 5 per cent free nitric acid. The followinglimits (4) were found: Macrotechnic: Limit concentration, 1 mg. zinc/cc. 0.05 mg. zinc Limit of identification, about Microtechnic: Limit concentration, 0 . 4 mg zinc/cc. 0.0006 mg. zinc Limit of identification, about (Droplets of 1 t o 4 cu. m. volume used)
Capillary Pipet
Reaqen t PLper
. ,p,y b. . T ’ I, ,I
C c b
a
b
1
c
FIGURE 1
MACROTECHNIC. B means of a glass rod transfer one drop of the test solution to tEe center of a piece of cobalticyanidepaper about 1 inch (2 cm.) square, which is held horizontal by means of a forceps. When the drop is completely absorbed, dry by holding the piece of paper high above a small Bunsen flame. A yellow line will a pear first along the outline of the drop. Continue drying untifthe center turns brown. Then light the paper. Place the ash on a clean porcelain dish or plate for inspection. In case zinc is present, a disk of green ash will be visible on the spot where the drop of solution had been added. The green disk is always surrounded by a circular zone containing very little ash. Sometimes it is connected with the surrounding sheet of black ash only by a delicate network of black fibers. This phenomenonis more pronounced when using the second procedure. MICROTECHNIC. The size of the cobalticyanide paper may be decreased to less than one fourth of the aforementioned. First take the test solution up into a micropipet, a glass capillary of 0.5- to I-mm. bore with a finely drawn out point of about 0.1mm. bore at the opening. Touching the center of the reagent paper with the point of the capillary pipet, the solution is sucked into the paper and is spread out in all directions. The arrows in Figure 1 indicate the direction of the flow of the liquid. It is self evident that the zone around the point where the solution enters the paper, a, will finally contain the zinc cobalticyanide precipitate and probably (and favorably) also some zinc nitrate in excess, because toward the end the potassium cobalticyanide originally present will already be used up. Zone b finally will
* Addition of potassium chlorate is not essential. The “cobalticyanide paper” may also he ordered from Microchemical Service, 30 Van Zandt Ave., Douplaston, N. Y.
But it must be pointed out that the Rinnmann’s green test is to be used only after separation of the zinc from most of the other elements. Tin and antimony can also give a green ash. A drop of 1 per cent titanium solution causes an intense blue color of the ash. More dilute (0.2 per cent) titanium solutions give a gray ash. The presence of larger amounts of other metals, such as manganese, iron, cobalt, nickel, or cadmium, interfere with the test, because they also give precipitates with potassium cobalticyanide and dark residues on ashing. The following limit proportions (4) may be mentioned: Zn : Cd = 1 : 5 ; Zn : Mn = 10 : 1; Zn : Co = 2 : 1; Zn : Ni = 1 : 1; Zn : Ti = 1 : 1; Zn : A1 = 1 : 2 (aluminum gives a light blue ash). It will be seen that i t is especially necessary to remove the manganese completely. Dark-colored zinc sulfide obtained in the course of a separation is best redissolved after washing. The solution is treated with sodium hydroxide and sodium peroxide in order to remove nickel, cobalt, manganese, and iron. From the filtrate the zinc is again precipitated as sulfide, which is finally dissolved in a small volume of 2 N nitric acid, and this solution used for the Rinnmann’s green test. It is not necessary to adjust the acidity of the solution very carefully, as the test works also with strongly acid solutions (30 per cent nitric acid). LITERATURE CITED (1) (2)
Benedetti-Pichler, A. A , 2. anal. Chem., 70, 257 (1927). Boettger, “Qualitative Analyse und ihre wissenschaftliche
Begruendung,” W. Engelmann, Leipzig, 1925. (3) Emich, F., “Mikrochemisches Praktikum,” p. 105, T. Bergmann, Munich. 1924. (4) Emich, F.; and Schneider, F., “Microchemical Laboratory Manual,” p. 1, Wiley, 1927. (5) Feigl, F., “Qualitative Analyse mit Hilfe von TBpfelreaktionen,” Akademische Verlagsgesellschaft, Leipzig, 1931. (6) Hedvall. J. A., Be?.. 45, 2095 (1912): 2. anmu. Chem.. 86. 201 .
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(1914); 96, 71 ( i g i s j . (7) Noyes, A. A., and Bray, W. C . , “A System of QualitativeAnalysis for the Rare Elements,” Maomillan, 1927. (8) Treadwell, F. P., and Hall, W. T., “Qualitative Analysis,” Wiley.
1930. RECEIVED December 7. 1031.
