Pure Chlorite Standard Developed

High-purity sodium chlorite (NaClO₂) is not (as of November 2002) commercially available by any manufacturer other than Inorganic Ventures. Its potential explosive instability makes it one of the most difficult anions to manufacture. Our in-house method for purification of the technical grade (approx. 80%) sodium chlorite was developed at our facilities between 1998 and 2001. The final product is a Certified Reference Material that is certified by Iodometric Titration and ion chromatography. Currently, we have one year of stability data on the first lot of 1K ClO₂^- and our study is ongoing.

Chloride Purification Levels

The difference in the chloride level before and after purification of the NaClO₂ is illustrated below in chromatograms I and II.

Chromatogram I: ~20µg/g ClO₂^- made from NaClO₂ before purification

Chromatogram II: ~20µg/g ClO₂^- made from NaClO₂ after purification

Certification

A. Total chloride determination:  Treating ClO₂^- with Sulfurous Acid reduces the ClO₂^- to Cl^- and it makes measuring the total Cl^- content (ClO₂^- and Cl^-) possible by Ion Chromatography. See Chromatogram III below.

ClO₂^- + 2H₂SO₃    Cl^- + 2H₂SO₄

Chromatogram III: ClO₂^- treated with H₂SO₃

This method measures the ClO₂^- and Cl^- content together. As it is visible on Chromatogram II (~20µg/g ClO₂^- made from NaClO₂ after purification) there is still a small amount of Cl^- left in the material.

B. Chlorite determination:  Iodometric titration in the presence of starch indicator versus a standard Thiosulfate (S₂O₃^-2) solution (standardized vs. NIST SRM 136e Potassium Dichromate).

Chemistry:

ClO₂^- + 4I^- + 4H⁺    Cl^- + 2H₂O + 2I₂

2I₂ + 4S₂O₃^-2    4I^- + 2S₄O₆^-2

Note that this method measures only the ClO₂^- content.

C. Chloride level:  (using collected data)

1. The ClO₂^- level obtained from method B (iodometric titration) is 983 µg/mL. This is equivalent to 983 * 0.5256 = 516.66 µg/mL Cl^-.
   
   The total Cl^- level obtained from method A is 525.33 µg/mL Cl^-. The difference of the total Cl^- level (525.33) and the ClO₂^- as Cl- (516.66) gives the Cl- level in the purified ClO2- lot which in this case is 8.67  9 µg/mL.

1. Another way of determining the Cl^- level in the ClO₂^- solution is to dilute sample with deionized water and compare to a known concentration (Cl^- NIST SRM 3182). See Chromatogram IV and conditions below.
   
   If 1.0mM NaHCO₃ is used as eluent, the separation of ClO₂^- and Cl^- is better.

Chromatogram IV: ~100µg/g ClO₂^-

Results obtained from C-1 and C-2 (two different methods) above agree with one another and no other peaks are detected in the chromatogram IV. Thus, it is safe to assume that there are no other species containing Cl (ClO₃^-, ClO₄^- ,ClO₂, ClO-, Cl₂) in our ClO₂^- CRM solution and our methodology is accurate.

Related Anions

In addition to chlorite (EPA methods 300.1, 317.1, 325.0), our facility also manufactures bromate (EPA methods 300.1, 317.1, 321.8, 325.0) and bromide (EPA methods 300.1, 317.1, 325.0), both of which are included in the group of Disinfectants and Disinfection Byproducts Anions.