A sensitive and selective test for the detection of mercury has been developed based on the strong catalytic action of mercuric ions on the decomposition of hexacyanoferrate (II) in the presence of nitrosobenzene. The sensitivity for the pure system expressed as limit of identification (in 0.05 ml.) is 0.002 microgram of mercury at a limiting concentration o f 1 to 25,000.000. Of the cations studied only silver and gold give an analogous reaction. By following an appropriate procedure, mercury can be detected even when it is present in mixtures with cations giving coloured precipitates with the hexacyanoferrnte (II)

I. Kraljić

M. Mate

English

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CR 0 AT I CA CHEM I CA ACT A 28 (1956) 273 <CCA-38 546.49 :544.192 A New Very Sensitive and Selective Spot Test for Mercury I. Kraljic and M. Mate .Institute of Inorganic, Anaiyticai, and Physicai Chemistry, Phacuity of Pharmacy, University of Zagreb, Zagreb, Croatia, Yugosiavia Received November 6, 1956 A sensitive and selective test for the detection of mercury has been deve1oped based on the strong catalytic action of mercuric ions on the decomposition of hexacyanoferrate (II) in the presence of nitrosobenzene. The sensiti'Vity for the pure ,system expressed as limit of identification (in 0.05 ml.) is 0.002 microgram of mercury at a limiting ooncentration of 1 to 25,0 C<O.OOO. Of the cations studied only silver and gold give an analogous reaction. By following an appropriate procedure, mercury can be detected even when it is present in mixtures with cations giving coloured precipitates with the hexacyanoferrnte (II). By cataly tic action of merouric ,ions on the hexacyanoferrate (II) in pre-.s ence of nitroso-benzene (NOPh), a violet water-soluble i-on [Fe(CN) ,,NOPhr3 :.is formed 1. [Fe(CN)u] 4- + Hg2+ + H2o = [Fe(CN)5H20J3- + Hg (CNr [Fe(CN)oHpp- + NOPh = [Fe{CN)oNOPh]3- + Hp Hg{CN)+ + H+ = Hg2+ + HCN (1) (2) (3) 'The maximum catalytic actfon of mernury .occurn at pH 4.F The reaction rate is cQlmpa·ratively low a:t room temiperature; by raisi ng the temperature to 60°C the ·react.ion ~s accelerated and the test becomes move s.en1sitive. By using .appropriate concentrations of the K 4 [Fe(CN).,] and NOPh solutions, by addition ·of acetate buffer (pH 4.1) <mid by heatililig, a great sensitivity of the test is obtained. EXPERIMENTAL 1. Reagents HgCL2 ·A 0.01 molar solution was used as stock soluUon. Diluted solutions were -obtained by drlutit1g the ·stock soluUon with distilled water. K 4[Fe(CN) 6] • 3 H20 · 0.20/o solut1on for the pure system and in the presence of small amounts of cations giving a precipitate with hexacyano.ferrate (II) . When _suoh caUons are present in higher concentrations, a 0.5°/o od l.00/o so·lution should be used. ln order to obtain greatest sensitivity, the solution should be made up fresh each day and protected from direct sunUght. C6H5NO. Saturated aqueous solution obtained by dissolving the substance in ·distilled water a t 7ooc, prepared fresh every week. Nitrosobenzene w as prepared by 'oxidation of .aniline with Caro's acid3. Acetate buffet. 1-molar solution, pH 4. 1. 274 I. KRALJIC AND M. MA TE Solution A. Mixture of acetate buffer and saturated solution of NOPh, 1 to 9~ prepared fresh daily. When the soluti~n to be tested is strongly acidic or alkaline, the concentration .of the buffer must of course be increased by raising the above-ratio to 1 : 4 or by adding a drop of pure acetate buffer. All the reagents, except the nitrosobenzene, were c. p. grade. 2. Working method All the experiments were performed on a porcelain· spot pJ.ate or in micro-test tubes by usJ.ng 1 drorp (0.05 ml.) of diluted solution of HgO~, then adding 1 drorp {0.05 ml.) of solution A, and next 1 drop (0 .02 ml.) of the solution of hexacyainoferrate (II). When the spot rplate is used, the warming about to 60°C cain. be carried 01Ut by moving the plate unifoomly aibo·ve a small and broad fl:ame of a Bunsen burner. In the micro-test tubes the appropriate temperature can be oibtained. by dipping the test tubes into a water bath (in a beaker) heated to 60°C. The procedure with the srpot plate taikes mOTe time than with the test. tubes, and as the nitrosobenzene is volatile, it is inecessary to add every 2 :minutes a new drorp of the saturnted solution of nitrosobenzene. When the· concen~ation of mercury in the solution to be tested is very low (below 10-6 M/1 · HgCl2) , a blank must be run simultaneously with the test. The blank serves also as .a chec<k on the temperature, since it gives a positive reaction when. the temperature is too high (above 60°C). In order to exclude the infliuence of sun!light, which has an effect similar-to that of mercury, all the experiments have been conducted in the shade or in the bght of an incandescent lamrp. The sensitiivity of the eye for faint pink or· violet colour is greater in suitable artifi.ciail li<ght than in daylight, which is important for the detection of mercury in concentrations nea.T the concentra,.... tion limJt of the test. 3. Sensitivity The sensitivity of the method has heen first ascertained for the pure system, followiing both ;procedures, i. e. with the porcelain spot plate and with. mii~ro-test 1iubes. In both cases the same sensJtivity was measrured, which was, expressed as limit of identification, 0.002 microgram of m ercury and, as con-centration limit, 1 : 25,000.000. TABLE I Relation of reaction-time to concentration; micro-test-tubes procedure Cone. HgC12 \ at 2ooc . \ At 600C 1 X 10-4 M/l. 1 X 10-s M/l. 3 X 10-6 M/l. 1 X 10-6 M/l. 5 X 10-7 M/l. 3 X 10-1 M/l. 2 X 10-1 M/l. 30-40 sec. 4 min. S-10 sec. 15-20 " 30 " 50--60 " <2 min. 2 " 2,5 " A NEW SPOT TEST FOR MERCURY 275 The procediuTe using micro-test tubes is quicker, du'e to the better heat transfer through the thin .glass walls: it taikes not more than 2.5 to 3 minutes, .as compared with !UIP to 10 minutes for the procedure u:sing the spot plate. By measuring the .time elaipsed from the moment when heat is applied to the first (pOOitive reaction, the concentration of the solution tested can be approxi-m.art:ely estimated, as appears also from Table I , embracing a range of con--centrations from 2 X 10-7 to 1 X 10-4 M/1 of mercury. 4. Interferences of cations By the procedl.llre with the porcelain •spot plate interference studies were ·made including cations of all six analytical .grouips, as well as some other -cations. The solutions of the cations were :prepared from c. p. grade salts : nfutrat>es, sulphates, acetates and chlorides. The results of these studies are .summarized in table II. TABLE II Interferences of cations Concentration of Hg2+: 0,02 µg. /0,05 ml. (2 X io-s M/l.) Me : Hg Cation (by Remark Ag Pb Bi eu2+ Cd Sn Sb .AJ:. (As02 -) Fes+ Al Cr Co Ni Mn Zn Ca, Sr, Ba Mg K, Na, NH4 U022+ Aus+ wei~t)a> 200 200 200 200 20 25 50 200 200 200 50 50 100 200 2.000 200 2 X 10-s molar soluti-on gives the same effect as mercury Cu2[Fe(CN)6] precipitate must be destroyed after warming( with NH40H) Prussian blue must be destroyed after warming (with NH40H) 0.1 molar solution is without affect These cations show no interferences (U02)2[Fe(CN)6] precipifate must be destroyed after warming (with NH40H) 1 X io-s molar solution gives the same effect as mercury a) Condition at which the test is slightly influenced by the presence of these cations. Of the catioins sturliied only silver aoo gold react similatly as mercury and hence interfere with the detection of mercuxy when present in concentrations greater than 2 X 10-e M/1 . Ag and 1 X 10-s M/1 ·Au. 276 I. KRALJIC AND M. MATE Fe3+, Cu2+ and U0~2+ give coioured 1preciipitates, hut their intederence is readily eliminated by maJkiing the solution alikaline with diluted NH40H, after it has been warmed. In the pOC'esence of greater amounts of trivaJent ilron it is reccrrnunenided to add a:lso a drorp of a 100/o solution of rpota:ssium~sodium-tarta­rate, and in the presence of greater amounts of corprper, a diluted solution of potaStSium cyanide. 5. Interference of anions Taible III 'Summarilzes the r esults of the study of interfer.ence irncLuding sorne anions (a:s potassium or sodium saJts) with the test for mercury. TABLE III Interferences of anions Concentration of Hg2+: 0,02 µg. /0.05 ml. ' {2 X 10-& M/ l.) Anion S04- - , N03-, C2Ha02-c1-Br-J -CN-CNS-soa- -HP04--C204- -Concentration of a nLona) 0.01 M/l. 0.01 M/l. 1 X 10-4 M / l. 0.01 M/1. 0.01 M/l. 0.01 M/l. 0.01 M/1. 0.01 M/l. Remark Without effect 2 X 10-4 M solution inhibits strongly the catalytic action a) Condition at which the test is slightly influenced by the presence of these anions. The majority of the anioillS studied does not affect si1gnifircantly the sensi-tivity orf the test. Iodides, when present in a higher amounts, im!Pair con-sideraibly the sensitivity of the test, and may even completely inhibit the catalytic action of mercury. Cyanides, which inhibit many reactions in which mercury takes part, affect adversely a:lso the action of mercury in this reaction, but only in the cold4• On hearting, the cyanides are volatilized as HCN, since the test is conducted in a feebly acidic medium; the sensitivity oif the test is therefore only insignificantly affected even by larger concentrations of cyanides. The detection of mercury by this method is possible also in the presence of suJrphides (H2S), if the amounts present are not excessive. This is due to the· fact that n.i.tr:osobenzene oxidilzes s2- to elemental suiphur, thus demasiking the mercury. 6. Procedure To 1 dro;p (0.05 ml.) of the solution to be tested,, ;placed in the hollow of a sipot plate or in a micro-test tUJbe, 1 drop (0 .05 mJ.) of solution A is added, followed by 1 drorp (0.02 ml.) of a 0.50/o sol1Ution of K 4 [Fe{CN)6] • 3 H 20. When the concentration of Hg is known to be very small, a pM"allel blank must be run. The ~eaiction is accelerated by heating to 60°C. When working with micro-test t1Uibes the heating taikes not more than 2.5 to 3 min1Utes, with the A NEW SPOT TEST FOR MERCURY 277 porcelain spot pJate it may taike up to 10 minutes, depending on the thickness of the porcelain wahl. When the spot plate is used, a new drop of nitrosobenzene must be added every 2 minutes as .long as the heating is going on. In the absence of silrver (2 X 10-6M/1. or mme) an:d .gold (1X10-5M/l. orr more) in the solution, a violet or 1p1Illk: coloration indicates the 1p:resence of mercury. If on addition of the hexacyanoferrrate (II) solution a coloured precipitate is formed (owing to the presence of Fe3+, Cu2+,UOz2+), the precipitate must be destroyed after heating by addition of 1 drop of diluted NH,OH; in the presence of meroury, the violet coloration of the sollllti.on then becomes apparent. If copper is [present in higher concentratiooo, it gives a bliue coloration with the NH40H; this is readily discharged by adding 1 drrop of a diluted soJiution of KCN. In the presence of greater amounts of Fe3+, the addition 0f 1 drop of a 100/o solution. of K-Na-tartarate wiH .prove helpfuJ. REFERENCES 1. S. Asper g er, I. Murat i and 0. Cup ah in, J. Chem. Soc. 1953, 1041. 2. S. Asper g er and D. Pav 1 o v i c, J. Chem. Soc. 1955, 1449. 3. L. Ga t term an n, Die Praxis des organischen Chemikers, 30 Aufl., Berlin 1943_ p. 178. 4. I. K r a 1 j i (: and M. Mate, Croat. Chem. Acta 28 (1956) 249. IZVOD Novo, vrlo osjetljivo i selektivno dokazivanje :Zive u kapi I. Kraljic i M. Mate Na temelju jakog katalitickog djelovanja Zi.vinih iona na raspad heksacijano-ferata (II) u prisutnosti nitrozobenzena razradena je osjetljiva i selektivna metoda za detekciju zive reatkcijom u kapi'. Osjetljivost metode za cisti sistem izrazena kao granica identifilkacije je 0.002 µg Hg (u 0.05 ml), a kao grani!ena koncentracija (con-centration 'limit) 1 : 25,000.000. Od i!spitanih kationa analognu reakciju daju jos srebro i zlato. Uz odgova.raju(:e uvjete metoda omogueuje dokaz zive i u prisutnosti kationa, koji sa heksacijanoferatom (II) daju bojadisane taloge. ZAVOD ZA ANORGANSKU, ANALITICKU I FIZIKALNU KEMIJU FARMACEUTSKOG FAKULTETA SVEUCILISTA U ZAGREBU Primljeno 6. studenoga 1956. 

A New Very Sensitive and Selective Spot Test for Mercury

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