ASTM D7319 Standard Test Method for Determination of Existent and Potential Sulfate and Inorganic Chloride in Fuel Ethanol by Direct Injection Suppressed Ion Chromatography
9. Preparation of Standard Solutions
9.1 Stock Solutions:
9.1.1 Sulfate Stock Solution, approximately 2000 mg/L - To ensure dryness, place anhydrous sodium sulfate (5 g) in a drying oven at 110°C for at least an hour, cool, and store in a desiccator. Accurately weigh 2.96 g anhydrous sodium sulfate to the nearest tenth of a milligram and transfer to a 1-L volumetric flask. Add water to dissolve the sodium sulfate, and make to volume. Calculate the concentration of sulfate in the solution in accordance with Eq 1. Other volumes of stock solution can be prepared using the appropriate ratio of reagents.
Stock Sulfate (mg/L) = (g Na2SO4)(0.6764)(1000 mg/g)/1 L
where:
g Na2SO4 = weight in grams of Na2SO4 dissolved in 1 L, and
0.6764 = weight percent sulfate in Na2SO4.
9.1.2 Chloride Stock Solution (approximately 2000 mg/L) - To ensure dryness, place sodium chloride (5 g) in a drying oven at 110°C for at least an hour, cool, and store in a desiccator. Accurately weigh 3.30 g dried sodium chloride to the nearest tenth of a milligram and transfer to a 1-L volumetric flask. Add water to dissolve the sodium chloride and make to volume. Calculate the concentration of chloride in the solution in accordance with Eq 2. Other volumes of stock solution can be prepared using the appropriate ratio of reagents.
Stock Chloride (mg/L) = (g NaCl)(0.6068)(1000 mg/g)/1 L
where:
g NaCl = weight in grams of NaCl dissolved in 1 L, and
0.6068 = weight percent chloride in NaCl.
9.2 Chloride and Sulfate Standards in Water - Add water and sulfate and chloride stock solutions are added to a 1-L glass volumetric flask in accordance with Table 1 to achieve the desired standard.
9.2.1 Chloride and sulfate stock solutions from 9.1 are added quantitatively into the flask and mixed quantitatively with water in accordance with Table 1. Be very careful to measure the exact volumes of the sulfate and chloride stock solutions that are added to the flask, and fill the flask to 1.00 L with water. The sulfate and chloride concentrations of each standard are calculated in accordance with Eq 3 and Eq 4.
Sulfate in Standard (mg/L) = (Va x Ca)/V
Chloride in Standard (mg/L) = (Vb x Cb)/V
where:
Va = volume of sulfate stock solution (9.1.1), in mL,
Ca = concentration of sulfate stock (Eq 1), in mg/L,
Vb = volume of chloride stock solution (9.1.2) in mL,
Cb = concentration of chloride stock (Eq 2), in mg/L,
V = final volume of standard solution, in L.
9.2.2 Multiples or fractions of Table 1 values can be used to prepare different volumes of standards, but Eq 3 and Eq 4 are still used to calculate standard ion concentrations.
NOTE 3 - Alternatively, commercially available stock calibration solutions can be used, provided that the solutions are traceable to primary stock solutions or certified reference materials and are free from other analytes.
10. Calibration
10.1 Set up the ion chromatograph in accordance with the manufacturer's instructions. No specific parameters are given here since different equipment will require differing eluent, flow conditions, and instrument settings. Calibrate the ion chromatograph with at least five levels of sulfate and chloride, starting near but above the minimum detection limit, and further defining the working range in samples subsequently to be analyzed. Use concentrations of calibrant solutions that bracket the expected range for the samples to be analyzed. Use one or more mid-range standards shall be used to verify the linearity of the calibration plot.
10.1.1 Typical ion chromatographic conditions:
Flow: 0.7 mL/min
Suppressor flow: 0.5 to 1.0 mL/min
Sample loop: 20 µL
10.1.2 Other analytical conditions can be used per the manufacturer's instructions. It is important that the resulting chromatogram contain chloride and sulfate peaks with baseline separation like that shown in Fig. 1. It is the user's responsibility to determine retention times for each analyte ion. If present in sufficient quantity, other anions can interfere with the chloride and sulfate measurements.
NOTE 4 - The sample loop volume will vary based on the column capacity, sensitivity, and other factors. Refer to ion chromatography equipment manuals and column information for machine-specific details.
10.1.3 Establish analytical curves at only one detector scale setting in order to prevent a change of slope affecting the analytical curve.
10.2 Verify the analytical calibration plot daily or whenever samples are to be run, prior to the analysis of samples to verify the system resolution, calibration, and sensitivity as part of the quality verification process (see Section 14).
10.3 Rerun the sulfate and chloride calibration plots after any change of the ion chromatography eluent solution from 8.3, to reestablish ion retention times and resolution.
10.4 Measurement of the calibration standards. Inject 20 µL of each calibration solution from 9.2 into the ion chromatograph, and measure the areas of the peaks corresponding to sulfate and chloride ions. An ion chromatogram of a 1 mg/L calibration solution is shown in Fig. 1 (other anions, if present, will elute as shown). It is the user's responsibility to determine retention times for each analyte ion.
10.5 Construct the sulfate and chloride calibration plots by plotting the peak area counts against the sulfate and chloride ion concentrations. Use linear regression to determine the best straight line calibration, the plots should each have a linear least squares correlation coefficient of 0.99 or greater, see Figs. 2 and 3. The response factor for each ion, Rf, is the slope of the calibration plot straight line, in mg/L/(area count).
NOTE 5 - If the plot of the peak area values against the ion concentrations is not linear (the correlation factor should be at least 0.99), the procedure should be checked for errors, and if necessary, the calibration should be repeated starting from Section 9.
