EXPERIMENTAL.
The apparatus shown in Figure 1 has been constructed to fit into the low-temperature accessory of a Jouan Dichrograph Model 185 11. To minimize the heat exchange between flask and buret, the connection was made as narrow as possible (3 mm; narrower connection made filling impossible) and thermostated at 25 “C. Any measuring error caused by shrinking of the flask, which was made of Duran 50, has not been taken into account. The volume of the flask was 18.0 ml; the buret was read to an accuracy of 0.1 ml. Standard deviations vary from 0.07 at room temperature to 0.35 at low-temperature, probably because at low-temperature, temperature control is less accurate. The solvents used were :
Figure 1. Apparatus for contraction measurements
ETHER,anhydrous, Baker, distilled and purified by chromatography on AlzOa (basic Merck). ISOPENTANE, Baker, distilled and purified by chromatography on silica gel. ETHANOL, absolute, p.a., Merck. METHYLCYCLOHEXANE, p.a., Baker, purified by chromatography on AgN03/A120a. METHANOL, Uvasol, Merck. GLYCEROL, redistilled, extra pure, Merck.
RECEIVED for review November 4, 1970. Accepted March 8, 1971.
Determination of Iodine and Fluorine in Selenium Ulla Westerlund-Helmerson Boliden Aktiebolag, Central Laboratory, Riinnskiir Works, Skelleftehamn, Sweden WHENproducing refined selenium, it is of great importance to have a rapid method for determination of halogens. Our experimental work was at first directed only toward the determination of iodine because it was the most interesting contaminant, but the method can also be used successfully for determination of fluorine. As our trials to separate iodine by dissolution of selenium in nitric acid in the presence of excess silver ions and also separation of iodine by distillation had failed to give acceptable results, our next step was to melt the selenium to be analyzed with an excess of sodium hydroxide . The reaction obtained (1, 2) 6 NaOH
+ 2 NaBe + 3 HzS04
+
3 Se
+ 3 NaZSO4+ 3 HzO
(1) D. M. Chizhikov and V. S. Shchastlivyi, “Selenium and Selenides,” London and Wellingsborough, 1968, p 882. (2) P. J. Durrant and E. Durrant, “Introduction to Advanced Inorganic Chemistry,” 2nd ed., 1970, p 40. 1120
4 5
10 2
10
2 2 2 2 2 2
20 30 40 50 60
54, 55, 56, 56 54 55 55 55 54 53 57
+ 3 Se = NaZSeO3+ 2 NazSe + 3 HzO
is rather rapid at 400-500 “C,and the melt can be dissolved in water. By neutralization with sulfuric acid you get NazSeO:,
Table I. Determination of I after Total Melting of Se with NaOH and after Melting Extraction of Se with NaOH Se, grams NaOH, grams J in ppm
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
Iodine present is bound as NaI. When the melting is undertaken in normal atmosphere, you get oxidation and still have small quantities of NazSe03in the solution after neutralization. To increase the sensitivity, we concentrated the halogens by using melting extraction. Thus we could reduce the amount of NaOH and increase the amount of Se to be melted. After leaching the melt with water, we found that iodine is totally transferred to the alkalic phase as NaI. Table I shows that melting extraction gives good conformity. The values in
the table were determined titrimetrically according to the method of Crouch (3). The determination of iodine by X-ray fluorescence, after precipitation with AgNOa, using AgCl as collector, failed because Se, precipitated as AgSeOa, interfered. Bromine, though, could easily be determined. We therefore used an ion-selective electrode for determination of iodine. When we adapted the amount of NdOH, H20, and in order to make the determinations in 0.5M NazS04by pH 3, we got a linear relation, with the theoretical slope, between the electrode potential and the logarithm of the concentration of iodine down to 8 X 10-8M, which means 0.05 ppm when 20 grams of selenium were melted with 4 grams of NaOH and the volume was 100 ml. To overcome the influence of divergent composition of test solution and standard solution, we used the method of “known addition”, and found it suitable to arrange the figures in a table (4-6) (Table 11). When using the suggested method in practice, we have found that when the leaching solution is neutralized to pH
