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D3227_9 (16)

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If the sample is to be measured volumetrically, determine the density by test method D1298 or test method D4052 at the temperature at which the test portion will be taken, either directly or from the density determined at a reference temperature and converted to the transfer temperature by use of the Petroleum Measurement Tables guide D1250. For the removal of hydrogen sulfide, reference section 9.2 through 9.2.3 of the written standard. Measure with a pipette or other suitable transfer device or weigh 20 to 50 milliliters of the original or treated sample into an appropriately sized beaker. For example, a 200-, 250-, or 300-millimeter-sized beaker is typically large enough, containing 100 milliliters of the appropriate titration solvent. Place the beaker on the titration stand or on the auto sampler of an automatic titration system. If an automatic titration system is used, set up the system to reproduce the experimental conditions specified in 9.3.1 through 9.3.3 of the written standard. Adjust the position of the tritration stand so that the electrodes are about half immersed. Fill the burette with 0.01 moles per liter alcoholic silver nitrate solution and position it in the tritration assembly so that the tip extends approximately 25 millimeters, 1 inch, below the surface of the liquid in the beaker. Adjust the speed of the stir to give vigorous stirring without spattering. Record the initial burette and cell potential readings. The usual meter readings for mercaptan presence are in the negative 250 millivolts to negative 350 millivolts range. At suitable small portions of 0.01 moles per liter silver nitrate solution and after waiting until a constant potential has been established, record the burette and meter readings. Consider the potential constant if it changes less than 6 millivolts per minute. If potential readings obtained with freshly prepared electrodes are erratic, it is possible that the electrodes are not properly conditioned. This difficulty usually disappears in succeeding titrations. When analyzing samples with a relatively high mercaptan concentration, the use of a 20 to 50 milliliter sample size can require an unreasonably long time titration time and a large quantity of the titrant. For such samples, a smaller sample size may be used, or the original sample may be diluted with the suitable solvent that is miscible with the sample and free of mercaptans into the 20- to 50-milliliter sample size range prior to titrating the sample. The precision of samples using less than 20 milliliter sample sizes or samples that have been diluted have not been determined in an inter-laboratory study. With certain instruments, the algebraic sign of the potentials may appear reversed. When the potential change is small for each increment of silver nitrate solution, add volumes as large as 0.5 millimeters. When the change of potential becomes greater than 6 millivolts per 0.1 milliliters, use 0.05 millimeter increments of 0.01 moles per liter silver nitrate solution. Near the end point of the titration, 5 or 10 minutes may elapse before a constant potential is obtained. Although it is important to wait for equilibrium conditions, it is also important that the duration of the titration be as short as possible to avoid oxidation of the sulfur compounds by atmospheric oxygen. Once started, a titration must never be interrupted and resumed later. Continue the titration until the meter reading change of the cell potential per 0.1 milliliters of 0.01 molar silver nitrate solution has become relatively constant. Consider the potential constant if it changes less than 6 millivolts per minute. Remove the titrated solution. Rinse the electrodes with alcohol, and wipe with a dry tissue. If an automatic titration system is used, rinsed the electrodes well with alcohol. Allow the excess alcohol to drain off the electrode, and then proceed with the next sample. Between successive determinations, or batches of determinations in the case of automatic titration systems, on the same day, store the electrodes in accordance with 8.1 and 8.2.5 of the written standard. As often as is required and preferably at least daily, when the test equipment is being used, perform a blank titration following 9.3 through 9.3.3 of the written standard without adding a test sample.

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Duration: 6 minutes and 36 seconds
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Language: English
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Posted by: abuckmaster on Aug 23, 2018

D3227_9 (16)

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