Analytical Method: Chloride Determination for COD Cell Tests for Seawater

Instruments and Reagents:

Cat.No. 173016  Spectroquant® VIS Spectrophotometer Prove 100 or

Cat.No. 173017  Spectroquant® UV/VIS Spectrophotometer Prove 300 or

Cat.No. 173018  Spectroquant® UV/VIS Spectrophotometer Prove 600 or

Cat.No. 109748  Spectroquant® Photometer NOVA 30 or

Cat.No. 109751  Spectroquant® Photometer NOVA 60 or

Cat.No. 173632  Spectroquant® Colorimeter Move 100

Cat.No. 114946  Rectangular cells 10 mm and/or

Cat.No. 114947  Rectangular cells 20 mm and/or

Cat.No. 114944  Rectangular cells 50 mm
Cat.No. 109136  Sodium hydroxide solution 2 mol/l Titripur®
Cat.No. 111132  MColortestTM Chloride Test

To determine the COD content in seawater, the chloride content of the seawater sample must be depleted. The tolerable chloride-content after depletion is 2000 mg/l (for Cat. No. 117058) resp. 250 mg/l (for Cat. No. 117059) in the supernatant.
In acidic solution chloride ions are titrated with mercury(II) nitrate solution against 1,5 diphenylcarbazone as the indicator, forming slightly dissociated mercury(II) chloride in the process. At the titration endpoint, excess mercury(II) ions react with the indicator to form a blue-violet complex. The chloride concentration is determined from the consumption of titration solution (mercurimetric determination).

Measuring range:
> 250 mg/l, 1 drop = 250 mg/l

Sample preparation:
Inject 5.0 ml sodium hydroxide solution 2 mol/l into the test vessel. 
Carefully allows 0.5 ml of depleted sample (according to the package insert COD Cell Test for seawater, Cat.No. 117058 and 117059, section 7 “chloride depletion”) to run from the test vessel onto the sodium hydroxide solution and mix (Wear eye protection! The cell becomes hot!).
Add 2 drops of reagent Cl-1 and swirl. The sample generally turns yellow in color. (Reagent Cl-2 is not required).
Holding the reagent bottle vertically, slowly add reagent Cl-3 dropwise to the sample while swirling until its color changes from yellow to blue-violet. Shortly before the color changes, wait a few seconds after adding each drop.
Result in mg/l Chloride = number of drops x 25

For use with
test kits:
  Measuring range in
mg/l COD
117058 Spectroquant® COD Cell Test for seawater/
high chloride-contents
5.0 - 60.0
117059 Spectroquant® COD Cell Test for seawater/
high chloride-contents
50 - 3000




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