Polarographic Microdetermination of Sulfate Ja’n Mager, Eugen Hluchai, and Emil Abel Research Institute for Hygiene, Bratislava, Czechoslovakia
SOMEINDIRECT METHODS for the polarographic determination of sulfate have been developed (1-5), but these methods require greater concentrations of sulfate and therefore prior evaporation of the sample may be necessary before the determination can be carried out. We searched for a quick and sensitive method with a wide range (0.1-200 mg/liter of modifying the volumetric method (6, 7) into a polarographic one. The principle consists in the exchange of the sulfate ion for chromate using a barium chromate reagent. The chromate ion represents the concentration of sulfate in the sample and will be determined from its polarographic wave. The polarographic techniques for chromate (8-11) have been somewhat modified in our study, using in the applied buffer
the first reduction step at -0.36 V, where the reduction of chromate produces Cr(II1). The diffusion current is in a good agreement with the Ilkovir equation for n = 3 and varies linearly with the concentration of the chromate. EXPERIMENTAL Reagents. BaCrOc stock reagent was prepared by dissolving 29.45 grams of K2Cr207and 20.00 grams of KHCOI separately in distilled water. The solutions were mixed and made up to about 700 ml with water. To this boiling solution 48.76 grams of BaClz.2H20in 200 ml of water were added. The precipitate settled and the supernatant liquid was sucked away with a syphon. The precipitate was made up to 500 ml with distilled water and transferred to a brown glass bottle. This reagent served as a stock reagent. The chromate reagent for polarography was prepared from the stock reagent as follows. To 50 ml of the stock reagent was added 1 gram of BaSO;, 30 grams of KCI, 100 ml of 25 HCl, and then the reagent was made up to 500 ml. Procedure. To a 10-ml aliquot of sample, 1 ml of the chromate reagent was added and after shaking, the solution was allowed to react for 10 minutes. Then 1 ml of 25% ammonium hydroxide was added, which changed the Cr207-2 into Cr0,-2 with a color change from orange t o green. After 5 minutes, without filtering off the precipitate, the solution was poured into the polarographic vessel so as not to add surplus sediment, although a n excess of the precipitate will not affect the height of the polarographic wave. After deoxygenation of the solution the polarographic determination was carried out a t -0.36 V. For deoxygenation a stream of Nz gas or a freshly prepared solution of Na2SOIwas used.
(1) C. Ruffs and A. Mackela, ANAL.CHEM., 25, 660 (1953). (2) H. Hohn, “Chemische Analysen mit dem Polarographen,” Springer, Berlin, 1937. (3) K. Matsumoto and M. Shibahara, J. Pharm. SOC.Japan, 73, 656 (1953). (4) 0 . Ohlweiler, Anal. Chim. Acta, 11, 590 (1954). (5) A. D. Horton and P. F. Thomason, ANAL.CHEM.,23, 1859 (1951). (6) A. Jfilek, “Odm&x4 analysa, Techn. vgd. vydfivatelstvi,” Prague, 1951.
(7) “Deutsche Einheitsverfahren zur Wasser, Abwasser und Schlammuntersuchung,” Chemie, Weinheim, 1954. (8) M. Demassieux and J. Heyrovsky, J . Chim. Phys., 26, 219 (1929). (9) L. Praysler, Collection Czech. Chem. Commun., 3, 404 (1931). (10) I. M. Kolthoff and J. Lingane, “Polarography,” Interscience. New York, 1952. (1 1) J. Proszt and V Cieleszky, “Polarografia,” Akadkmia, Budapest, 1965.
Table I. Results of the Determination of Sulfate in Synthetic Known Samples Gravimetric method Taken mg/liter
Found mg/liter
so4- * 21 .o 32.0 41.5 49.3 57.1
Volumetric method
Re1 std dev
SO4-2
mg
20.8 32.3 41 .O 48.2 56.4
-0.2 +O. 3 -0.5 -0.9 -0.7
z
-0.95 $0.94 -1.20 -1.82 -1.22
Found mgiliter
Polarographic method Found mg/liter Re1 std dev sod-* mg +0.1 +O .47 20.9
Re1 std dev-~
SO4-2
mg
z
21.4 32.4 41.1 50.5 56.1
+0.4 +0.4 -0.4 +1.2 -1.0
+1.90 +1.25 -0.96 $2.44 1.75
z
31.8 41.7 49.0 57.3
-0.2 1-0.2 -0.3 +0.2
-0.62 +0.48 -0.61 +O, 35
Table 11. Results of the Determination of Sulfates in Different Types of Water Type of water
Interfering substances, mg/liter
Gravimetric method, mg/liter found
Volumetric method, mg/liter found
Polarographic method, mg/liter found
...
41.6 64.1 1525.-
41.3 63.2 1370.-
41.5 63.8 1535.-
56.0
20.5
57.2
48.1
10.6
49.5
Drinking water Drinking water Spa “SIiaE” Spring “Adam” Spa “Smrdaky”
...
Fe2 6.23 Caz 650.Fez 1.1 HzSa 240.-
Waste water a
Organic substances
S-2 preliminary filtered off as CdS.
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ANALYTICAL CHEMISTRY
DISCUSSION OF RESULTS
Synthetic known samples (Table I) and different types of water (Table 11) have been analyzed for sulfate and the results obtained polarographically have been compared with those from the gravimetric and volumetric determinations. For the synthetic samples and drinking water, without interfering substances (Fe+2,S2, organic substances in high concentration, etc.) there was in all cases good agreement. In samples with the above mentioned reducing substances, a preliminary oxidation step is needed for the polarographic procedure. After that the polarography gives uniform results with gravimetry. Interferences in water are caused by formation of chromate compounds of a minute solubility, reduction of chromate before its polarography, or sorption of the chromate. The first type of interference arises especially in concentrations of Cat2 higher than 200 mg/liter. In this case a less soluble CaCrOl diminishes the concentration of free C r 0 4 + for polarography. To remove the excess Ca+2, we used the cation exchanger Dowex 50Wx8 in the Na cycle. None of the common cations Li‘, Rb+, Cs+ (100 mgjliter), Na+ and K+
(5000 mg/liter) Be+2,Mgf2 (100 mg/liter), Ca+2(200 mg/liter) and anions F-, Br-, I- (100 mgjliter), C1- (10,000 mgjliter), Nos- (5000 mg/liter), Si03-2 and B03-3 (10 mgjliter) will interfere. Reducing agents interfere by reducing the Cr04-2to Cr(II1). All types of reducing agents can be easily eliminated by using oxidizing compounds. In this work we used chlorine in a weak alkaline solution or KBr03 in an acid solution at the boiling point. After preliminary oxidation of the reducing agents the analysis was carried out in the above manner. By comparison with the volumetric method, the polarography can be carried out in the presence of these quoted oxidants, because of the different values of the reduction potentials. The effect of sorption has been considered, especially on the BaS04 formed. This was investigated using freshly prepared BaS04 and a solution of Na2Cr04. Up to a concentration of 200 mg/liter S04-2,there was no interference observed. RECEIVED for review September 19, 1966. Accepted May 22, 1967.
Carrier Gas Effects in Gas-Liquid Chromatography with Packed Columns John R. Conderl and Stanley H. Langer
Chemical Engineering Department, Unicersity of Wisconsin, Madison, Wis. THE RETENTION VOLUM~. in gas-liquid chromatography depends on molecular interactions in both the liquid and gaseous phases. Usually retention in the liquid is at least an order of magnitude larger than in the gas phase, but gas imperfection must nevertheless be taken into account for the most accurate determination of thermodynamic parameters in solution (1-4). The effect of changing the carrier gas on the partition coefficient is likely to be greater in capillary, than in packed, columns ( 5 ) . Desty, Goldup, Luckhurst, and Swanton took advantage of this and used capillary columns to measure the mixed second virial coefficient, B23, for the solute-carrier gas (respectively denoted by subscripts 2 and 3) interaction by varying the mean column pressure (6). For practical reasons, activity coefficients in solution are most often measured on packed columns. Although it is possible to eliminate the effect of gas imperfection by varying the total pressure and extrapolating to zero pressure (7), it is experimentally much more convenient to work at atmospheric pressure only and
calculate the correction to be applied for gas imperfection. The difficulty with this approach is that few of the relevant data are at present available, especially for B23. The matter is made more urgent insofar as activity coefficients and specific retention volumes have been measured by a number of workers using different carrier gases ; the results cannot be compared unless the relative contributions from gas imperfection in the carrier gases are known. We have therefore measured retentions on packed columns for several liquid phases at elevated temperatures using nitrogen and helium as carrier gases. The comparison in these two gases is of special interest, because while helium is the most frequently used carrier gas in the United States, nitrogen is most often used elsewhere and for preparative work. The results correlate well with theoretically calculated values of B23 and further indicate no appreciable effects on retention due to solubility of the carrier gas.
Present address, Department of Chemistry, University College of Swansea, Singleton Park, Swansea. Wales
An expression is first required relating the specific retention volume, V p T ,at column temperature to equilibrium thermodynamic parameters in the gas and liquid phases. The compressibility-corrected retention volume, VRO,is related to the liquid phase volume, VL,by
( I ) J. K. Conder, Ph.D. Thesis, Cambridge, England, 1965. (2) D. H. Everett and C. T. H. Stoddart, Trans. Faraday SOC.,57, 746 (1961). (3) S. H. Langer and H. Purnell, J . Phys. Chem., 67, 262 (1963). (4) S. H. Langer and H. Purnell, Ibid., 70,904 (1966). (5) A. Goldup, G. R. Luckhurst, and W. T. Swanton, Nature, 193, 333 (1962). (6) D. H. Desty, A. Goldup, G. R. Luckhurst, and W. T. Swanton, “Gas Chromatography, 1962,” M. van Swaay, Ed., Butterworths, London, 1962, p. 67. (7) D. 11. Everett, Trans. Faraday SOC.,61, 1637 (1965).
THEORY
V.qo
KVL
(1)
where K is a partition coefficient. Following a procedure similar to that used by Everett (7) in deriving the mass distribution coefficient, k ‘ , we may show that In V’,
=
C
+ KP,J
(2)
where VOL. 39, NO. 12, OCTOBER 1967
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