Instrument Prep: IC Anion Analysis

Ion chromatography offers a fairly simple method for performing multiple anion analyses simultaneously. While the outlay for the initial instrumentation is fairly steep, the applicability of the instrument for both lower and upper level classes is extensive. The method used in this experiment for anion analysis is EPA Method 300.0, Determination of Inorganic Anions by Ion Chromatography (United States Environmental Protection Agency, Revision 2.1, 1993, Cincinnati, OH)

Instrument Parameters

There are several brands of ion chromatographs on the market. We used a Dionex instrument with the operating parameters shown in Table 1.

Table 1: Instrument Parameters for Anion Analysis

Type of column

Dionex AS4A anion column/AG4 guard column

Type of detector

Conductivity

Eluant

100% 2.25 mM NaHCO3/2.25 mM Na2CO3

Pressure/Flow rates

800-1200 psi, 2 mL/min

Run times

5-7 minutes, depending on age of column

Order of elution

F- > Cl- > NO3- > PO4-3 > SO4-2

Other interfering anions

Br-, I-, ClO-, AcO-, SO3-2, NO2-

A sample chromatogram is shown on p. 106 in the laboratory manual and an overhead transparency master is provided in Appendix C of the instructor's manual.

Preparation of Standards

Commercial standards can be purchased from Dionex (see Appendix A for address and phone) if time is more precious than money. If you wish to prepare your own, begin by purchasing the following salts in as high a purity as possible: NaCl, NaF, NaNO3, Na2SO4, and NaH2PO4. The water quality standards report nitrate as "N", so the concentration of that anion should be calculated and reported as N, not NO3-. A mix of the five anions is prepared as a concentrated stock solution and diluted to make the calibration standards. Table 2 provides the recommended masses of the salts for one liter of the concentrated stock standard. Dissolve all salts in a small volume of deionized water in a one liter volumetric flask and dilute to volume with deionized water. Since it is difficult to weigh the exact masses, get as close as you can and recalculate the correct concentrations. Remember -- mg/L ppm in dilute aqueous solution. Store the concentrated standard in a well rinsed, well labeled nalgene bottle. This primary standard should easily last one year.

Table 2: Preparation of Concentrated Stock Standard

Salt

Mass (g)

Mass of the anion alone

NaF

0.442

200 mg F-

NaCl

0.495

300 mg Cl-

NaNO3

6.073

1,000 mg N as NO3-

NaH2PO4

1.895

1,500 mg PO4-3

Na2SO4

2.218

1,500 mg SO4-2

Dilute 10 mL of the primary stock standard with deionized water to a final volume of 100 mL. This is the secondary stock solution. The concentrations should now be:

To make the final calibration standards, dilute the required amount of secondary stock solution to 100 mL. Exact volumes for each standard and final concentrations of each anion are given in Table 3.

Table 3: Preparation of Final Calibration Standards

Standard

mL of secondary stock solution

Final volume

F- (ppm)

Cl- (ppm)

N as NO3- (ppm)

PO42- (ppm)

SO4- (ppm)

1

10

100 mL

2.0

3.0

10.0

15.0

15.0

2

4

100 mL

0.8

1.2

4.0

6.0

6.0

3

2

100 mL

0.4

0.6

2.0

3.0

3.0

4

1

1 mL

0.2

0.3

1.0

1.5

1.5

5

0.5

0.5 mL

0.1

0.15

0.5

0.75

0.75

If the original masses of the salts were not exactly according to Table 1, you will need to correct for the exact mass of the salts you used to obtain accurate concentrations.

The final calibration standards should be placed in very clean plastic bottles and refrigerated when not in use. They will stay good for approximately one-two months if kept refrigerated and tightly capped.

Preparation of Eluant Solution

Sodium Carbonate: 2.25 mM, Sodium Bicarbonate: 2.12 mM


Homepage / Instructor Manual Contents / The Chemistry of Water