ANALYTICAL CHEMISTRY
1122
The precipitate filtered off before titration was dissolved in dilute hydrochloric acid, and the solution was treated as described in the foregoing paragraph, this time discarding the precipitate. The milliliters of sodium hydroxide used in both titrations were added for the calculation of the boron cont’ent of the sample. ACKNOWLEDGMENT
This work was part of a metallurgical investigation spoiiroretl by the U. S. Office of Kava1 Research. Permission to publish the results is gratefully acknowledged. LITERATURE CITED
(1) Blumenthal, Herman, AXIL. CHEM.,23, 992 (1951); Poicdci, Met. Bull. 6, 80 (1951); J . Metals, 4, 140 (1952).
Brunishola,G.,andBomirt, J., Helv. Chim. Acta, 34,2074 (19.51). Chapin, TV. H., J . Am. Chem. Soc., 30, 1691 (1908). Foote, F. J., I d . Eng. Chem.. 4, 39 (1932). Hazel, W. H., and Ogilvie, G. H., A N a L . CHEM.,22, 697 (1950). (6) Hollander, >I., and Rieman, W., ISD. ENG.CHEM.,ANAL.ED., (2) (3) (4) (5)
18,788 (1946).
(7) Lindgren, J. &I., J . Ani. Clicvi. SOC.,37, 1137 (1915). (8) Martin, J. R., and Hayes, J. R., A s a L . CHEM.,24, 182 (19.52). (9) Sabinina, L. Y., and Styunkel, T. V., Zavodslcaye Lab., 13, 752. (1947). 110) Tanabe, H., and Hidakii, IC., AWL.R e p . Takeda Res. Lnb., 9, 68 (1950). (11) Tschischewski, K.,I x d . E X Q Chem., . 18, 607 (1926).
RECEIVED for review Novenibcr 8, 1952. Accepted March 9, 1953. P I C scnted before the Division of Analstical Cheniistry a t the 122nd Meeting or tlie AMERICAS CHEXICAL SOCIETY, .4t!antic City, N. J.
Modification of the Test for Zinc by Its Coprecipitation with Cobalt( II)-Mercury( II) Thiocyanate F. E. BROWN . ~ N DJAMES S. PROCTOR Department of Chemistry, Zowa State College, Amps. Zotcn is no entirely satisfactory qualitative test for ZIIIC Zinc sulfide and many other insoluble compounds of zinc arc‘ white and are easily mashe 1 or simulated by any of the many other white precipitates and especially by a white precipitate of free sulfur formed by the oxidation of hydrogen sulfide. Zinc may be retained by precipitates formed earlier in thc analysis. Soyes, Bray, and Spear (6) reported that when iron( 111) and manganese( IV) are precipitated by sodium hvdroxide and sodium peroxide, as much as 7 mg. of zinc might be retained by 500 mg. of iron(II1) hydroxide, and 21 mg. of zinc, by 500 mg. of manganese(1V) oxide. If zinc could be identified by a colored precipitate before its separation from iron and manganese, two important sources of error would be avoidable. Korenman ( 3 ) reported that nhen cobalt(I1) is mixed with ammonium-mercury(I1) thiocyanate, a blue precipitate forms; the time required for the formation of the precipitate varies from immediately to several minutes, as the concentration of the cobalt(I1) is decreased; and the presence of small concentrations of zinc ions decreases the time required to form thiq precipitate and increases the sensitiveness of the reaction 25-fold. Krumholz and Sanchez ( 4 ) suggested that the difference in time required for the appearance of this blue precipitate in the absence of zinc, and the time required when zinc is present be used as a test for the presence of zinc. I n describing this test for zinc, Feigl ( 2 ) stated that the limit of identification in a neutral solution is 0.2 microgram of zinc at a concentration of 1 to 250,000; that iron(II1) forms a red iron(II1) thiocyanate R hich obscures the blue precipitate, that other ions of the ion-zinc group, except nickel which is not mentioned, decrease the sensitivity but not unduly; and “According to the amount of zinc present, either a t once, or at longest after two minutes a blue precipitate is formed. In the absence of zinc the precipitation begins after two or three minutes.” Later Feigl ( 1 ) described a yellow-green precipitate formed immediately upon the precipitation of zinc-mercury(I1) thiocyanate in the presence oi an excess of nickel(I1). Since its composition is 4Zn[Hg(CNS)a].Si[Hg(CNS)r] it is assumed to be a compound ( 7 ) . The announcement of this precipitate and the statement that cobalt(I1) salts act like nickel(I1) salts are ascribed to Korenman ($), but neither statement is found in the article cited. Feigl’s directions ( 2 ) provide for the removal of the red color due to the formation of iron(II1) thiocyanate by a later addition of an alkali fluoride. These reports raised three questions. How does the presence of nickel(1J) affect the precipitation of cobalt(I1) by ammoniummercury(I1) thiocyanate when zinc is absent and when zinc I S 7~~~~
present? How can the formation of the blue cobalt(II)-mercurj.(11)thiocyanate in the absence of zinc be delayed for B period so long, that the prompt appearancr of the blue precipitate proves the presence of zinc? How can the formation of the interfering red iron(II1) thiocyanate be avoided? EXPERI.UENTAL
Preventing Interference by Red Iron(II1) Thiocyanate. Thr iron-zinc group is often precipitated by hydrogen sulfide in an alkaline solution. The precipitate is treated for a short time with 1 to 2 M hydrochloric acid to dissolve all metal ioiis except nickel(I1) and cobalt(I1). Preliminary experiments in this laboratory showed that such a solut,ion made within a short time after the precipitation by hydrogen sulfide, contained so little iron(II1) that no red color due to iron(II1) thiocyan:tte formed. .Iged precipitat,es produced some red color due to iron(111). The addition of a moderat,e amount of phosphovic acid prevented the formation of iron(II1) thiocyanate in the prrac‘iice of 0.02 JI iron(II1).
52o
3
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I R A T 1 0 ,KO3 : C N U Figure 1. Precipitation of Cobalt-MercuryThiocyanate from a Solution 0.13 M in Hydrochloric Acid Final concentration of NiII: 1. 0.00051 M , 2. 0.0013 M , . and 3. 0.0026 M
1123
V O L U M E 25, NO. 7, J U L Y 1 9 5 3 COMPOSITION OF A U X I L I A R Y SOLUTlONS C O N C E N T R A T I O N M PER L I T E R Con NlU HCI H3P04
60-
0 I
0
-
00025
0.5
300025 0 0 0 2
0 5
0 7
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v)
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04
06
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40
60
100
200
400
F I N A L C O N C E N T R A T I O N OF ZlNC(P1, M I L L I M O L E S / L I T E R
Figure 2. Precipitation of Cobalt-Mercury Thioq unate in the Presence of Zinc
Formation of Blue Cobalt Precipitate in Presence of Nickel When Zinc Is Absent. Other experiments showed that the presence of nickel(I1) in dilute solutions of cobalt(I1) containing no zinc, delayed, even for hours, the formation of the blue cobalt. precipitate; but if zinc was added to other saniples of these same solutions, t'he blue cobalt precipitate appeared in a short time. If these effects of nickel(I1) coultl be regulated, the presence of nickel(I1) could be used to control the time required for the appearance of t'he blue precipitate iu the absence of zinc. The first systematic series of experiments xas conducted to determine the time required to produce the blue precipitat,e in solutions containing both cobalt(I1) and nickel(I1) but no zinc. Tn-o solutions mere prepared-the precipitating solution, a 0.277 .I/ solut'ion of ammonium-mercury(I1) thiocyanate, made by tlissolving 90 grams of ammonium thiocyanate and 90 grams of Inercury thiocyanate in enough water to make 1 liter; and the ausiliary solution whirh contained varying concentrations of cobalt(II), nickel(II), and hytlrochloric acid, or hydrochloric : ~ n dphosphoric acide. Some significant data are shon-n by the curves in Figure 1. These data were secured by mixing in a depression of a white spot plate. 1 drop of an auxiliary solution and 1 drop of water (a substitute for the unknown solution). Then 1 drop of the precipitating solution was added, and niixt,ure was effected b)- a rotnry motion. The concentrations in the auxiliaq- solution iyere adjusted so that the final concentrations on the spot plate for these experiments were hydrochloric acid, 0.13 -11; ni(akel(II),, 0.00051 X,or 0.0013 31, or 0.0026 -11; and cobalt(I1) varying froin 4- to 100-fold the concentration of the iiickel(I1). For each experiment, the time elapsing between the addition of the grecipiiating solution and the formation of a blue precipitate was plotted against the ratio of the concentration of cobalt(I1) to t,lie concentration of nickel(T1). Logarithmic coordinates were upeci. These curves show that as this ratio decreases, the time required to forin the blue precipitate increases, slowly at first, but mpitll>~ niter the ratio becomes less than 10 to 1. The maximum concentration of cobalt(II\ Jvhich must be kept, from falsely indicating the presence of zinc by producing a quickly formed blue precipitate is the largest, concentrat'ion which does not proclaim its presence by producing a pink solution. One milliliter of a 0.005 Ad solution of cobalt(I1) is distinctly pink. The presence of nickel in a concentration 0.10 to 0.25 :is great (about 0.001 M ) should prevent the rapid formation of the blue cobalt, precipitate in such a solution. Khen 1 drop of t'he precipitating solution was added to a misture of 1 drop of water arid 1 drop of auxiliary solution containing cobalt (soln. 1, Figure 2), which contained no nickel(II), the blue precipitate appeared in 1.5 minutes. In sis experiments, 1 drop of a solution containing cobalt (soln. 4, Figure 2 ) > which was 0.001 M in nickel(II), was substituted for 1 drop of solutio11 1.
I n not one of these did a precipitate appear within 10 minutes. It is evident that small concentrations of nickel(I1) will delay the appearance of the blue precipitate in the absence of zinc. Formation of Blue Cobalt Precipitate in Presence of Nickel When Zinc Is Present. I t must also be shown that in the presence of zinc, the appearance of the precipitate is not delayed unduly. To determine the extent of the delaying effect of nickel(I1) in the presence of zinc, each of the four auxiliary solutions described in Figure 2 was used lyith solutions of zinc such that the final concentrations of zinc on the spot plate varied from 0.0004 M to 0.04 .TI. Phosphoric acid was omitted from two solutions i o determine its effect on the rates of formation of the precipitate. Figure 2 reports data, by showing a plot of time in seconds required for the formation of the blue precipitate against the final concentration of zinc in millimoles per liter. Logarithmic coordinates are used. The auxiliary solution in which the concentrations n-ere nickel(II), 0.001 M, hydrochloric acid, 0.5 -11, 2nd phosphoric acid, 0.7 JI, was more sensitive to zinc than the other solutions which contained nickel(I1). Sensitivity. The pipets used in these esperimmts deliverctl drops d i o s e volumes were 0.04 ml. Then 3 drops, on the spot plate, 0.0004 ;If in zinc would contain 3 micrograms of zinc in 120,000 micrograms of wat.er. A precipitate would form within 2 minutes when the concentration of zinc is considerably smaller than the sinallest, concentration recorded on these curves. Then the sensitivity is better thnn 3 micrograms a t a concentration of 1 part in 40,000. Obtaining Sample Solution from Iron-Zinc Group. The test for zinc should be made in the solution containing a11 members of the iron-zinc group, escept the major parts of cobalt and nickel, in the scheme for analysis JThich includes thr separation of all other nirmbers of thtl iron-zinc group from the cobalt(I1) and nickel(I1) sulfides by dissolviag them in 1 to 2 31 hydrochloric acid. The test will fail if the solution contains a high concentration of nickel; the hlue precipitate requires more than two minutea to appear. Since from 5 to 20% of the cobalt(I1) and nickel(I1) always dissolve ( 5 ) ,if they are originally present in large amounts, they may produce distinct'ly colored solutions. I n such cases the solution should he separated from the major part of the nickel(I1) and cobalt(I1). the group should be reprecipitated by hydrogen sulfide. and the other members of the group should be dissolved from the much qmaller nniount' of nickel(I1) anti cobnlt(I1) in the second precaipitiLtr. RECOM \IENDED PROCEDURE
1Lakc. the test for zinc on any solution of the iron-zinc group which is not, distinctly colored by nickel(I1) or cobalt(I1). Mix 1 drop of the solution of the iron-zinc group with 1 drop of an auxiliary solution which is 0.0025 .M in cobalt(II), 0.001 M in nickel(I1). 0.5 Jf in hydrochloric acid, and 0.7 J1 in phosphoric acid in a depression of a x-hite spot plate. Add 1 drop of a 0.277 J I solution of anmioriium-rnrrcury( TI) thioc>-anate and mix \vith 3, rotary motion. A blue precipitate will appear within 2 minutes if as much as 0.2 millimole of zinc is present in a liter of the solution. The appearance of a small amount of dark precipitate which does not spread through the whole volunie of the solution should be disregardett . LITERATURE CITED
(1) Feigl, F., "Chemistry of Specific, Selective and Sensitive Reactions," p. 160, Sew 1-ork, Academic Press Inc., 1949. ( 2 ) Feigl, F., "Qualitative Analysis by Spot Tests," p. 111, 2nd English ed. translated from the 3rd German ed. by Janet W. Nathews, Ken. Tork, Nordemann Publishiug r o . , 1939. ( 3 ) Iiorennian, I. XI., Z. trnnl. Chenz., 95, 44 (1933). (4)Kruniholz, P., and T n z q u e z Sanchez, J., M i k r o c k e m i e , 15, 114 (1934). ( 5 ) Soyeu, A. d., Bray, I T , C., and Spear, E. E., .J. d ? n . C h e m . Soc., 30, 518 (1908). ( G ) Ibid.,p. 544. ( 7 ) Weidenfield, I,.. Dissertation, Vienna, 1930; test not available. Seep. l(i1 of Feigl ( 1 ) . RECEIVED for r e v i r w
.hirrl-t
i , 1Rj.).
Accepted February Z i , IR,i8.
