| abstract |
An apparatus for determining the concentration of an analyte ion in the presence of an interfering ion of the same ionic charge comprises a first ion-selective electrode or ISE (3A), sensitive to the analyte, and a second ISE (3B), sensitive to the interferent. To calibrate the apparatus, the first ISE is brought into contact with first, second and third standard solutions, each containing known concentrations of analyte and interferent. Because the ratio of analyte concentration to interferent in the first standard solution is identical to that in the second standard solution but different to that in the third, the Nicholsky-Eisenmann equation is easily solved to yield the standard potential, E 8 , the slope a and the sensitivity coefficient K MN for the ISE. Thereafter the analyte concentration can be obtained from measurements of the electrode potentials using the values determined in this way. In a preferred embodiment, the apparatus takes the form of a flow cell, and the method finds particular application in the measurement of Mg2 + in the presence of Ca2 +, or NH 4 + in the presence of K+. |