number of determinations in the same milligram range were made without diverse ions present. Results showed an average relative error of 0.5 Z. The given pH range must be maintained in order to keep stable the mercury TMPI complex and to keep effective the masking of As(V), Cu(II), Fe(III), Sn(IV), and Te(1V) with sodium citrate. Selenium, thallium, and the elements not masked by sodium citrate within the solid blocks in Table I interfere. EDTA cannot be used as a masking agent. In strongly acidic solutions it precipitates as the free acid, and in weakly acidic solutions it prevents the precipitation of mercury. Lead. Table 11 shows the results for lead samples containing diverse ions. An average relative error of 0.7z was obtained. The same quantities of lead without diverse ions present were determined, and an average relative error of 0.5z was found. Lead is precipitated best with TMPI from 2-6 vjv hydrochloric acid solutions. Sulfurous acid is added to solutions containing As(V), Fe(III), and Pt(1V) to reduce their valence state, thus preventing coprecipitation with the lead. Samples containing no barium or strontium can be fumed to dryness using a mixture of sulfuric and hydrobromic acids, thus avoiding interference from antimony, mercury, selenium, and tin. The lead sulfate resulting from the fuming is dissolved in 5 ml of hydrochloric acid followed by 40 ml of water. The general procedure is now resumed. A distinct advantage of the proposed method over the classical lead sulfate methods is that small quantities of lead can be determined in the presence of relatively large quantities of barium and strontium. This aspect becomes most attractive, for example, when analyzing for lead in a leadbarium-zinc silicate. A sodium carbonate fusion, leached in hydrochloric acid, followed by a silica dehydration separation can provide the conditions under which an accurate deter-
z
mination of lead is made possible in the presence of barium. The volume of the solution containing lead ions should be kept under 50 ml before the ice cube and the precipitant are added. Platinum. Table I1 shows the results for platinum samples containing diverse ions. An average relative error of 0.5 % was obtained. Twenty additional determinations were made with no diverse ions present. The results again showed a mean relative error of 0.5 %. It is necessary to heat the chloroplatinic acid solutions in the presence of excess iodide to get complete transition from a chloro to an iodo complex. Iridium and ruthenium were found to be even more sluggish in forming complexes with iodide. Applications for the proposed method may be found in platinum-plated materials and in alloys containing gold and silver where silver can be separated as the chloride and gold reduced to the metallic state. The reducing agent should be destroyed and the platinum oxidized back to the quadrivalent state. Copper and iron, along with many other cations, can be easily separated from platinum by passing a weakly acidic chloride solution through a suitable cation-exchanger in the hydrogen form (15, 16). The effluent can be evaporated and the platinum precipitated with TMPI as described in the procedure. Table I shows the elements that are known to coprecipitate with platinum if no separations are made. Selenium and thallium interfere. RECEIVED for review September 9,1966. 2, 1966.
Accepted November
(15) C. K. Butler, Znd. Eng. Chem., 48, 711 (1956). (16) H. G. Coburn, F. E. Beamish, and C. L. Lewis, ANAL.CHEM., 28, 1297 (1956).
Polarographic Determination of Sulfur Dioxide in Mercury(l1)-Sulfur Dioxide Complex Leonard L. Ciacciol and Thano CotsisZ Research Department, T . J . Lipton, Inc., Englewood Cliffs,N . J .
THEUSE OF SODIUM TETRACHLOROMERCURATE(~~)for an SOz absorber for atmospheric samples as proposed by West and Gaeke (1) was extended to use in the detection of SOnin foodstuffs (2, 3, 4). SOn in the sodium tetrachlorate mercurate solutions was determined by the pararosaniline method ( I ) . However, a much larger range of SOn concentrations can be Present address, Central Research, Wallace & Tiernan, Inc., Newark, N. J. 2 Present address, Avon Products, Inc., Suffern, N. Y. (1) P. W. West and G. C. Gaeke, ANAL.CHEM., 28, 1816 (1956). (2) E. B. Beetch and L. J. Oetzel, J. Agr. Food Chern.,5,951 (1957). (3) J. E. Brekke, J. Assoc. Ofic.Agr. Chemists, 44,641 (1961). (4) F. S. Nurv. D. H. Taylor, J. Agr. Food Chem., . . and J. E. Brekke,.~ 7, 351 (1959). (5) I. M. Kolthoff and C. S. Miller, J . Am. Chem. Soc., 63, 2818 (1941). \
,
260
ANALYTICAL CHEMISTRY
accommodated by the polarographic analysis (10 to 1000 pg per ml) than by the colorimetric method (0.2 to 2 pg per ml), although the colorimetric method is much more sensitive. For a sampling situation where the SOz content is not known, a polarographic procedure capable of such wide ranges in concentration would be of great advantage. In this laboratory, the reported distillation procedure (2) was used in modified form for the determination of SOyin foodstuffs. The polarographic technique herein reported was found particularly advantageous where concentration levels in the samples were unknown. The polarography of SOz in acidic solutions was reported by Kolthoff and Miller (5). However, the polarographic wave (6) ] in a for SOn {present in the complex ion [HgClzS0~.]-~ solution of NazHnC1, was obscured by the reduction wave of (6) R. V. Nauman, P. W. West, F. Tron, and G. C. Gaeke, ANAL. CHEM., 32,1307 (1960).
the [HgCl4]-Z moiety. Thus, a procedure was developed for the reduction of this complex to Hgo by means of hydrazine in a basic medium. After removal of the Hg", the solution was adjusted to a pH 0:' 1.O and a reduction wave for SO2 was obtained. EXPERIMENTAL
Reagents. Distilled water is rectified over K M n 0 4 and used for the preparation of all reagents. A 0.3M NazHgC14 (11) solution is prepared in the usual manner ( I ) , remembering to dissolve the NaCl completely before addition of HgClZ. This solution is used for collecting SOs samples and making polarographic standard solutions (containing 10 to 1000 pg per ml of SOs) using NaHS03. Equipment. A Metrohm Polarecord was used along with D.M.E. and an Ag/AgCl(s), KCl(s) reference electrode in a cell containing a small stirring bar. Polarograms were determined at solution i.emperatures of 25.0" + 0.2"C. Polarographic Analysis. Place 20.0 ml of the 25 ml, 0.3M Na2HgCli absorption solution, containing 10 to 1000 pg per ml of SOs, in ;I 50-ml centrifuge tube. Add 2.0 ml of hydrazine hydrate and shake intermittently for 2 minutes. (Vent frequently to release Nz formed during the reaction.) Centrifuge at 2000 rpm for 10 minutes. Transfer 10.0 ml of the clarified solution to the dry polarographic cell. Sweep the solution with Nz for 2 minutes. Allow Nz to sweep over the solution and add 5M HC1 cooled to 0°C and free of dissolved oxygen. (Add 2.0 ml of 5M HCl if SOz is present or 3.0 ml if suliite standard solution is used directly. In any event, the final p H must be 1.Os) Mix, using a magnetic stirrer, and subject the quiet solution to polarography in the voltage range 0 to - 1.O volt us. Ag/AgCl(s), KCl(s) electrode. RESULTS AND DISCUSSION
During the development of the procedure it was discovered that the id/C (pa/pg of SOz/ml solution) was not constant in time for standard solutions of NaHS03 in 0.1M NazHgC14. Stability was achieved by using water redistilled over KMn04 and 0.3MNaaHgC14as the solvent. In Table I, a comparison is given of the value of the id/C,the source of HgC12,time, and the concentration of SOs. The first two samples of HgC12 indicate poor stability +-orthe 0.7 pg per ml solution and it seemed better to restric; the method to the final concentration range of 7 to 700 pg per ml (or sample range of 10 to 1000 pg per ml of SO2). However, in general, stability is maintained at least over a 4-hour period. An average experimental El,z value of -0.32 + 0.01 volt us. Ag/AgCl(s), KCl(s) electrode at 25.0 O h 0.2 O C (capillary constants, m = 1.79 mg per second and t = 4.30 seconds) was obtained and compares favorably with the reported values of -0.37 volt cs. SCE (5). Values for the standard calibration curves were determined on three different occasions and are as follows: for concentrations of 700,550,350,140,55, and 7 pg per ml, the id/C values were 0.246, 0.243, 0.226, 0.218, 0.212, and 0.162 pa/pg/ml, respectively; for concentrations of 700, 350, 140, 70, 35, and 7 pg per ml, the id/C values were 0.247, 0.252, 0.226, 0.209, 0.203, and 0.193 pa/pg/ml, respectively; and for concentrations of 140, 112, 70, 35, and 21 pg per ml, the idJC values were 0.206, 0.206, 0.200, 0.192, and 0.181 pal pg/ml, respectively. A plot of microamperes us. concentration shows a curve with two slopes with the inflection point at an SOaconcentration o!' 140 pg per ml. The average id/C values of 0.23 and 0.20 pa/pg/ml for SOz concentration ranges of 700 to 140 and 140 to 7 pg per ml, respectively, show good experimental agreement with values of Kolthoff and Miller
Table I. Stability of SOn in 0.3M NasHgClr Solutions SOz*,mlml 140 7 0.7c HgClz Time," source hours i d / C I.ta/I.tg/ml Mallinckrodt 0 0.227 0.187 0.222 WLYB 4 0.206 0.174 0.122 J. T. Baker 25418
0 4
0.232 0.238
0.193 0.190
0.171 0.157
J. T. Baker 23047
0 4
0.213 0.214
0.187 0.193
0.157 0.207
J. T. Baker 23308
0 4.5
0.222 0.213
0.170 0.189
0.207 0.209
a Time elapsed between preparation of the solution and the polarographic analysis. * Final solution concentration of SOzutilizing NaHS03. Other values obtained were 0.320 and 0.193.
(3) of 0.21 to 0.24 pa/pg/ml for concentrations of 31 to 149 pg per ml. Since atmospheric samples collected in NagHgCl4and containing SOzand nitrogen(1V) oxides might be analyzed by this technique, knowledge of the effect of these oxides on the polarographic method was desirable. It is energetically possible for hydrazine to oxidize nitrogen tetroxide to nitrate ion in an alkaline medium, thereby eliminating this gaseous contaminant (7). Also, the nitrogen(1V) oxides would probably be partially removed by the sweep of nitrogen gas formed during the analytical procedure by the interaction of hydrazine and mercuric mercury. Thus it is felt that the initial presence of nitrogen(1V) oxides would not present a serious problem, since their elimination during the procedure seems likely. A few experiments were performed to shed some light on the possible influence of nitrogen(1V) oxides on this analytical procedure. Several test tube experiments indicated that hydrazine reacts with nitrogen(1V) oxides and nitrite ion in alkaline water solutions, eliminating these two species completely. Furthermore, in comparing Na2HgC14 solutions containing 200 pg per ml of SOs with a composite NazHgCll solution of 200 pg per ml of SO2and 1000 pg per ml of NO9, no differencesin or id/C values could be detected when the polarographic analytical method was applied. Thus at these concentration levels, the polarographic method is applicable to the determination of SO2 in the presence of nitrogen(1V) oxides and there is strong indication that other concentration ratios of SOz to nitrogen(1V) oxides will present no great analytical problem. ACKNOWLEDGMENT
The authors are indebted to Marga Wohl for her conscientious and accurate technical assistance.
RECEIVED for review August 2, 1966. Accepted November 14, 1966. Part of a presentation at the Fourteenth Annual Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1963.
(7) T. Moeller, "Inorganic Chemistry," pp. 289, 583, Wiley, New York, 1952. VOL. 39, NO. 2, FEBRUARY 1967
261
