WILLIAM E. MORRELL University of Illinois, Urbana, Illinois
ZINC is one of the ions missed most frequently by students in qualitative analysis, although many methods for detecting zinc have been proposed.l,2 A desirable qualitative test should he simple, sensitive, and specific. I t should give clear-cut results while permitting wide tolerances in conditions and quantities of materials used. For use in qualitative analysis courses i t should utilize reactions readily understood and reagents commonly available and familiar to elementary students. I t is believed that the test described below meets these requirements. A principal objection to many of the tests which have been proposed for zino is that they are not specific. Similar results are commonly given by ions closely associated with zinc in the usual analytical procedures. The test here described overcomes this difficulty. In fact it is applicable in the presence of almost any combination of the substances traditionally included in qualitative analysis courses. The following ions, in hundreds of combinations with and without Zn++, have been investigated: Ag+, Hg,++, Pb++, Bi+++, Cu++, Cd++, Hg++, As+++, Sb+++, Sn++, Sn++++, Mn++, Ni++, Co++, Fe++, Fe+++, Al+++, Cr+++, Ba++, Sr++, Ca++, Mg++, K+, Na+, NH4+, H+, S; CN-, F~(CN)B=, Fe(CN)c=, SCN-, I-, Br-, C1-, 103-, BrOa-, ClOa-, C10; SO&-, Wa-, Sz03-, CrO4-, C204-, P-, Mn04-, NO8-, NO2-, R0,-, AsOIE, AsO,=, PO,=, C2Ha02-, C03; SiOa; OH-. No interferences other than those discussed below have been found. Under the conditions to be described, Zn++gives a whitish precipitate. Large concentrations of Ph++ and Sn++ give similar precipitates, which are, however, much more soluble and can easily be avoided. In the presence of large quantities of ammonia Cu++ also gives a precipitate, but it is reddish-brown and it, too, can easily he avoided. Since copper, tin, and lead differ so greatly from zinc in other reactions commonly employed in qualitative procedures (reactions with H2Sor chloride, for example), chances of confusing zinc with copper, tin, or lead are practically negligible in the method to be described. There is practically no chance of confusion with any of the ions commonly found in the same traditional sulfide group as zinc. Aluminum ion, for instance, causes no difficultywhatever. Strong 1 ''Tables of Reagents for Inorganio Analysis, Reports of the 'International Committee on New Annlytieal Reactions and Reagents' of the 'Union internetionale de Chimie! " First Report, 1938, p. 176; Third Report, 1948, p. 114. WENGEE, P.,R.DUCKERT, AND D. RIETH, Hdu. chim. Aeta, 25,40&15 (1942).
oxidizing agents may need to he reduced, but they provide their own warning. The procedure is based on the fact that of all the common ions, zinc is the only one which gives a ferrocyanide precipitate readily soluble in sodium hydroxide solution but insoluble in dilute hydrochloric acid. Essentially, the test consists of adding sodium hydroxide solution to an aqueous solution of the unknown until any arnphoteric Zn(0H)z is at least partly dissolved, adding potassium ferrocyanide solution in excess, filtering or centrifuging the mixture if any precipitate is present, and adding a few drops of the filtrate to dilute hydrochloric acid. A precipitate indicates the presence of zinc. The following more detailed directions take care of all interferences which have been observed. PROCEDURE
(a) Make a sample of the unknown somewhat acidic with dilute hydrochloric acid, or if already acidic, add sodium chloride solution. Make conditions such that any zinc present would be in solution. Centrifuge or filter off any undissolved solid or precipitate and discard it. This step reduces the concentration of Pb++, if lead is present, to a point where it can cause no interference. The Ag+ and Hg2++ which will also precipitate here, if present, would cause no later difficulty even if not removed. If in this step a precipitate which could be colloidal sulfur should appear, boil the mixture to coagulate the sulfur somewhat, cool to reduce the solubility of PhCle, and then centrifuge or filter as b e fore. Finely divided sulfur, if a large quantity were allowed to remain, could be confused with a zino precipitate in the final test. (b) T o the filtrate from the preceding step, add NaOH., (3-6M ) until any Zn(OH)2 would be at least partly dissolved. The concentration and amount of sodium hydroxide solution used are not rtt all critical. A very dilute solution or too large an amount would dilute the unknown unnecessarily. Too large an amount of hydroxide increases somewhat the chance of stannous ion-or lead ion if it were still present in high concentration-giving a final precipitate which could be confused with t h a t of zinc. There is considerable leeway, however, in the quantity of NaOH which can safely be used, but in general too large an excess should be avoided. (c) Add K,Fe(Cn)e., in excess. There is even greater latitude in the quantity of ferrocyanide which can be used. Increased excess helps somewhat in diminishing possible interferences, but the advantages
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finally become counterbalanced-if the concentration of zinc is already smaU-by the disadvantage of increased dilution of the substance sought. (d) If any precipitate is now present, filler, or centrifuge the mixture or a fiaction of it. The precipitate will be discarded, so if partial settling has occurred, much of the precipitate can be left behind by decantation into the funnel or centrifuge tube. The resulting filtrate should be clear. If Cr+++or MnOk- be present in the unknown, the filtrate will be green. If CrOa- or Fe(CN)e', it will be bright yellow. Otherwise it should be practically colorless, or light yellow due to the presence of the ferrocyanide ion. (e) Add five drops of the filtrate to five milliliters of 8 M HCL and shake enough to insure mixing. A whitish precipitate shws the prescence of zinc. Use of a medicine dropper is convenient. Although there is again considerable leeway in concentration and amounts, the concentration suggested is advantageous. Repeated experiments have shown the zinc ferrocyanide to give practically as heavy a precipitate in 3 M HC1 as in 0.3 M, but to be appreciably more soluble in 6 M acid. Precipitates of ions such as Sn++ and Pb++ appear much less readily in 3 M HC1 than in more dilute acid. However, if the various ions are present a t small concentrationsi. e., the possible concentration of zinc being of the order of magnitude of 0.001 M to 0.010 M in the original solution-the test can be made somewhat more sensitive by using 1.5 M HC1 instead of 3 M. No advantage is gained by going to still more dilute acid, however, and in most cases where there is any difference the 3 M is preferable. Considerable variation in relative amounts of filtrate and acid is permissible, but again the relative quantities suggested-me drop of filtrate for each milliliter of acid-are advantageous. With these relative amounts, interfering precipitates very seldom occur, while the zinc precipitate appears immediately or within a few seconds after shaking. The precipitate is probably a double salt, MaZna(Fe (CN)&, where M might be H+, Na+, K+, orprobably any of various other cations which might be presenL3 The precipitate is of the nature of a turbidity, which may vary from very heavy to light. By comparison with standards it might even be used to estimate the concentration of zinc in the original unknown, providing enough NaOH is used to dissolve all zinc from the alkaline precipitate. If the original concentration of zinc is about 0.1 M or greater, the final -precipitate is opaque. If the final test solution (e) is bright yellow, the original unknown ~robablvcontains a strone oxidizine agent, and this might prevent the precipitation of zinc. Procedures to handle this situation are discussed in the next section.
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the final test solution (e) indicates the presence of Fe (CN)6'. The ferricyanide may all have been present in the original unknown, in which case no harm is done to the zinc test except that it is given the yellow color. On the other hand, some or all of the Fe(CNh' may have been formed from the added Fe(CN)#-- by a strong and rapid oxidizing agent in the unknown. If t,he oxidant is chromate, bromate, or hypochlorite, oxidation of the ferrocyanide to ferricyanide may be complete, in which case no zinc test can be obtained. If the oxidant is iodate, permanganate, or nitrite, the oxidation is likely ,to be only partial, and the zinc precipitate can still appear in the yellow solution. If the oxidant is chlorate, the test will at first be unaffected but will become yellow on standing. The ferrocyanide is oxidized in the alkaline solution of step (c) by permanganate (giving the green manganate), but in the other cases the oxidation of the Fe(CN)8= is mostly delayed until the mixture is dropped into the acid in the final step. Nitrate, even in large concentrations, has no apparent effect and does not harm the zinc test in the slightest. A yellowish precipitate in these yellow test solutions shows zinc to be present. If no precipitate appears in such a solution, zinc might still be present. The addition of more ferrocyanide to this solution can complete the reduction of the oxidizing agent and zinc can then precipitate with the excess ferrocyanide. However, cleaner tests without discoloration can be obtained by reduction of the oxidant in step (a). The reduction may be a t least partially accomplished by the chloride ion in the acidic solution in step (a). But the resulting chlorine must be mostly removed or else it in turn can provide continued source of oxidation. Reduction can also be accomplished by sulfide or HPSin the acidic solution. Rut unless the unknown is being put through the traditional acidic-solutionH2S-group separation anyway-in which case the reduction will occur incidentally-the easiest and cleanest reduction can be made with sulfite. Addition of solid N&SO3 in excess to the acidic unknown in step (a) will destroy any harmful oxidizing agent (including the chlorine) conveniently and quickly, and will leave no product which need be eliminated. Such addition of sulfite can be included as a routine part of step (a) in cases of unknowns in which strong oxidizing agents are possible constituents. The test can then be carried through from step (b) as described above. INTERFERENCES
In the procedure outlined above, there is little chance of any of the substances commonly used in qualitative analysis being confused with zinc. A light turbidity in the final test might be the result of a large concentration of Sn++ rather than a small concentration of Zn++. A EZTECT OF OXIDIZING AGENTS precipitate or even a turbidity due to Sn++ practically A brieht vellow color with a sliehtlv greenish tinge in never appears, however, unless a relatively larger volume of the filtrate than suggested in (e) is added to a SNTO, S., Bull. Znsl. Phys. Chem. Research (Tokyo),8,921-51 (1929); 10, 31627, 703-7 (1931). (Abstracts, 2 , 107-9 (1929); the HC1. Furthermore, turbidities caused by Zn++ and Sn++ can easily be distinguished. With the tur4 , 3 6 7 , 69-70 (1931).)
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bidity in 5 ml. of 3 M HCl, the addition of 1 ml. of 6 M HC1 will clear it up if it is caused by Sn++ but will have practically no apparent effectif zinc is present. Lead in high concentrations acts much like Sn++ in the final test, and can be distinguished from zinc in the same manner. Enough Pb++ is removed as a chloride precipitate in (a), however, so that no turbidities due to lead appear in the final test. If both zinc and copper are in the unknown, the final zinc precipitate may he discolored bluish or tannish. Copper present without zinc may result in a tannish or brownish tinge in the final solution. Copper in the presence of large concentrations of ammonia will finally give a discolored solution or a reddish-brown precipitate. This can be prevented by removing the Cu++ with HzS or by volatilizing some of the ammonia. If the deep blue color of the copper-ammonia complex ion is noticed in the original solution or in step (b) in the above procedure, the alkaline sample can be heated in an evaporating dish, boiling off some of the ammonia. A p r e cipitate will form and will probably contain the zinc, if present in the unknown, so after the dish has cooled, hydrochloric acid should be added and the procedure followed through from step ( a ) . Cobalt in the presence of a large concentration of ammonia can result in a purplish discoloration or even a slight turbidity in the final test solution. In this case warning of trouble is given by a purple or brown filtrate in step (d) and the odor of ammonia in the alkaline solutions. Again, any interference can be prevented by volatilizing off some of the ammonia, proceeding as in the case of copper. It is not necessary to remove all of the ammonia to avoid a copper or cobalt precipitate or turbidity. In fact, the ammonia from the amount of NH4+ often included as a constituent in qualitative unknowns is seldom enough to result in a copper or cobalt turbidity. In any case, warning of possible trouble would be given by a rather highly colored filtrate in step (d). In general, the filtrate resulting from (d) should be clear and practically colorless, or a light yellow due to the excess ferrocyanide ion which has been added. For the ions investigated, a highly colored filtrate is a warning of probable trouble in all cases except two: if the unknown contains Fe (CN)6=,the liltrate will be yellow; if Cr+++, green; neither of these causes difficulty. However, yellow or intense green will also be obtained if the unknown contains Crop- or MnO4-, respectively, and if no provision has been made for their reduction. In the resulting final test solution, a zinc precipitate can probably still form in the case of Mn04-, but will likely be missed in the case of CrOl-. Therefore a highly colored filtrate (particularly if bright yellow) which does not give a precipitate in the final test is suspect. But if a highly colored filtrate does give a precipitate, it is also suspect. If a rather highly colored but sufficiently alkaline Htrate does give a precipitate, there are four most likely possibilities: (1) The unknown may contain con-
JOURNAL OF CHEMICAL EDUCATION
siderable ammonia and either copper or cobalt. (2) The filtrate may contain particles of solid due to ineffective filtering or centrifuging. (3) Insufficient ferrocyanide may have been added, resulting in incomplete precipitation of some ion which should have been removed. This possibility can be investigated by adding a small amount of filtrate to 6 M HNOa. If excess &Fe(CN)6 has correctly been added, the slightly greenish yellow of dilute ferricyanide will soon appear. (4) If none of the preceding accounts for the precipitate, the filtrate is probably green or yellow and the unknown probably contains zinc together with MnO4-, Cr+++, or Fe(CN)ea. The final test solution (e) in turn should also be practically colorless. The zinc precipitate, if present, should be almost white with just a tinge of slightly yellowish gray. Tinges of other colors, particularly bluish, may appear, due to the formation of traces of compounds such as Prnssiau Blue. A flesh-to-brownish discoloration usually indicates copper which has not been completely removed. A bright yellow color in the final test usually means that a strong oxidizing agent is present, and this has been discussed above. An unknown which gives oolloidal sulfur when made acidic could be mistakenly thought to contain zinc. Such could be unknowns containing thio~ulfateion or sulfide ion. (The latter will commonly contain free sulfur due to oxidation by the air.) The confusion which might be caused by colloidal sulfur is forestalled by step (a) in the procedure. It is not necessary that the removal of sulfur be complete. Considerable turbidity due to sulfur can be tolerated in filtrate (d) without being noticeable in the final test solution ( e ) . In the presence of cyanide, precipitates are given by several ions in addition to zinc, particularly by silver, copper, cobalt, nickel, and to a lesser extent, cadmium. This probably makes little difference, however, since cyanide ion is seldom included in unknowns in qualitative analysis. If cyanide ion is present, the safest procedure is to remove it before proceeding with the test. ALTERNATEPROCEDURFS
It will be noted that the principal interferences (Pb++, Sn++, Cu++, c0l10idd sulfur, strong oxidizing agents) discussed above would be avoided by a preliminary precipitation by H2S in acidic solution. Applied to the alkaline sulfide group, this test for zinc is practically foolproof. Specific instructions concerning concentrations and amounts have mostly been avoided in this paper because of the wide variations which still give excellent results. Examples of satisfactory quantities are: approximately equal volumes of unknown solution, 6 M NaOH, and 0.5 M K4Fe(CN)~relatively less NaOH if no precipitate appears when i t is added; finally, one drop of the resulting filtrate added to each milliliter of 3 M HCI. Either macro or micro methods can he used.
MAY, 1950 ACKNOWLEDGMENT
of this test which was apA preliminary plicable in the presence of a limited number of ions was
m developed by Melvin L. Ott when he was a student in the author's freshman chemistry course at Wright Junior College in 1939.
