ASTM D7328 Standard Test Method for Determination of Existent and Potential Inorganic Sulfate and Total Inorganic Chloride in Fuel Ethanol by Ion Chromatography Using Aqueous Sample Injection
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 Type II water to dissolve the sodium sulfate and make to volume. Calculate the concentration of sulfate in the solution according to Eq 1. Other volumes of stock solution may 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 ofa milligram and transfer to a 1 L volumetric flask. Add Type II water to dissolve the sodium chloride and make to volume. Calculate the concentration of chloride in the solution according to Eq 2. Other volumes of stock solution may 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 - Type II water and sulfate and chloride stock solutions are added to a 1 L glass volumetric flask according to Table 1 to achieve the desired standard. These standard solutions should be discarded and remade every month.
9.2.1 Chloride and sulfate stock solutions from 9.1 are added quantitatively into the flask, mixed quantitatively with Type II water according to Table 1. Be very careful to measure the exact volumes of the sulfate and chloride stock solutions that are added to the flask, and to fill the flask to 1.00 L with Type II water. The sulfate and chloride concentrations of each standard are calculated according to Eq 3(a) and Eq 3(b).
sulfate in standard (mg/L) = Va x Ca/V (a)
chloride in standard (mg/L) = Vb x Cb/V (b)
where:
Va = volume ofsulfate stock solution (for example, Table 1, Column 3), in mL,
Ca = concentration of sulfate stock (Eq 1), in mg/L,
Vb = volume of chloride stock solution (for example, Table 1, Column 2), in mL,
Cb = concentration of chloride stock (Eq 2), in mg/L, and
V = final volume of standard solution, in L.

9.2.2 Multiples or fractions of Table 1 values can be used to prepare different total volumes (V) of standards, using Eq 3(a) and Eq 3(b) to calculate standard ion concentrations.

NOTE 1 - 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.

9.3 Hydrogen Peroxide Solution, 0.90 % - Carefully pipette 3.0 mL of the 30 % hydrogen peroxide reagent into a 100 mL volumetric. Add Type II water to make to volume.

NOTE 2 - Hydrogen peroxide will degrade in aqueous solution, therefore prepare fresh 0.90 % solution daily.

10. Calibration
10.1 Set up the ion chromatograph according to the manufacturer's instructions. No specific parameters are given here since different instruments may require different eluents, 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 to be analyzed. The concentrations of calibrant solutions used shall bracket the expected range for the samples to be analyzed. One or more mid-range standards shall be used to verify the linearity of the calibration plot.
10.1.1 Typical ion chromatographic conditions (conditions used to generate Fig. 1 chromatograms):
Flow: 1.5 mL/min
Sample loop: 25 µL
Suppressor flow: 2 mL/min
10.1.1.1 Other analytical conditions may 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. If present in sufficient quantity, other anions can interfere with the chloride and sulfate measurements.

NOTE 3 - The sample loop volume will vary based on the column capacity, sensitivity, and other factors. Refer to vendor ion chromatography equipment manuals and column information for machine-specific details.

10.1.2 Analytical calibration plots shall be established at only one detector scale setting in order to prevent a change of slope affecting the analytical curve.

10.2 The analytical calibration plot shall be verified 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 The sulfate and chloride calibration plots shall be rerun after any change of the ion chromatography eluent solution (8.3), to reestablish ion retention times and resolution.

10.4 Measurement of the calibration standards. Inject 25 µL of each calibration standard solution (9.2) into the ion chromatograph, and measure the areas of the peaks corresponding to the 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).

10.5 Construct the calibration plots from the peak area measurements 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 coefficients 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 4 - If the plot of the peak area values least squares correlation is less than 0.99, the instrument and calibration solutions should be checked for errors, and if necessary, the calibration procedure should be repeated starting from Section 9.