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Measurements of the refractive index, its temperature dependence, and the optical transmission of black liquors produced during sulfate pulping are reported for soluble solid residue concentrations up to k 60%. The design features of a commercial refractometer for monitoring the concentration of black liquor are examined briefly. A procedure is proposed for laboratory calibration of commercial sensors that employs black liquor solutions in highly refractive organic liquids as reference samples.Introduction. Refractometric methods and instrumentation are used quite successfully for determining the concentration of soluble solids in black liquors produced during sulfate pulping [1][2][3][4][5]. After several stages of thermal evaporation, these alkalis are fired in coregeneration boilers at most plants at solid residue concentrations of up to k = 70-75% or above [6] with partial recovery of the cooking residue in the form of green lye. Commercial refractometric sensors can monitor the solid residue in the black liquors at any stage of the evaporation process if their optical properties are reliably known. Only limited amounts of the required data can be found in the available scientific and technical literature, and, at best, only at a qualitative level [7]. Thus, the purpose of this article is to study the optical properties of black liquors at levels corresponding to the specifications for refractometric technologies for solid residue monitoring.Experiments and Discussion. Ten reference samples of black liquor from the Segezhskii cellulose plant with concentrations up to k = 60% (as determined by weighing the dry residue, i.e., by gravimetry) were used for measuring the refractive index n of the test samples. The refractive index of the test solutions was measured with an Expert pro digital laboratory refractometer at a wavelength of λ = 589 nm for residue concentrations up to k 60%, as well as with an Abbe refractometer at λ = 633 nm for k up to 32%, where measuring the visually observable light-dark interface did not significantly reduce the accuracy of the measurements: the absolute error Δn in measuring n for the test samples with both instruments was within ±0.0005. The measured concentration dependence k(n) at t = 20

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