Thermo Fisher Scientific Ion Selective Electrodes Chloride - Manual (Page 12)

Instruction Manual Chloride Electrode
1. Acidify with 1M HNO
3
to pH 4 to remove hydroxide interference.
2. These substances represent complexing species whose maximum level can be exceeded
without electrode damage. Value shown represents a 1% error.
3. Add CISA to solutions containing mixed halides to remove interferences. See the
procedure below.
4. Add CISA or a solution of Ni
+2
to remove sulfide or cyanide interferences.
Removal of Various Interferences with CISA
CISA is an oxidizing agent which will oxidize up to a 100-fold excess of CN
-1
over Cl
-1
, 100 ppm
NH
3
, 100 ppm Br
-1
or I
-1
, or 500 ppm S
-2
. Chloride measurement interferences may be removed by
using CISA. The reagents used to prepare CISA are strong oxidizing agents and should be handled
in a fume hood.
To prepare CISA, add approximately 800 ml of distilled water to a 1 liter volumetric flask. Add
15.1 grams of NaBrO
3
to the flask and swirl to dissolve the solid. Slowly add 75 ml of concentrated
nitric acid (70% w/w or 15.9N), mix, and dilute to the mark with distilled water.
To use CISA, mix equal amounts of CISA and sample. Solutions should be allowed to stand for ten
minutes before measuring. Since chloride will be oxidized upon prolonged standing, all standards
or samples mixed with CISA should be discarded after measuring. A fresh mixture of CISA and
standard should be prepared for each calibration. After adding CISA, follow the procedures for
direct measurement.
Complexation
Total concentration (Ct) consists of free ions (Cf) and complexed or bound ions (Cc) in solution:
Ct = Cf + Cc
Since the electrode only responds to free ions, any complexing agent in the solution reduces the
measured concentration of ions.
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