In order to screen samples of return underground local-water of oil and gas condensate fields for the presence of dissolved hydrogen sulfide, it was proposed to use single comparison sample for visual binary testing of H2S. Two indicator reactions occurring in solutions were selected — the formation of a stabilized suspension of MnS and CdS. Suspensions prepared in the presence of borate buffer (pH 9.18) and gelatin (stabilizer) are stable during the twenty four hours; a difference in the turbidity of suspensions can be observed near the normalized concentration of hydrogen sulfide (сlim = 15 mg L-1). When establishing the concentration of H2S in the comparison samples, a statistical approach was applied; the estimated values of the threshold concentration of H2S (ccomp.) are less than the normalized level on the value which providing the risk of a false-negative test result not more than 5% - 11.6 mg L-1 for MnS and 13.4 mg L-1 for CdS. Preference was given to a test system based on a stabilized suspension of CdS, because for this reaction, a narrower interval of unreliability and the value of ccomp. is nearer to the value of the сlim. The correctness of the visual binary testing of H2S in the samples of the analyzed water was confirmed by spectrophotometric method with p-phenylenediamine.

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The method of the methanol content control in samples of return underground local-water of oil and gas condensate fields is described. The determination is based on a preliminary estimation of the chemical oxygen demand index, using an accelerated technique with potentiometric redox titration of potassium dichromate excess. On water-methanol solutions of known composition using the “added-found” method, it was proved that under the conditions of the chosen procedure, the methanol is completely oxidized; the value of the relative standard deviation of the determination results does not exceed 3 %. The interfering influence of the components of highly mineralized water samples on the integral assessment of chemical oxygen demand was studied. The accuracy of the determination of methanol in real water samples is proved by the method of standard additives.