Can one use a Potassium Permanganate standard to verify the DPD method?

Dokument-ID

Dokument-ID TE1846

Veröffentlichungsdatum

Veröffentlichungsdatum 05.07.2022
Frage
Can one use a Potassium Permanganate standard to verify the DPD method?
Zusammenfassung
Potassium permanganate (KMnO4) and DPD verification
Antwort
Hach does not advise using Potassium Permanganate to verify the DPD chlorine method. Hach advises the use of a primary chlorine standard.

Dilute solutions of Potassium Permanganate are used in Standard Methods as equivalent standards for establishing a chlorine calibration. It should be emphasized that permanganate is a stronger oxidant than chlorine and certain precautions on its use and storage should be acknowledged. As noted by Gordon, et al., permanganate oxidizes DPD to both the colored and colorless oxidation product. Hach Company researchers have noted the order of adding reagent to sample also will affect the ratio of oxidized DPD products.

A few precautions in the preparation and use of permanganate standards are noted:
  • Glassware used in the preparation and dilution of permanganate solutions should be treated with chromic acid cleaning solution to remove any organic contamination. The glassware should then be rinsed copiously with pure water, which is low in organics.
  • Water used for dilution of stock permanganate solution should be low in organics and should exceed American Society for Testing and Standards (ASTM) Type I quality specifications. Dilution water for permanganate should never be stored in plastic containers or exposed to airborne contamination. The stock solution should be standardized routinely with dried sodium oxalate.
  • Dilute equivalent standards are not stable and should be prepared as needed. Never store dilute permanganate in plastic containers.
Because of these constraints, Hach Company does not recommend the use of permanganate equivalent standards with Hach DPD reagents.

Alternatively, Hach Company recommends the standard additions technique using Chlorine Voluette® Standards for routine verification of pre-programmed calibrations.

The Chlorine ampuled standards are pure aqueous free chlorine standards prepared in two ranges: 20 - 30 mg/L or 50 - 75 mg/L chlorine. The actual value is provided for each lot of standards. Hach Company research has shown that the ampuled chlorine standards exhibit excellent stability, when stored at temperatures between 2 to 8 °C (33 to 47° F).  A simple procedure is used to verify the accuracy of the chlorine calibration. For example:
  1. Snap the top off a Chlorine PourRite® or Voluette® Ampule Standard Solution.
  2. Use the TenSette® pipet to add 0.1, 0.2 and 0.3 mL of standard to three 25-mL samples. Swirl gently to mix.
  3. Analyze each sample immediately per the Hach DPD colorimetric procedures.
  4. Each 0.1 mL of standard will cause an incremental increase in chlorine. The exact value depends on the Voluette concentration. Check the certificate enclosed with the Voluette Ampules for this value.

More information can be found in Chlorination, Chloramination And Chlorine Measurement_DOC180.53.20183 document.

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