Thermo Fisher Scientific Ion Selective Electrodes Cyanide - Manual (Page 6)

Cyanide Electrode Instruction Manual
6
All samples must be aqueous and not contain organics which can dissolve the epoxy electrode body
and/or the cement bonding the sensing crystal to the electrode body. Infrequent measurements in
solutions containing methanol, benzene, or acetonitrile are permitted. Highly polar solvents slowly
attack the epoxy body electrode. Please check with Eutech Instruments Pte Ltd. before using the
electrode in other solvents.
The temperature of the sample solutions and of the standard solutions should be the same and below
80
o
C.
Interferences should be absent. If they are present, use the procedure found in the
Interferences
and
Electrode Response
sections to remove them.
Measurements above 1X10
-3
M should be done infrequently, as cyanide ion slowly erodes the
membrane. (See section
Electrode Life
.) It may be necessary to polish the membrane occasionally
with polishing paper or jeweller's rouge as the electrode is used. Samples should be diluted below
1X10
-3
M if possible.
Proper pH is ensured if ISA is used. The pH should be above 10 so that cyanide is present as CN
-1
rather than as HCN in all standards and samples.
Units of Measurement
Cyanide ions can be measured in units of ppm, moles per liter, or any other convenient
concentration unit. Table 1 indicates some concentration units and conversion factors.
TABLE
1: Concentration Unit Conversion Factors
ppm CN- moles/liter
26.00
1.0X10
-3
10.00 3.8X10
-4
2.60 1.0X10
-4
1.00 3.8X10
-5
0.26 1.0X10
-5
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with ISA. The temperature of both sample solutions and standard
solutions should be the same.
Direct Measurement of Cyanide (using a pH/mV meter)
1. By serial dilution, prepare 10
-3
, 10
-4
, and 10
-5
M or 10, 1 and 0.1 ppm standards for the
cyanide ion from the 0.01M or 1,000 ppm standards. Prepare standards with a composition
similar to the samples if the samples have an ionic strength above 0.1M.
2. Place 100 ml of the most dilute standard solution in a 150 ml plastic beaker. Place the
beaker on the magnetic stirrer and begin stirring at a constant rate. Add 1 ml of 10M