Addition of polysulfide to pulping liquor has long been known to increase the yield of wood pulp from the kraft process. Methods heretofore available for determining polysulfide in pulping liquors require either spectrophotometric measurements or indirect, multistep, titrimetric procedures.
Absorbance measurements at 285 nm' or 297 nmz have been employed for polysulfide determinations, but they cannot be used on colored spent liquors.
Indirect methods for polysulfide in which thiosulfate is measured before and after the polysulfide is decomposed with sulfite are used for analysis of white and black (spent) liquors3,4. An acidimetric titration may also be used to determine polysulfide from consumption of sulfite in the same decomposition reactions. Polysulfide may be reacted with copper gauze to form copper sulfide; the sulfide is decomposed in acid and the hydrogen sulfide (H$) evolved is determined iodonietrically6. In a newer method, polysulfide is represented by the increase in the liquor's sulfide content following reduction of the polysulfide with sodium amalgam'. Reaction of dissolved organics with the amalgam prevents use of this technique on black liquors. Results from this method are influenced by a high thiosulfate content in the liquor and also by the extent to which the sample is diluted for contact with the amalgams.
Many of the indirect methods lose precision at low polysulfide levels because the analytical result is a small difference between large numbers. Furthermore, results of all of these procedures are strongly influenced by the analyst's ability to avoid oxidative losses of polysulfide from the sample before and during the determination. Oxygen must be rigorously excluded during preparation of standards for the spectrophotometric methods.
This report describes the development of a gas chromatographic (GC) method for determining polysulfide in all types of kraft pulping liquors. Polysulfide is decomposed to elemental sulfur in buffer (pH 5.5), and the elemental sulfur is derivatized with triphenylphosphine. The resulting triphenylphosphine sulfide is determined by flame ionizationGC.
In contrast with spectrophotometric methods, the new procedure is calibrated with a pure, commercially available compound, triphenylphosphine sulfide. All operations prior to GC are performed in sealed septum vials; this effectively eliminates loss of polysulfide due to air oxidation. This direct method of measuring po-