Stripping voltammetric method for determining the degree of substitution in carboxymethylcellulose

The main indicators of quality that determine the practical value of the salt of carboxymethylcellulose (CMC) and the area of its use are the degree of substitution by carboxymethyl groups and the degree of polymerization. The degree of substitution is determined by the number of methyl carboxyl groups in one hundred units of the CMC macromolecule, where hydrogen is replaced by the -CHCOONa group. The degree of substitution by carboxymethyl groups affects such an important property of the CMC salt as its solubility in water, therefore reliable and rapid methods are needed to control the degree of substitution. The currently used methods for determining the degree of substitution in carboxymethylcellulose (CMC) have a number of significant drawbacks: long analysis time, high labor intensity, the need for preliminary sample preparation, affecting the quality of the results. The article presents a method for determining the degree of substitution of carboxymethylcellulose by stripping voltamme-try. The proposed method is based on the formation of a non-electroactive complex of carboxymethyl cellulose with copper ions and the determination of free copper ions by stripping voltammetry. The conditions for the formation of a complex of copper with carboxymethylcellulose (pH 4.0 - 4.2) and the determination of copper unbound in the complex by stripping voltammetry have been optimized. It is proposed to use a graphite electrode modified with a solid polymer electrolyte based on copolymers of methyl methacrylate and methacrylic acid with encapsulated mercury as an indicator electrode for the determination of copper. The fixing of the polymer film on the surface of the electrode occurs due to physical adsorption. The modified electrode retains its performance for 50 cycles. By varying the thickness of the polymer film deposited on the electrode, it is possible to significantly expand the range of detectable concentrations of unbound copper. The optimum potential for electroconcentration of copper is -0.8 V; when accumulating in a more negative potential region, the copper complex with the CMC is destroyed. It is established that sodium glycolate, which is usually present in technical CMC, does not interfere with the determination. The stages of sample preparation and analysis are combined in one cell. The method allows to determine the degree of substitution in CMC in the range from 10 to 270%, which completely covers the range of products. Carrying out a complete analysis to determine the degree of substitution of CMC does not exceed 40 minutes. Comparison of the results obtained by the proposed method with TU 2231-002-50277563-2000 showed no significant discrepancies. The expressiveness, simplicity and high repro-ducibility of the results allow the analysis of the degree of substitution of CMC in a streaming mode, which makes it possible to implement this method of analysis in production laboratories.

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