10
Analytical Procedures
Analytical Techniques
A variety of analytical techniques are available to the analyst.
The following is a description of these techniques.
Direct Calibration
is a simple procedure for measuring a large
number of samples. Only one meter reading is required for each
sample. Calibration is performed in a series of standards. The
concentration of the samples is determined by comparison to the
standards. ISA is added to all solutions to ensure that samples and
standards have similar ionic strength.
Incremental Techniques
provide a useful method for measuring
samples, since calibration is not required. As in direct calibration, any
convenient concentration unit can be used. The different incremental
techniques are described below. They can be used to measure the
total concentration of a specific ion in the presence of a large
(50-100 times) excess of complexing agents.
Known Addition
is an alternate method useful for measuring
dilute samples, checking the results of direct calibration (when
no complexing agents are present), or measuring the total
concentration of an ion in the presence of an excess complexing
agent. The electrodes are immersed in the sample solution and
an aliquot of a standard solution containing the measured
species is added to the sample. From the change in potential
before and after the addition, the original sample
concentration is determined.
Direct Calibration
Set-up
1. Connect electrodes to the meter.
2. Prepare two standards which bracket the expected sample
range and differ in concentration by a factor of ten. Standards
can be prepared in any concentration unit to suit the particular
analysis requirement. All standards should be at the same
temperature as the samples. (For details on temperature effects
on electrode performance, refer to
Temperature Effects
.)
