ASTM D7318 Standard Test Method for Existent Inorganic Sulfate in Ethanol by Potentiometric Titration
11. Procedure
11.1 Sample Analysis - 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. Samples 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.
11.2 Blank Measurement:
11.2.1 Determine the blank value by analysis of a denatured ethanol sample used for making standards (100 g) using the procedure described in 11.3. An aliquot of aqueous sulfate blank solution (8.5) is added to all samples as part of the blank. This is necessary to provide a stable blank value.
11.2.2 The average of three blank measurements is recorded as Vb for use in the calculation of sulfate content of samples.
11.3 Sample Measurement:
11.3.1 Accurately weigh an ethanol sample or blank (approximately 100 g) into a 150-mL beaker, and record the weight as G.
11.3.2 Carefully pipet 0.200 mL of aqueous sulfate blank solution (8.5) into the beaker containing the ethanol sample.
11.3.3 Add 1 mL of 0.1 M perchloric acid (8.6). 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 mL of 0.1 M perchloric acid is insufficient to achieve a pH in the range of 3 to 5, add an additional millilitre of acid and recheck the pH.
11.3.4 To prepare the autotitrator 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.
11.3.5 Duplicate determinations should agree to within the repeatability limits in Section 15 and Table 2. If not, additional titrations should be performed until agreement is achieved.
11.3.6 If satisfactory titration curves are not obtained, the lead electrode should be polished (9.2). Typical titration curves for sulfate standards in denatured ethanol are shown in Fig. 1, and examples from commercial denatured ethanol samples are shown in Fig. 2.
12. Calculation or Interpretation of Results
12.1 Calculate the inorganic sulfate content of the ethanol sample as follows in accordance with Eq 5:
Sulfate (mg/kg) = [(V - Vb)(MPb)(F)]/G
where:
Sulfate = sulfate concentration, mg/kg,
V = volume of lead titrant for sample, mL (11.3.4),
Vb = average volume of lead titrant for blank, mL (11.2.2),
MPb = molarity of lead titrant, mol lead/L (as calculated by Eq 4),
F = factor, 96058 mg Sulfate/mol Sulfate, and
G = weight of ethanol sample taken for analysis, g (11.3.1).
12.2 The units of mg/kg can be converted to mg/L by multiplying the result by the density of denatured ethanol using Eq 6. Denatured ethanol density may be measured using Test Method D4052. However, for the purposes of this test method, all calculations should use mg/kg, weight/weight units.
Sulfate (mg/L) = Sulfate (mg/kg) x D
where:
D = density of denatured ethanol, g/mL.
