Electrode Characteristics; Reproducibility; Interferences; Precipitation And Complexation - EUTECH INSTRUMENTS CADMIUM EPOXY Instruction Manual

Instruction Manual Cadmium Electrode
+2
M Cd = ————
+2
V Cd
where:
+2
M Cd = concentration of cadmium ion in the sample (moles/liter)
V t = volume of EDTA added at endpoint
M t = EDTA concentration (moles/liter)
+2
= volume of unknown sample (100 ml)
Cd

ELECTRODE CHARACTERISTICS

Reproducibility

Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors like temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is
independent of concentration within the electrode's operating range.

Interferences

A surface layer of silver metal may be formed by strongly reducing solutions. A layer of silver salt
may be deposited on the membrane if high levels of ions forming very insoluble salts are present in
Electrode
the sample. Proper performance can be restored by polishing. See the section entitled
Response
for proper polishing procedure.
The cadmium ion electrodes do not respond to anions or most cations. The electrode membrane is
poisoned by solutions containing copper, mercury, and silver. These ions must be absent from the
solution.
If the level of ferric ion or lead ion exceeds the level of cadmium ion in the sample, the
measurement will be affected. Sodium fluoride added to the sample will eliminate ferric ion.

Precipitation and Complexation

Sulfide, carbonate, oxalate, phosphate, hydroxide, and other ions precipitate insoluble cadmium
salts. The level of cadmium ion, the level of the precipitating ion, and the pH of the sample
determine formation of a precipitate.
A wide variety of species, including acetate, ammonia, bromide, chloride, citrate, cyanide, and
EDTA, form complexes with cadmium. The total cadmium concentration, the concentration of the
complexing species, the solution pH, and the ionic strength all determine the extent of
complexation. Complexation reduces the free cadmium ion concentration and, since the electrode
responds only to free cadmium ions, a false reading results.
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