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Separation and determination of sulfides and sulfur dioxide Authors: Lü Nan, Zhang Hanqi, Wei Li, Jiang Yanxia, Jin Qinhuan, Fu Chunyan, Zhuang Hongmei. At room temperature, the volatile sulfur-containing compounds are mainly hydrogen sulfide and sulfur dioxide; their determination is important for the morphological analysis of sulfur-containing compounds. Since natural gas and industrial exhaust gases usually contain these two different forms of sulfides, developing a simple, rapid, and accurate analysis method is a prerequisite for effectively controlling pollution and making full use of chemical waste. We have previously used the dual-wavelength method to determine the contents of hydrogen sulfide and sulfur dioxide in mixtures, but this method has certain limitations. This paper uses the pre-enrichment method to separate two different forms of sulfides, and an atomic absorption spectrophotometer is employed to determine each form of sulfide separately. Through the analysis of mixed samples, it can be seen that this is a quick and effective method for separately measuring hydrogen sulfide and sulfur dioxide, featuring simple instrumentation and easy operation. It is particularly important that the introduction of this method laid the foundation for the development of sulfide testing instruments. 1 Experimental Section 1.1 Instruments HG-9001 atomic absorption spectrophotometer (Shenyang Analytical Instrument Factory). LZ-1010 modular flow injection analyzer (Shenyang Film Mirror Factory). The absorption cell is a quartz tube, 120 mm long, with an inner diameter of 5 mm and an outer diameter of 7 mm, and quartz windows on both sides. The absorption cell is fixed to the combustion head of the flame atomizer. Reactor: With an inner diameter of 10 mm and a height of 150 mm, a peristaltic pump was used for sample injection in the experiments, and the volume of the mixture inside the reactor (sample solution + reaction solution) was controlled by adjusting the liquid level within the reactor. Enricher: inner diameter 15 mm, height 200 mm. Concentrated H3PO4 cell: inner diameter 10 mm, length 100 mm, containing 7 mL of concentrated H3PO4. 1.2 Reagents The reaction solution is a dilute HCl solution. S2-standard stock solution (1.000×103 mg/L): Prepared using Na2S·9H2O and calibrated by the iodimetry method, with the calibration procedure being the same as that described in the literature. SO2 standard stock solution (1.00×103 mg/mL): Prepared using Na2SO3 and calibrated by the iodimetric method, with the calibration procedure being the same as that described in the literature. The water used in the experiments was distilled water, and all reagents were of high-purity or analytical grade. The carrier gas is nitrogen with a purity of 99.99%. 1.3 Experimental Method The experimental setup is shown in Figure 1. A peristaltic pump is used to feed in the blank sample and the reaction solution, and the liquid level is adjusted to optimize the volume of liquid inside the reactor. After the carrier gas was blown until the absorbance value reached 0.000, a certain volume of BaCl2 solution was added to the c(SO2 enricher) ； A certain volume of NaOH solution is added to the d(H2S pre-concentrator). Since barium chloride can react with sulfur dioxide under certain conditions but not with hydrogen sulfide, hydrogen sulfide can enter the d concentrator via the c concentrator. Replace the blank sample with the sample and continue injecting. After a certain volume of the sample has been introduced, turn off the pump and allow the carrier gas to be replaced with air. Add a certain volume of hydrochloric acid to d, set e to 2, use a carrier gas to carry the generated H2S into the absorption cell, read the absorbance value, and then discharge the solution in d into the waste tank. Set e to 1, add a certain amount of hydrochloric acid to c; set e to 2, and the carrier gas carries the released SO2 gas into the absorption cell, after which the absorbance value is read. Figure 1 Experimental setup for the separation and determination of H2S and SO2 a Flow meter ； b. Reactor ； c. SO2 pre-concentrator ； d.H2S pre-enricher ； e. Three-way piston ； f. Mixing tube ； g Concentrated H3PO4 tank ； h. Absorption cell ； i. Quartz window ； j. Liquid seal tube ； k, l injection port. 2 Results and Discussion Based on the absorption spectra of H2S and SO2 in the ultraviolet-visible region, a magnesium hollow cathode lamp was selected as the light source, with a wavelength of 202.6 nm. 2.1 Selection of experimental conditions for SO2 determination 2.1.1 Effect of the pH value of the enrichment solution on absorbance value The barium chloride solution is used as the SO2 enrichment solution, and the effect of the pH value of this solution on absorbance is shown in Figure 2. To control the pH of the barium chloride solution, we used an HAc-NaAc buffer for adjustment. The methods for preparing buffers at different pH values are shown in Table 1. Figure 2 Effect of the pH value of the SO2-enriched solution on the absorbance value. The SO2 concentration was 27.0 μg/mL, the concentration of HCl in the reaction solution was 1 mol/L, the flow rate of the carrier gas was 300 mL/min, the enriching agent was BaCl2, and the pH was adjusted using a buffer solution prepared from HAc and NaAc. Table 1 Preparation of buffer solutions pH VNaAc/mLVHAc/mLVNaAc/VHAc 4.10 1.0 4.0 0.25 4.65 1.0 1.0 1.0 0.52 64.0 1.0 4.0 0.58 516.0 1.0 16.0 6.0 0.24 51.0 24.5 * The concentrations of NaAc and HAc are 0.1 mol/L. As can be seen from Figure 2, the absorbance value increases as the pH value rises; at lower pH values, SO2 is not fully absorbed, resulting in a lower absorbance value ； At pH = 6, the absorbance value is maximum ； When pH > 6, part of the H2S is adsorbed, failing to achieve the purpose of separation. In the HAc-NaAc buffer system, the maximum pH is 6; even when only NaAc solution is used, since the reaction solution is HCl solution, HCl will always be carried into the enriched solution, thereby forming an HAc-NaAc buffer solution and lowering the pH. Therefore, the pH value chosen in this paper is 6. 2.1.2 Effect of carrier gas flow rate on absorbance value during SO2 separation and enrichment. Figure 3 shows the relationship between carrier gas flow rate and absorbance. The results showed that the absorbance value was highest when the carrier gas flow rate was between 300 and 400 mL/min ； If the flow rate is too low, the absorbance value will also be low, and it is possible that some of the SO2 is not carried away ； When the flow rate of the carrier gas is too high, the reaction between SO2 and the enrichment solution is incomplete due to the short residence time in the enricher. Figure 3: Effect of carrier gas flow rate on absorbance value. The concentration of SO2 is 28.0 μg/mL, the concentration of HCl in the reaction solution is 1 mol/L, the volume of the enriched solution is 1.5 mL, and the pH of the enriched solution is 6.2. 1.3 Effect of hydrochloric acid concentration in the reaction solution on the absorbance of sulfur dioxide. Figure 4 shows the effect of hydrochloric acid concentration in the reaction solution on absorbance. Figure 4 shows that the absorbance value is highest when the HCl concentration in the reaction solution is 1 mol/L ； If the HCl content is too low, the reaction is incomplete, resulting in a decrease in the absorbance value ； If the HCl content is too high, some of the HCl is carried into the enricher, which lowers the pH of the enriched solution; as a result, SO2 is not adsorbed completely, and the measured absorbance value is low. Figure 4: Effect of hydrochloric acid content in the reaction solution on absorbance value. The concentration of SO2 is 28 μg/mL, the flow rate of the carrier gas is 300 mL/min, the volume of the enriched solution is 1.5 mL, and the concentration of BaCl2 in the enriched solution is 0.1 mol/L. 2.1.4: Effect of HCl content used for releasing SO2 on absorbance. The test results show that the absorbance value increases as the HCl content increases; when the HCl content reaches 1.0 mol/L, the absorbance value no longer changes significantly with further increases in HCl content. 2.1.5 Effect of BaCl2 content in the enrichment solution When the BaCl2 content in the enrichment solution is greater than 0.05 mol/L, it has no effect on the measurement values. The concentration of the BaCl2 solution we used was 0.1 mol/L. 2.2 Selection of experimental conditions for H2S determination 2.2.1 Effect of carrier gas flow rate on absorbance value Figure 5 shows the effect of the flow rate of the enrichment and separation carrier gas on absorbance. The results in Figure 5 show that the absorbance value is highest at a flow rate of 400 mL/min. When the carrier gas flow rate is too low, H2S is not carried away completely, resulting in a decrease in absorbance ； If the carrier gas flow rate is too high, the residence time of H2S gas in the enricher is short, resulting in incomplete reaction and thus a decrease in the absorbance value. Considering the aforementioned determination of SO2, the carrier gas flow rate selected in this paper is 400 mL/min. Figure 5 Effect of the carrier gas flow rate in enrichment and separation on absorbance. The S2- concentration is 1.9 μg/mL, and the HCl concentration is 1 mol/mL. 2.2.2 Effect of the NaOH concentration in the enriched solution on absorbance value: When the HCl concentration in the reaction solution is twice that of the NaOH concentration and the volumes are equal, the absorbance increases slightly as the NaOH concentration rises. This is because an increase in concentration leads to an increase in the heat of neutralization, thereby accelerating the release of H2S. 2.2.3 Effect of HCl content in the reaction solution on absorbance: During the enrichment process, when the NaOH concentration in the enriched solution is 0.5 mol/L and the HCl concentration is between 1 and 2 mol/L, the absorbance value is at its maximum. When the HCl content is low, the reaction is incomplete, and the absorbance value decreases. When the HCl content is high, the enrichment reaction is incomplete, resulting in a decrease in absorbance. Based on the above experimental results, the following experimental conditions were selected: the pH of the BaCl2 solution in the SO2-enriched solution was set at 6 (NaAc-HAc); the concentration of BaCl2 in the SO2-enriched solution was 0.1 mol/L; the flow rate of the carrier gas (N2) was 400 mL/min; the hydrochloric acid concentration in the reaction solution was 1.0 mol/L, and the same concentration applied to the release solution; the hydroxide ion concentration in the H2S-enriched solution was 0.5 mol/L; the volume of the SO2-enriched solution was 1.5 mL, and the volume of the H2S-enriched solution was also 1.5 mL. Table 2 lists the detection limits, linear ranges, and precision of S2- and SO2. The concentration of the analyte used to determine precision was 100 times the detection limit, with 8 measurements taken. Table 2 Analytical performance for sulfur and sulfur dioxide Sample linear range/ (μg·mL-1) Detection limit/ (μg·mL-1) RSD (%) Enrichment factor S2-: 0.04–50.0, 22.1%, 2.3% SO2: 0.2–400.0, 8.1%, 1.9%, 2.3% Interferences At 202.6 nm, the interference of some co-existing anions on the determination of S2- and SO2-3 was investigated. When the concentration of S2- is 1.02 μg/mL and that of SO2-3 is 10 μg/mL, 100 times the concentrations of Br-, CO2-3, SCN-, and NO-3, as well as 10 times the concentration of NO-2, do not interfere with the determination. 2.4 Analysis of mixed samples Several synthetic samples were analyzed under the experimental conditions described above, and the test results are listed in Table 3. As can be seen from Table 3, the ratio of SO2 to H2S in the mixed samples ranges from 2 to 100 times; this method can be used for determination as long as the concentration of the sample being tested falls within its linear range. The results show that this method can be used for the separation and determination of S2- and SO2-3. Table 3 Sample analysis results Mixed sample/(μg·mL-1) Measured value/(μg·mL-1) Recovery rate (%) H2SSO2 H2SSO2 H2SSO2 0.10 10.00 0.10 9.40 100 94 0.20 10.00 0.21 9.60 105 96 0.50 10.00 0.54 9.20 108 92 1.00 10.00 1.00 9.80 100 98 2.00 10.00 2.01 9.90 101 99 5.00 10.00 4.90 10.01 98 101 (end)