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D7318_11

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Obtain an ethanol sample in accordance with practice D4057 or practice D4177. The sample should be well mixed to ensure homogeneity. A representative portion shall be taken for analysis. Samples should be collected in containers with closures that seal well to prevent evaporation of ethanol. Sample containers should not contain any residual sulfate. If containers have been cleaned and rinsed with water, they must be rinsed with type III water and dried prior to use. Determine the blank value by analysis of a denatured ethanol sample used for making standards, 100 grams, using the procedure described in 11.3 of the written standard. An aliquot of aqueous sulfate blank solution, 8.5 of the written standard, is added to all samples as part of the blank. This is necessary to provide a stable blank value. The average of three blank measurements is recorded as v sub b for use in the calculation of sulfate content of samples. Accurately weigh an ethanol sample or blank, approximately 100 grams, into a 150-milliliter beaker and record the weight as g. Carefully pipette 0.200 milliliters of aqueous sulfate blank solution, 8.5 of the written standard, into the beaker containing the ethanol sample. Add 1 milliliter of 0.1 molar perchloric acid, 8.6 of the written standard. The sample should be slightly acidic to pH paper before titrating. pH should be in the range of 3 to 5. The pH can be checked by dipping a glass stirring rod into the sample and testing a droplet on the rod with a moistened pH test strip. If 1 milliliter of 0.1 molar perchloric acid is insufficient to achieve a pH in the range of 3 to 5, add an additional milliliter of acid and recheck the pH. To prepare the auto-titrator for operation, immerse the electrodes in the titration solution. Initiate stirring and titrate to the potentiometric endpoint with lead titrant. Record the volume of titrant used as v. Duplicate determinations should agree to within the repeatability limits in section 15 and table 2 of the written standard. If not, additional titrations should be performed until agreement is achieved. If satisfactory titration curves are not obtained, the lead electrode should be polished, 9.2 of the written standard. Typical titration curves for sulfate standards in denatured ethanol are shown in figure 1 of the written standard, and examples from commercial denatured ethanol samples are shown in figure 2 of the written standard.

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Posted by: abuckmaster on Sep 14, 2018

D7318_11

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