ANEW reagent, (2E,2'Z) -2,2'- (3- ((E)- (2-hydroxyphenyl) ..........diazenyl) pentane-2,4- diylidene) bis (hydrazinecarbothio-amide) (OPTS) was synthesized and studied. Acid-base, spectrophotometric properties of OPTS were studied in 50% ethanol-water mixture solutions at pH 3–11. Dissociation constants of the reagent OPTS have been determined by spectrophotometry and potentiometric titrations: pK