ASTM D7318 Standard Test Method for Existent Inorganic Sulfate in Ethanol by Potentiometric Titration
8. Preparation of Standard Solutions
8.1 Lead Nitrate Titrant, 0.0025 M - Dissolve 0.833 g lead nitrate in 300 mL water. Pour into a 1-L bottle and fill with denatured ethanol and mix well. Standardize in accordance with 10.1.
8.1.1 Alternatively, this solution may be purchased from a commercial vendor, and its exact molarity shall be determined in accordance with 10.1.
8.2 Aqueous Sulfate Standard, 0.01 M - Dry 5 g anhydrous sodium sulfate at 110°C for 1 h. Remove it from the oven, and allow it to cool in a desiccator over anhydrous calcium sulfate. Accurately weigh about 0.70 g on an analytical balance to the nearest tenth of a milligram, and place it in a 500-mL volumetric flask. Add Type III water to dissolve the sodium sulfate, then dilute to volume. Calculate the exact concentration in accordance with Eq 1.
G/[(142.02)(0.500)] = Molarity
Molarity = molarity of sulfate standard solution, mol/L,
G = weight in grams of Na2SO4, dissolved in 500 mL, and
142.02 = gram molecular weight of Na2SO4.
8.3 Aqueous Sulfate Stock Solution for Standards in Ethanol, 2000 mg/L - Accurately weigh 2.95 g anhydrous sodium sulfate to the nearest tenth of a milligram and transfer it to a 1-L volumetric flask. (Dried anhydrous sodium sulfate should be stored in a desiccator.) Add Type III water to dissolve the sodium sulfate, and make to volume. Calculate the concentration of sulfate in the solution in accordance with Eq 2.
Aqueous Stock Sulfate (mg/L) = (g Na2SO4)(0.6764)(1000 mg/g)/1 L
g Na2SO4 = weight in grams of Na2SO4 dissolved in 1 L, and
0.6764 = fraction of sulfate in Na2SO4.
8.4 Sulfate Standards in Ethanol - Ethanol (denatured containing no measurable sulfate) is weighed into a container (equipped with a closure to prevent evaporation) in accordance with Table 1 to achieve the desired standard. Aqueous sulfate stock solution from 8.3 is added to the solution in accordance with Table 1, and the final weight of the solution recorded. Standards should be remade weekly or if recovery of less than 90% is noted. The concentration of the standard is calculated by dividing the number of milligrams sulfate from the sulfate stock solution and dividing by the final solution weight in accordance with Eq 3.
EtOH Sulfate Standard (mg/kg) = (V x C)/W
V = volume of aqueous sulfate stock (8.3), mL,
C = concentration of aqueous sulfate stock (8.3), mg/L, and
W = final weight of ethanol and aqueous sulfate stock aliquot, g.
8.5 Aqueous Sulfate Blank Solution, 0.01 M - This is the same solution as in 8.2. This solution contains sulfate and will be added to all samples to allow a measurable sulfate blank to be measured.
8.6 Dilute Perchloric Acid, 0.1 M - Dissolve 8.8 mL perchloric acid (7.3) in 250-mL water in a 1-L volumetric flask. Mix well and dilute to the mark with Type III water.
8.7 Lithium Chloride in Ethanol, 1 M - Add lithium chloride (8.49 g) to absolute ethanol (200 mL) while stirring until it is dissolved.
9. Titration Equipment Preparation
9.1 Titrator - Prepare the titrator by filling the titrator reservoir with lead titrant (8.1). Follow the manufacturer's procedure for filling the buret. Perform titrations in monotonic titration mode, using either 25-µL or 50-µL titrant addition increments. Use a 10 mV/min drift condition or 20-s wait time between additions (whichever is achieved first).
9.2 Electrode Preparation - Proper care of the lead–selective electrode is essential for obtaining high–quality titration curves. Preparation of the lead electrode should be performed as specified by the manufacturer. A lead electrode utilizing a solid crystal sensor requires polishing when performance deteriorates. The voltage range for a 10 ppm sulfate in ethanol titration should span at least 50 mV and produce an acceptably shaped titration curve (see Fig. 1). If this level of electrode performance is not met, this indicates a need for lead electrode polishing. The lead electrode normally comes supplied with a polishing grit and a cloth rectangle. The polishing grit medium is placed on the cloth, wet with ethanol or water, moved over the surface of the cloth for a minute, rinsed with water to remove the polishing medium, and wiped dry with a tissue. The electrode is then soaked in lead titrant (8.1) for 2 min to re-equilibrate the electrode surface. The double junction reference electrode is filled with 1 M lithium chloride in absolute ethanol (8.7) in the outer chamber.
10. Standardization
10.1 Measure the exact concentration of the lead nitrate titrant by titration of the sulfate standard (8.2).
10.1.1 Transfer 1.00 mL of the 0.01 M aqueous sulfate standard solution (8.2) into a 150-mL beaker. Record this volume as V.
10.1.2 Add approximately 100 mL of denatured ethanol (7.4) and 1 mL of 0.1 M perchloric acid (8.6).
10.1.3 Prepare the autotitrator for operation, immerse the electrodes in the titration solution, initiate stirring, and titrate to the potentiometric endpoint with lead titrant. Titration should require approximately 4.0-mL of lead solution to reach the endpoint. Record this volume as T.
10.2 Titrate a blank denatured ethanol sample in the same manner without addition of sulfate standard. Record this volume as Tb.
10.3 Calculate the concentration of the lead nitrate titrant as shown in Eq 4. The standardization should be repeated until precision comparable to that shown in Section 15 and Table 2 has been achieved.
MPb = (V x S)/(T - Tb)
where:
MPb = molarity of lead in the titrant, mol/L,
V = volume of sulfate standard solution added, mL (10.1.1),
S = molarity of the sulfate solution from 8.2, mol/L,
T = volume of lead titrant used to titrate the sulfate solution, mL, and
Tb = volume of lead titrant used for the blank, mL (10.2).
