1、 uop IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO DETERMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH AND SAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THIS PROCEDURE IN THE MANNER PRES
2、CRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS (MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED IN THIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTION EQUIPMENT (PPE). COPYRIGHT 1964, 1967, 1987, 1989, 2005, 2010
3、 UOP LLC. All rights reserved. Nonconfidential UOP Methods are available from ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. The UOP Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service at servicea
4、stm.org, 610.832.9555 FAX, or 610.832.9585 PHONE. Hydrogen Sulfide and Mercaptan Sulfur in Liquid Hydrocarbons by Potentiometric Titration UOP Method 163-10 Scope This method is for determining hydrogen sulfide and mercaptan sulfur in hydrocarbons by potentiometric titration. Typical samples include
5、 gasoline, naphtha, light cycle oils and similar distillates that are liquid at ambient temperature and pressure. The lower limit of quantitation is 0.2 mass-ppm mercaptan (as sulfur) and 1.0 mass-ppm hydrogen sulfide (as sulfur). References UOP Method 41, “Doctor Test for Petroleum Distillates,” ww
6、w.astm.org UOP Method 999, “Precision Statements in UOP Methods,” www.astm.org Outline of Method The liquid hydrocarbon sample is weighed into 2-propanol containing a small amount of ammonium hydroxide. The solution is titrated potentiometrically with alcoholic silver nitrate using a glass reference
7、 and silver-silver sulfide indicating electrode system. Hydrogen sulfide and mercaptan sulfur concentrations are calculated as mass-ppm. Free sulfur complicates the potentiometric curve, and instructions are given for interpreting the curve when free sulfur is present. Apparatus References to catalo
8、g numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Balance, readability 0.1-mg Beaker, Berzelius, tall form, 400-mL, Fisher Scientific, Cat. No. 10-310-9 Beakers, electrolytic, 250-mL, Brinkmann Instruments, Cat. No. 020212209, available from Metro
9、hm USA, two or more required Cleaning pad, synthetic, mildly abrasive, Scotch-Brite, Runco Office Supply, Cat. No. MMM-96 Cylinder, graduated, 100-mL, Class B, VWR, Cat. No. 89000-270 2 of 7 163-10 Desiccator, 160-mm ID, and porcelain plate, VWR, Cat. Nos. 25035-005 and 89038-068, respectively Elect
10、rode, combination silver/glass Titrode, 6.0430.100, Brinkmann Instruments, Cat. No. 020948507, available from Metrohm USA. The electrode should be dedicated to sulfur analysis. Flasks, volumetric, Class A, 500-mL, VWR, Cat. No. 89090-748, two required Pipets, volumetric transfer, Class A, 1-, 8-, an
11、d 50-mL, VWR, Cat. Nos. 89045-056, -070, and -086, respectively Regulator, nitrogen, two-stage, high-purity, delivery pressure range 15-200 kPa (2-30 psi), Matheson Tri-Gas, Model 3121-580 Rubber bulb-type pipet filler, VWR, Cat. No. 53502-205 Titrator, potentiometric, recording, 2000-mV range, 1-mV
12、 resolution, capable of reducing the titration rate to a minimum in the vicinity of the endpoint, with dispenser having a volume readout of 0.00 - 99.99 mL, 0.0001 of the buret volume resolution, Metrohm Model 836 Titrando system with optional sample changer, and two 20-mL burets, Brinkmann Instrume
13、nts, Metrohm USA Automatic titration using the titrator is recommended. However, titration can be performed manually using the following equipment. Buret, 50-mL, Class A, with PTFE stopcock, VWR, Cat. No. 89001-624 pH meter, digital, Mettler Toledo, Model S20K, VWR, Cat. No. 11238-592 Stirrer, magne
14、tic, with stir bar, VWR, Cat. No. 12365-380 Reagents and Materials References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Unqualified references to solutions mean aqueous solutions. Ammonium hydroxide, 28.0-30.0 mass-% NH3, Certifie
15、d ACS Plus grade, Fisher Scientific, Cat. No. A669-212 Desiccant, 8-mesh, indicating, Drierite, VWR, Cat. No. 22891-028 Detergent, LiquiNox, VWR, Cat. No. 21837-005 Nitrogen, high purity, 99.99% Paper, test, lead acetate, strips in vials, VWR, Cat. No. 60792-009 2-Propanol, Certified ACS grade, VWR,
16、 Cat. No. VW5520-3. Purge with nitrogen for 10 to 15 minutes before using. Silver nitrate, 0.1000-N solution, VWR, Cat. No. VW3214-2 Silver nitrate, alcoholic solution, 0.0100-N. Prepare a 0.0100-N alcoholic silver nitrate solution by pipetting 50 mL of the standardized 0.1000-N solution into a 500-
17、mL volumetric flask. Dilute to the mark with 2-propanol, cap and invert several times to mix thoroughly. This solution is stable for two months, after which a new solution is prepared. Silver nitrate, alcoholic solution, 0.0010-N. Prepare a 0.0010-N alcoholic silver nitrate solution by pipetting 50
18、mL of the standardized 0.0100-N solution into a 500-mL volumetric flask. Dilute to 3 of 7 163-10 the mark with 2-propanol, cap and invert several times to mix thoroughly. This solution is stable for two months, after which a new solution is prepared. Sodium sulfide nonahydrate, VWR, Cat. No. EM-SX07
19、70-1 Sodium sulfide nonahydrate, 1% solution. Dissolve 1.0 g 1.0 mg of sodium sulfide in 100 mL of water. Toluene, 99.8% minimum purity, VWR, Cat. No. AA43061-K2. Purge with nitrogen for 10 to 15 minutes before using. Water, deionized or distilled Wipers, Kimwipes Ex-L, VWR, Cat. No. 21905-026 Proce
20、dure The analyst is expected to be familiar with general laboratory practices, the technique of titration, and with the equipment being used. Electrode Preparation 1. Prepare the silver electrode by cleaning the surface of the silver band on the electrode with a mild detergent. Prepare this electrod
21、e fresh daily or as required. Proper electrode preparation is essential to obtain reproducible and noise-free titration curves having good distinguishable endpoints. 2. Rinse the electrode with water and wipe dry with a wiper. 3. Polish the surface of the electrode gently with a cleaning pad until t
22、he surface is smooth to the touch. 4. Rinse the electrode with water again and wipe dry with a wiper. 5. Immerse the electrode in a solution consisting of 1 mL of concentrated ammonium hydroxide, 96 mL of 2-propanol and 8 mL of 1% aqueous sodium sulfide solution. Add slowly from a buret over a perio
23、d of about 10 minutes, with stirring, approximately 10 mL of 0.0100-N alcoholic silver nitrate solution. A film of silver sulfide will be deposited on the silver. Wipe off the excess silver sulfide on the electrode with a wiper. The automatic titrator may be used instead of the buret, titrating a mi
24、nimum of 10 mL of titrant. During the titration of samples containing sulfide, the electrode will be coated with AgS. It is necessary to repeat the electrode preparation when the coating begins to peel, becomes rough and uneven, or if the titration curves become noisy. To remove any residue from pre
25、vious titrations, it is good practice to dip and swirl the electrode in mild detergent and rinse with water at the at the beginning of each set of samples. This aids in keeping the titration curves noise free and reduces the chance of the appearance of false endpoints. Titration of Sample Mercaptans
26、 oxidize easily. It is important that samples are handled quickly in order to minimize their exposure to air. Titrate samples immediately after they are pipetted while covering the beaker with a nitrogen blanket during the analysis. 1. Prepare the titrator for operation with the proper electrode in
27、place and with the appropriate molarity alcoholic silver nitrate solution (see Table 1) as the titrant. Set the operating parameters on the instrument to reduce the rate of titrant addition in the vicinity of the endpoints. 4 of 7 163-10 2. Before titrating, check the contents of the sample qualitat
28、ively for hydrogen sulfide by suspending lead acetate paper in the vapor space above the sample. Moisten a piece of lead acetate paper with water and fasten one end of the paper to the stopper of a bottle containing the sample. Do not allow the paper to contact the hydrocarbon sample. Allow it to st
29、and for about one minute. If the paper turns silver-black, hydrogen sulfide is present, and the test is positive. This test is sensitive to approximately 1 mass-ppm of hydrogen sulfide. Some mercaptans will cause lead acetate paper to darken with a tan to yellow coloration. This is not to be confuse
30、d with a positive hydrogen sulfide test. The Doctor Test, UOP Method 41, “Doctor Test for Petroleum Distillates,” may be run for confirmation if the lead acetate paper test is difficult to read. 3. Add the recommended volume of 2-propanol to the appropriate size beaker (see Table 1). Add approximate
31、ly 1 mL of concentrated ammonium hydroxide. 4. Weigh a portion of the sample, to the nearest 0.1-mg, into the beaker while maintaining the solution under a nitrogen blanket. Use Table 1 as a guide in selecting the proper sample size. The sample size should be selected according to Table 1 to give a
32、titration of at least 2 mL. When the sample contains more than 500 ppm sulfur, select a sample size sufficient to give a 5- to 10-mL titration. If the sample is insoluble in the 2-propanol, add enough nitrogen-purged toluene to the beaker containing isopropyl alcohol to obtain a solution. Solutions
33、containing high levels of mercapans and low levels of hydrogen sulfide are titrated separately for the two components. A sample is titrated as above to the mercaptide endpoint, and then a larger sample is titrated only to the hydrogen sulfide endpoint. The mercaptan concentration is determined by di
34、fference between concentration determined from the total titration to the mercaptide endpoint, and the hydrogen sulfide concentration determined from the larger sample. If the ratio of sulfide sulfur to mercaptan sulfur is greater than 10:1, then the mercaptan result is considered to be qualitative
35、because the H2S endpoint overshadows the mercaptan endpoint. Table 1 Selection of Equipment, Reagents, and Sample Volumes Expected mercaptan concentration, ppm, S Beaker size, mL 2-Propanol, mL Suggested sample mass, g Normality of Titrant, N 0.5-1 400 150 100 0.001 1-100 250 100 50 0.01 100-300 250
36、 100 10 0.01 300-500 250 100 5 0.01 5. Add a stirring bar, and place the beaker in the titration apparatus. Insert the electrode and stir, covering the solution with a nitrogen blanket to prevent mercaptan oxidation. Adjust the stirring speed to create a vortex in the solvent such that bubbles do no
37、t develop at its center. Titrate with the alcoholic silver nitrate solution, manually or automatically depending on the equipment available. Observe the respective inflections and note the corresponding volumes of titrant. If the titration is to be performed manually, add the silver nitrate solution
38、 in 1-mL increments, recording the volume and emf after each addition. Near the endpoints, add the silver nitrate solution in 0.1-mL increments. Allow enough time for the titration cell to reach equilibrium before recording the volume of silver nitrate solution and the emf. The endpoints are found w
39、here the rate of change in potential with respect to the volume of added titrant is a maximum. Plot the titration data as emf vs. volume of silver nitrate solution. Estimate the mid-point of each inflection by inspecting the titration curve. Alternatively, the endpoints may be determined by 5 of 7 1
40、63-10 examining the titration data for the emf/mL maxima, thereby obviating the task of plotting the data. This procedure can be followed by an experienced operator. Interpretation of Titration Curves The Figure shows titration curves that can be obtained for gasolines having different ratios of hyd
41、rogen sulfide, mercaptan and free sulfur (including polysulfides). Curve A is an example of a titration where only hydrogen sulfide is present. Curve B is an example of a titration where only mercaptan is present. Curve C is an example of a titration where both hydrogen sulfide and mercaptan are pre
42、sent. Curves A, B, and C are the titration curves most commonly encountered. When free sulfur is present in the sample, it reacts with some of the mercaptan forming polysulfide, the titration of which complicates the interpretation of the titration curve. Curve D is an example of a titration where f
43、ree sulfur has reacted with a portion of the mercaptan to form polysulfide and, thus, three inflections are observed: hydrogen sulfide, polysulfide and the remaining mercaptan. Curve E is an example of a titration where the free sulfur has reacted with a portion of the mercaptan to form a polysulfid
44、e but no hydrogen sulfide was present and, thus, only two inflections are observed: the first for polysulfide and the second for mercaptan. Disulfides do not show an inflection. One Inflection (Curve A or B) When the lead acetate test is positive for hydrogen sulfide and only one inflection occurs,
45、the only species present is hydrogen sulfide (Curve A). The hydrogen sulfide content is determined from the total titration, from zero volume to the inflection point. When the lead acetate test is negative, the only species present is mercaptan (Curve B). The mercaptan content is determined from the
46、 total titration, from zero volume to the inflection point. Two Inflections (Curve C or E) When there are two inflections and the lead acetate test is positive, the first inflection is hydrogen sulfide, and the second inflection is mercaptan (Curve C). The hydrogen sulfide content is determined from
47、 the volume of titrant from zero volume to the first inflection point. The mercaptan 6 of 7 163-10 content is determined from the volume of titrant used from the first inflection point to the second inflection point. When the lead acetate test is negative, and the absence of H2S is confirmed by UOP
48、Method 41, Doctor Test, disregard the first inflection point. The mercaptan content is determined from the total titration, from zero volume to the second inflection point (Curve E). Three Inflections (Curve D) When there are three inflections, and the lead acetate test is positive, the first inflec
49、tion point is hydrogen sulfide. Disregard the second inflection point. The mercaptan content is determined from the volume of titrant from the first inflection point to the third inflection point. Three inflections are not normally seen when the lead acetate test is negative. Verify the sulfur types in the sample using UOP Method 41, Doctor Test, and check the equipment for proper operation. Calculations Calculate the concentration of hydrogen sulfide and mercaptan, as sulfur, using Equations 1 and 2, respectively. Report the results to the neare
