Results of an experimental study of the metastable zone width of pure potassium nitrate aqueous solutions contained in 4-cm³ capacity cuvettes of a UV−Vis spectrophotometer investigated as functions of cooling rate R and saturation temperature T₀ of the solution by the polythermal method are described and discussed. The metastable zone width was measured as the maximum supercooling ΔT_max defined as the difference between the saturation temperature T₀ and the temperature T_lim at which first crystals are detected in the solution, i.e. DT_max = T₀−T_lim. The changes occurring in the studied solutions during their cooling at constant predefined rates between 5 and 40 K/h were followed by registering changes in the intensity of transmitted light using the UV-Vis spectrophotometer. It was observed that the maximum supercooling ΔT_max for potassium nitrate aqueous solutions increases with increasing cooling rate R andsaturation temperature T₀. The experimental data on maximum supercooling ΔT_max corresponding to saturation temperature T₀ as a function of cooling rate R are analyzed by employing the following two approaches: self-consistent Nývlt-like approach and a novel approach based on the classical three-dimensional nucleation theory. The above results of the metastable zone width of potassium nitrate solutions contained in cuvettes are compared with the literature data on the metastable zone width of solutions of relatively large volume in the absence and presence of seeds.

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