Applications of Ion-Selective Electrodes Analyte Ammonia Carbon dioxide Chloride Chlorine residual Cyanide Fluoride Nitrate Nitrogen oxide/nitrite Oxygen, dissloved Sulfide Electrode type Gas sensing Gas sensing Solid State Liquid Membrane Combination Solid state Solid state Combination Solid state Liquid membrane Gas sensing Liquid membrane Solid state Ionic strength adjuster ph adjusting ISA CO 2 buffer 5 M NaNO 3 ----- ----- Iodide reagent 5 M NaNO 3 TISAB ----- 2 M (NH 4 )SO 4 Acid buffer ----- SAOB
Ion-Selective Electrodes (ISE) Analysis The principle of potentiometric analysis by ISE is based on the Nernst equation which can be written as: E= E 0 + S log C where: E = the measured voltage E 0 = reference potential S = the slope of the electrode C = the concentration of the ion in the solution Different types of sensor electrodes including solid state, liquid membrane, and gas-sensing types are available. The reference electrodes can either be a single junction or double junction type that contains the filling solution of KCl, AgCl, or KNO 3
The voltage difference between the sensing and the reference electrodes can be a measure of the concentration of the analyte ion, selective to the sensor electrode. This can be written as follows: C x = C s * 10 E/S Where: C x and C s = the concentrations of the unknown and the standard solutions, respectively. E = the difference of potentials between that of the standardizing solution and the sample solution. S = the slope of the electrode that measures the change in electrode potential per tenfold change in concentration.
Standard Addition Method The concentration of the standard can be calculated as follows: Conc. Of the standard to be spiked = Sample volume * Estimated sample conc. Volume of standard to be spiked
The concentration, Cx, of the analyte ion in the sample may be calculated from the following equation: Cx = ρcy [(1+ρ) * 10 E/S 1] Where: C y = the concentration of the standard ρ = the ratio of the volume of spiked standard to the volume of sample taken. S = the electrode slope = the millivolt difference, E 2 E 1
Example Five ml of 75 ppm chloride standard was added to 100 ml of a wasstewater sample for chloride analysis. The initial millivolt reading for the sample was 17.5 mv. After adding the standard, the reading was 35.3 mv. The slope was measured previously as 58.1 mv. Calculate the concentration of chloride in the sample.
Sample Addition Method Conc. Of the standard = Volume of sample to be spiked * Estimated sample conc. Volume of standard taken
Determine the concentration Cx of the analyte in the sample from the following equation: C x = [(1+ρ) * 10 E/S ρ] * C y Where: C y = the concentration of the standard ρ = the ratio of the volume of the standard taken to the volume of sample spiked E = the millivolt difference E 2 E 1 S = electrode slope
A wastewater sample was found to contain 3.8 mg/l cyanide. A 100 ml aliquot of this sample was spiked with 10 ml of 50 mg/l cyanide standard solution. The concentration of this spiked solution was measured to be 8.1 mg/l.
C y = 75 ppm ρ= 5 ml =0.05 100 ml E = 35.3 mv 17.5 mv= 17.8 mv S = 58.1 mv C x = 0.05 * 75 ppm [(1+0.05) * 10 17.8 mv/58.1 mv 1] = 3.75 ppm [(1.05 * 10 0.306 ) 1] = 3.75 ppm [(1.05 * 2.02) 1] = 3.75 ppm 1.124 = 3.34 ppm
Relative Percent Deviation (RPD) RPD = ( a 1 a2) or( a2 ( a1 + a2) 2 a 1 ) *100% Where a 1 and a 2 are the results of duplicate analysis of a sample
RPD = (11.1mg (11.1mg / / L L + 9.7mg 9.7mg 2 / / L) L) *100% = (1.4mg / L) 10.4mg / L *100% = 13.5
s = x 2 n ( 1 n x ) 2 Where Σx 2 = sum of the squares of individual measurements Σx = sum of individual measurements, and n = number of individual measurements RSD = s x *100 %
The sum of x = 30.5 The sum of x 2 = 155.45 Σx = 30.5 Σx 2 = 155.45 (Σx) 2 = (30.5) 2 = 930.25; n = 6 s = 15545 5 93025 6 mg/l = 2.86 mg/l = 0.41 5 = 0.29mg = 0.29 mg
% Recovery = 100(X s X u ) / K Where X s = measured value for the spiked sample X u = measured value for the unspiked sample adjusted for dilution of the spike, and K = known value of the spike in the sample
% Recovery = Measured concentration Theoretical concentration * 100% Theoretical concentration can be calculated as: (C u *V u ) (C s * V s ) + (V u + V s ) (V u + V s ) Where C u = measured concentration of the unknown sample, and C s = concentration of the standard While V u and V s are the volumes of the unknown sample and standard, respectively