N. V. Sverkunova, V. M. Aksenenko, R. D. Glukhovskaya

Mercurimetric Microdetermination of Chlorine and Bromine in Organic Compounds after Their Decomposition in an Oxygen Flask

(Presented by the scientific seminar of the Department of Analytical Chemistry)

The determination of halogens in organic compounds is reduced, in most cases, to decomposition of a weighed sample of the substance by one method or another, followed by determination of the halide in solution. Among the various decomposition methods, combustion of organic substances in an oxygen flask [1, 2] deserves considerable attention; it is distinguished by the simplicity of the apparatus and convenience in routine work. A large number of papers have been published concerning modifications of this method for the determination of elements in organic compounds, including halogens [3, 4, 5]. Various authors improve the decomposition technique [9, 12], introduce new absorbents [6, 7, 9, 10, 12, 13], and modify the final stage of the analysis [8, 10, 12].

We set ourselves the goal of determining the possibility of titrating halides with mercuric nitrate after combustion of the weighed sample in an oxygen flask in the presence of a mixed indicator. The latter was first applied by one of us for the determination of halides in dilute solutions [14] and of halogens in organic compounds after decomposition of the weighed sample with metallic sodium or potassium [15, 16]. The mixed indicator, which is a mixture in alcohol of two mercurimetric indicators—β-nitroso-α-naphthol and diphenylcarbazone—has high sensitivity and changes color sharply at the equivalence point, even in solutions of considerable acidity.

Before resolving the main question, it was necessary to select an absorbent that would not only absorb the gaseous products but also direct secondary reactions toward the formation of halides. The mixture of potassium hydroxide and hydrogen peroxide solutions recommended in the literature [3, 4], according to our data, is quite suitable for the determination of chlorine. However, in the analysis of bromine-containing materials we obtained, as a rule, low results for bromine. We explain this fact by the circumstance that, during combustion of the substances being analyzed, along with the principal product, hydrogen bromide, elemental bromine and even positively charged bromine are formed. By using a sodium sulfite solution as the absorbent—which simultaneously creates an alkaline medium, convenient for absorbing acidic products, and readily reduces the by-products of bromine conversion to bromide ions—we obtained stable results in the determination of bromine and chlorine. Like other investigators, we used hydroge-

hydrogen peroxide, but not as a component of the absorbing solution; rather, it is used to oxidize the excess sodium sulfite and to improve the subsequent titration. Oxidation of sodium sulfite with hydrogen peroxide is not accompanied by an increase in the acidity of the solution, which thereby becomes neutral. The introduced excess of hydrogen peroxide not only does not interfere with the determination but, on the contrary, is necessary for the titration of halides, since under the conditions of the experiment described the mixed indicator changes color more sharply at the equivalence point.

Experimental Part

Apparatus

1. A 250 ml flask with a ground-glass stopper (Fig. 1). A hook is fused into the lower end of the stopper; from it is suspended a platinum wire 0.5–1 mm in diameter and 4–5 cm long. A platinum gauze measuring 1.5 × 3 mm is attached to the lower end of the wire.

2. A microburet with a reservoir, GOST 1770-51.

Fig. 1.

Reagents

1. Sodium sulfite, analytical grade, 2 N solution.
2. Hydrogen peroxide, medical, 30% solution.
3. Sodium chloride, chemically pure, 0.02 N solution.
4. Mercuric nitrate, chemically pure, GOST 4521-48, 0.2 N solution.
5. Ethyl alcohol, rectified, GOST 5962-51.
6. Diphenylcarbazone, pure, GOST 2536-51, 1% alcoholic solution.
7. β-Nitroso-α-naphthol, pure, GOST 3492-52, 1% alcoholic solution.

Procedure of Analysis

A sample of the substance (5–9 mg) is taken in a weighing tube and poured into the center of filter paper (a packet), previously folded as shown in Fig. 2.

The packet is clamped in the platinum gauze so that the long strip of paper is directed upward, and the whole assembly is suspended from the hook of the stopper.

Into the flask are poured 2.5 ml of bidistillate and 1 ml of 2 N sodium sulfite solution, and oxygen from a cylinder is passed into it for 1–2 min at a rate of 100 ml/min. The flask is wrapped in a towel, the protruding end of the filter paper is ignited, and the stopper is quickly inserted into the flask. The latter is pressed down by hand, and the point of contact between the stopper and the neck of the flask is flooded with several drops of bidistillate, thereby creating a hydraulic seal. After combustion of the sample, the flask is left to stand for 45–60 min for complete absorption of the combustion products. The stopper is opened, it and the walls of the flask are washed with 7–10 ml of bidistillate, 0.5 ml of hydrogen peroxide is added, along with 4–5 ml of ethyl alcohol and one drop each of 1% alcoholic

Fig. 2.

solutions of β-nitroso-α-naphthol and diphenylcarbazone and titrate with 0.02 N mercuric nitrate solution until the color changes from yellow-green to pink-violet. In parallel, a blank experiment is carried out, burning an empty packet instead of the weighed sample.

The halogen content is calculated by the formula:

\[ \% \ \text{hal.}=\frac{(a-b)\cdot T\cdot 100}{v}, \]

where

\(a\) — the consumption of mercuric nitrate, in ml, used for titration of the halide;
\(b\) — the amount of mercuric nitrate, in ml, consumed in the blank experiment;
\(T\) — the titer of mercuric nitrate with respect to the halide;
\(v\) — the weighed sample of substance, in g.

Some results of the determination of chlorine and bromine in organic compounds are given in the table.

Table

Determination of halogens in organic halogen derivatives by the method of combustion in a flask with oxygen *

*) The purity of the substances was checked by melting point and by determination of the elemental composition for carbon, hydrogen, and nitrogen.

As the data in the table show, the mercurimetric method using a mixed indicator proved very suitable for the determination of chlorine and bromine in organic compounds after combustion in a flask with oxygen.

Conclusions

1. A modification is proposed for the mercurimetric determination of chlorine and bromine in organic compounds with a mixed indicator.

2. It has been shown that, for a sharper change in the color of the indicator at the equivalence point, the presence of hydrogen peroxide is necessary.

Literature

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