Method of determining degree of reduction in a sulfate liquor using IR and UV spectroscopy

A method for determining the degree of reduction in a sulfate liquor or a sulfate smelt solution, said method comprising the steps of:

• (a) measuring the light absorption of the sulfate liquor or the smelt solution in the infrared range of 1500 to 800 cm.sup.-1 to produce an IR absorption spectra;
• (b) measuring the light absorption of the sulfate liquor or the smelt solution in the ultraviolet range of 200 to 250 nm to produce a UV absorption spectra, without separation of the sulfate liquor or the solution into different components;
• (c) calculating the concentrations of sulfate, thiosulfate and optionally sulfite and/or carbonate in the sulfate liquor or the smelt solution from the IR absorption spectra by using a computer-implemented mathematical model in combination with a multivariate calibration technique;
• (d) calculating the concentration of sulfide and optionally polysulfide in the sulfate liquor or the smelt solution from the UV absorption spectra by using a computer-implemented mathematical model in combination with a multivariate calibration technique;
• and
• (e) determining the degree of reduction of the sulfate liquor or the smelt solution from the concentration of sulfide and optionally polysulfide, and the concentrations of sulfate, thiosulfate and optionally sulfite and/or carbonate calculated in steps (c) and (d).