ASTM E1787-16 Standard test method for Anions in Caustic Soda and Caustic Potash by Ion Chromatography
Anionic impurities in caustic soda and caustic potash are monitored by manufacturers and users for quality control of the products. Anions of primary interest are chloride, chlorate, and sulfate. The ASTM E1878-16 test method can determine precision estimates for these three impurities.
ASTM E1787-16 is used to determine anionic impurities in 50% Caustic Soda (Sodium hydroxide) and 50 % Caustic potash (Potassium hydroxide) solutions using ion chromatography (IC). Anions that can be determined at a concentration of 0.1 to 100 ppm include bromide, chlorate, chloride, fluoride, nitrate, phosphate, and sulfate. By varying sample size we can use this method for anhydrous caustic soda and caustic potash.
In ASTM E1787-16 initially the ion chromatograph and neutralization device is set as per the manufacturer’s instructions. The detector range is changed as per concentration of anions in the sample. The system is equilibrated by pumping eluent through the columns and the detector until a stable baseline is obtained (approximately 15 to 20 min). Sample solutions are prepared by taking the known weight of samples in a 100 mL volumetric flask and diluted to volume using water. The dilution ratio depends upon the anions concentrations in the sample. The neutralization module is capable of neutralizing a solution containing 25% sodium hydroxide when the solution is cycled through the neutralizer at least twice. Sample solutions containing up to 50 g of 50 % product or 25 g of anhydrous sample, therefore, can be neutralized effectively. The sample is injected into an ion chromatograph and the ion chromatogram of anions is recorded. Figure 1 shows an IC chromatogram of 45 % caustic potash that was diluted 20 g/100mL.
Figure 1: Typical Chromatogram of 45 % Caustic Potash (Diluted 20 g/100 mL) Using AS-12A Separator Column and 2.7 mmol/L Na2CO3/ 0.3 mmol/L NaHCO3 Eluent.
50 % concentration solution of Caustic soda and Caustic potash.
ASTM E1787-16 calculates the concentration of each anion of interest using the peak area of that anion from the sample chromatogram and response factor derived from “best-fit” linear equation obtained during the calibration procedure.
A = area of anion peak from the chromatogram of the diluted sample, area counts,
B = final volume of diluted sample, mL,
R = response factor from “best-fit” calibration line, (μg/mL)/area counts, and
G = sample weight, g.
Concentration of the sodium or potassium salt of the anion can be calculated as follows:
Hence, we can determine the anions concentration in caustic soda and caustic potash using ion Chromatography using ASTM E1787-16 method.
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