linearity of the calibration curve. The ionic strength of a solution is the collective effect of the charge and
concentration of all the ions present and is expressed as the activity coefficient. Total Ionic Strength Adjustment
Buffers (TISAB) contain an ionic strength adjuster, as well as a pH buffer, thus eliminating H
+
and
OH
--
interference, and undesired chemical reactions. Decomplexing agents are also added to complex or
precipitate interfering ions which may be present in sample solution.
Direct Potentiometry (mV mode)
Absolute and Relative mV
This is the most straightforward method to obtain results. A series of serial dilutions of a standard solution is
measured. The electrode potential of an unknown following calibration of the system by a series of serial
dilutions. A small volume of ISAB is added to both standard and test solutions to eliminate effects of differences
in activity.
The temperature must be kept constant between samples and standard. Within the linear range of the electrode
two standards are sufficient or additional points if low concentration expected outside linear range.
Procedure:
refer to electrode manual for specific electrode preparation.
prepare at least two standards within the linear range of the electrode, with concentration differing by a
factor of 10.
add constant volume of ISAB to sample and standard solutions.
ensure meter is reading millivolts.
place electrode in most dilute standard, record value when reading is stable.
rinse electrode and measure the next standard, following procedure as above.
repeat for each additional standard solution.
rinse electrode and place in sample.
repeat the above for each sample.
recheck calibration of the meter using one standard solution.
Fluoride
mV
slope
1ppm
-400
10ppm
-459
-59
100ppm
-518
Calcium
mV
slope
1ppm
332
100ppm
361
+29
1000ppm
390
3345/REV A/08-99
25
