Spectrophotometric determination of sulfite with soluble mercury(II) compounds

MICROCHEMICAL

JOURNAL

27, 351-356

Spectrophotometric with Soluble

(1982)

Determination of Sulfite Mercury(ll) Compounds

RAY E. HUMPHREY, GEORGE S. INGRAM, DRUCE K. CRUMP

AND

Depurtment of Chemistry, Sum Houston State University, Huntsville, Texus 77341

ReceivedJuly 25, 1981 INTRODUCTION

Solutions of mercury(I1) nitrate, an ionic compound (I) were reported to react with sulfurous acid in 0.1 M sulfuric acid to form a 2: 1 complex which had maximum absorption at 237 nm and was very stable (4). The absorbing species obeyed Beer’s law. Complexes with different stoichiometry were formed under different conditions. The mercury(I1) nitrate reaction with sulfite was made the basis for an indirect method in which the excess mercuric ion was measured with diphenylcarbazone (5). The mercury(I1) was assumed to form a complex with sulfite. Solutions of mercury(I1) chloride, an essentially covalent compound (I), are used as a trapping system in the West-Gaeke method for atmospheric sulfur dioxide (7). The solution contains excess chloride ion so that HgC&- is supposedly present. This reacts with sulfite to form a complex, Hg(S0,)C1,2-, which is stable. The sulfite is later released for calorimetric measurement. Insoluble mercury(I) chloride, Hg,C12, reacts with aqueous sulfite solutions to form a soluble mercury(I1) sulfite complex and elemental mercury. The absorbance of the sulfite complex or other mercurate complexes, HgX, ‘-, is measured in the ultraviolet region and related to sulfite concentration (3). Our investigation of the reaction of solutions containing mercury(I1) ion with acidic solutions of sodium sulfite showed that a species is formed with an intense absorption maximum at 237 nm. The absorbing species is believed to be mercury(I) ion dimer, Hg, 2+. This ion is reported to absorb at 237 nm with a molar absorptivity of 28,000 (2). The mercury(I) ion is stable for months in perchloric acid solutions. We have found sulfite can be determined accurately and rapidly by reaction with mercury(I1) ion in acid solution. The mercury(I) formed is stable for weeks. Covalent mercury(I1) compounds react with sulfite ion to produce a which has a maximum absorbance at 230 soluble complex, Hg(SO&P, 3.51 0026-265X/82/03035 l-06$01.00/0 Copyright @ 1982 by Academic Press. Inc. All rights of reproduction in any form reserved.

352

HUMPHREY,

INGRAM,

AND

CRUMP

nm. The stability constant for formation of the Hg(SO&~ species is very high with log K approximately 23 (6). Our results show that the complex is relatively stable in a buffer with pH 6.86 but rather unstable in water only. Mercury(H) chloride reacts with acidic sulfite solutions to form a species which absorbs at 230 nm and is rather unstable. It is likely that the mercury(H) species is reduced to mercury(I) and precipitated as Hg,Cl, in acidic and unbuffered solutions. MATERIALS

AND METHODS

Equipment. Absorption measurements were made with a Beckman ACTA III spectrophotometer. Chemicals and solutions. All chemicals used were the best available reagent grade. The buffer employed was 0.025 M in KH,PO, and Na,HPO,, pH 6.86. Sodium sulfite solutions contained 5% glycerol. Mercury(I1) sulfate was dissolved in 25 ml of 1 M H,SO, and diluted to 250 ml with 0.1 M H,SO,. Procedure. Reaction solutions were prepared by adding appropriate volumes of the mercury(I1) solution, sodium sulfite solution, and acid, buffer, or water. For the Beer’s law studies the sulfite solution was added last. The total volume of reaction solution was usually 20 ml. Absorbance values were read in 1.0 cm cells at various time intervals. Typical Beer’s law studies for the mercuric ion-sulfite reaction involved adding from 0.5-5.0 ml of approximately 4 x 1O-4M sodium sulfite solution to an acidic solution containing 5 ml of 2 x 10m3M HgSO,. The total volume was 20 ml and solutions were about 0.1 M in H&SO,. RESULTS Reaction

AND DISCUSSION

of Mercury(ZZ) Ion with Sulfite Solutions

Mercury(I1) ion shows a broad absorption in the uv region beginning around 250 nm and increasing sharply from 230 nm toward shorter wavelengths. Addition of sulfite solution to an acidic solution of mercuric ion results in an intense absorption with a maximum at 237 nm. The wavelength maximum and intensity agree with that reported for the mercury(1) ion (2). The absorption spectra for the reaction solutions were identical to the spectrum of mercury(I) sulfate solution in 0.1 M sulfuric acid recorded in our work. In earlier work on the reaction of mercuric ion with sulfite in acid solution it was assumed that some type of complex ion was formed (4). Several stoichiometries were reported. Mole ratio experiments in this work showed a combining ratio of one SO,‘- to two Hg”+ ions and two Hg’+ ions to one sulfite (Table 1). Solutions which had sulfite in excess of the 1:2 SO,‘-/Hg” ratio showed decreasing absorbance after a period of time. It is likely that the Hg,‘+ ion

DETERMINATION

TABLE I MOLE RATIO DATA FOR MERCURIC ION-SULFITE Moles SO,*-/Moles 0.1 I 0.23 0.34 0.45 0.57 0.68 0.79 0.90 I.1 I.4

Hg”

353

OF SULFITE

REACTION

Moles Hg’+/Moles

A” 0.23 0.44 0.64 0.83 1.04 I .04 1.02 1.06 I.12 I.13

SO:,y

A”

0.24 0.48 0.72 0.96 1.20 1.44 1.92 2.40 2.88

0.22 0.47 0.71 1.00 1.28 I.3 I .90 1.93 2.01

U Absorbance measured at 237 nm. Absorbance values for solutions in which the mole ratio of sulfite to mercuric ion was greater than 0.S started to decrease after 15-20 min.

is reduced further to elemental mercury. Reaction between mercuric ion and sulfite in acid solution apparently proceeds according to the following equation: 2Hg’+ + H,SO:, + H,O+ Hg,‘+ + H,SO, + 2HS. Solutions with mercuric ion in excess (Hg”+/SO,, > 2) showed constant absorbance for a long period of time. Mercury(I) ion is reported to be stable for months (2) and our results with such solutions are in agreement. Absorbance at 237 nm increases linearly with increase in sulfite concentration with mercuric ion in considerable excess. The lowest ratio of Hg2+/S0,“P was approximately 7-8. Beer’s law data is presented in Table 2 and recovery data in Table 3. The absorbance values increase for about TABLE 2 BEER’S LAW DATA FOR MERCURIC ION~SUL.FITE REACTIOF;

so:’ (mm)

A”

E

0.83 I.7 2.5 3.3 4.2 5.0

0.33 0.65 0.96 I .28 I.58 1.83

25,000 25,000 25,000 24,000 24,000 23,000

LI Reaction in 0.1 M H,SO,. h Wavelength 237 nm. Blank correction

0.25

354

HUMPHREY,

RECOVERY

DATA

INGRAM, TABLE FOK M~KCUKIC

AND

CRUMP

3 ION-SULFITE

REACTION

SO, presenP (w-d

SO, found (mm)

0.80 1.6 2.4 3.2 4.0 4.8

0.80 1.5 2.5 3.1 4.0 4.6

n Reaction in 0.1 M H,SO,, absorbance measured at 237 nm.

30 min and remain essentially constant for days. Reaction is 95% complete in 25 min. There seems to be a very slow and perhaps insignificant increase on standing. This reaction would appear to be a simple, rapid, and sensitive method for sulfite and presumably sulfur dioxide. Those anions which form insoluble mercury(I) compounds, including chloride (4), could not be tolerated. If such anions can be avoided, the system is exceptionally stable and the sensitivity very good.

Reuction of Covulent Mercury(IZ) Compounds with Sulfite The compounds involved in these experiments were mercury(I1) acetate, bromide, chloride and thiocyanate. An excess of the respective anion was added to each solution, corresponding to the composition HgXd2-, to see if there was any effect on complex formation or stability of the species formed. Mole ratio studies for all of these compounds (Table 4) showed two sulfite ions for one mercury(I1) presumably forming Hg(SO&-. This reaction is shown in the following equation, where X represents either acetate, bromide, chloride, or thiocyanate. HgXz + 2S03’-

G Hg(SO,),‘-

+ 2X-.

Continuous variations study of the mercuric chloride-sulfite reaction in a pH 6.86 buffer showed a 2:l ratio of SO,“-/Hg with some indication of a 1:l species. The presence of excess of the anion involved did not seem to have any particular effect on the results obtained. Absorbance values for these reaction solutions in buffer reached a maximum in a few minutes and decreased slowly with time. The loss after standing for one day was of the order of 10%. Beer’s law data for the mercury(H) sulfite complex formed from the different compounds is shown in Table 5. Response is linear and the sensitivity is less than one-half that of the mercury(I1) ion procedure. The molar absorptivity of the Hg(SO&species is about 25,000. Since the S0,2-/Hg(II) ratio is 2:l as compared to one Hgz2+

DETERMINATION

OF

355

SULFITE

TABLE 4 MOLE RATIO DATA FOR MERCURIC CHLORIDE-SULFITE Moles SO,*-/Moles 1.00 1.25 1.50 1.75 2.00 2.50 3.00 4.00 6.00

HgCI,

A”

Moles HgCl,/Moles

0.51

0.10

0.66 0.83 0.99

0.20 0.30

REACTION A*

SO,*-

0.12 0.25 0.36

0.40 0.50 0.60 0.80

1.11 1.22 1.24 1.26 1.27

0.45 0.49 0.53 0.57 0.59 0.61

1.00 2.00

n Wavelength 230 nm. Extrapolation of the plot showed intersection at 2.0 moles of sulfite to 1.0 mole of mercuric chloride in 6.86 buffer. b Wavelength 230 nm. Extrapolation of the plot showed intersection at 0.5 moles of mercuric chloride to 1.O mole of sulfite in 6.86 buffer.

formed for each sulfite on reacting with the Hg2+ ion, the significantly lower sensitivity of the complex for determination of sulfite is to be expected. Complex formation, as indicated by absorption at 230 nm, may also occur when mercury(I1) chloride solutions react with excess sulfite in distilled water or 0.1 M H&SO,. The modern absorptivity values are about the same as in the buffer. There is no definite peak in the spectrum, however. Acidic solutions of sulfite compounds would probably not contain any appreciable amount of SO:,2pion to form an Hg(SOJ,“- species. However, obviously some interaction takes place to form an absorbing species. These solutions show loss of absorption in an hour or so and deteriorate rapidly. Some precipitation was noted after one day. Possibly reduction occurs and Hg,Cl, forms. SUMMARY Sulfite ion reacts with mercury(I1) ion in acid solution to form the mercury(I) ion. The reaction is rapid and quantitative. The mercury(I) ion absorbs at 237 nm with a molar TABLE 5 BEER’S LAW DATA FOR SULFITE COMPLEXES OF COVALENT MERCURY(II) COMPOUNDS

so, (pw)

eHgC1,”

2.0 4.0 6.0 8.0

12,500 12,500 12,500 12,000

l HgBr, 10,000

11,500

rHg(A&* 10,000 10,000

11,500

10,000

11,000

10,500

l Hg(SCN), 9,200 9,000 9,200 9,800

a Molar absorptivity based on sulfite ion at 230 nm. Solution was 6.86 buffer. b Mercuric acetate solutions seemed to be somewhat unstable.

356

HUMPHREY,

INGRAM,

AND

CRUMP

absorptivity of about 25,000. The absorbance is linear over a range of approximately 0.5-5.0 ppm as SO,. Covalent mercury(H) compounds form a complex with sulfite, Hg(SO&‘- , which absorbs at 230 nm and shows a linear response over a range of l-8 ppm as SO,.

ACKNOWLEDGMENT This research was supported in part by the Faculty Research Fund of Sam Houston State University.

REFERENCES I. Cotton, F. A., and Wilkinson, G., “Advanced Inorganic Chemistry,” 4th ed., p. 603-604. Wiley, New York, 1980. 2. Higginson, W. C. E., The ultraviolet absorption spectra of mercurous perchlorate solutions. J. C1zem.Sot. 1951, 1438-1443. 3. Hinze, W. L., Kippenberger, D. J., and Humphrey, R. E., Determination of sulfite by reaction with mercury(I) chloride and spectrophotometric measurement of mercury(I1) complexes. Microchem. 1. 20, 43-49 (1975). 4. Okutani, T., Ito, S., and Utsumi, S., Ultraviolet spectrophotometric determination of micro amounts of sulfite. Nippon Kagaku Zasshi 88, 129661299 (1967). 5. Okutani, T., and Utsumi, S., A new spectrophotometric method for the determination of micro amounts of sulfite. Bull. Chem. Sot. Jupan 40, 1386-1391 (1967). 6. Sillen, L. G., and Martell, A. E., Stability constants of metal-ion complexes. Spec. Puh[. Chem. Sot. 17 (1964). 7. West, P. W., and Gaeke, G. C., Fixation of sulfur dioxide as disulfitomercurate(I1) and subsequent calorimetric estimation. Anal. Chem. 28, 1816-1819 (1956).