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Procedure for determining the concentration of Sulphate (SO4-2) in WATER

Although there is no electrode sensitive to the Sulphate ion this can be determined by reacting with an excess of Barium Chloride standard and precipitating as BaSO4. The amount of Barium removed from solution is monitored with a Barium electrode and is exactly equivalent to the amount of Sulphate in the sample solution. The result is calculated using the Standard Subtraction method.

Notes on Procedures and calculations.

  1. In order to make the Standard Subtraction calculations it is necessary to work in molar concentrations rather than ppm.
  2. It is important to choose an appropriate concentration of standard, and volume of standard and sample, to ensure that there is a detectable change in the Barium concentration (i.e. not too much excess of Ba), and that all the Sulphate in the sample is precipitated (i.e. there must be an excess of Ba).
  3. The concentration of the standard and volumes of standard and sample recommended here are suitable for sulphate concentrations in the range 1 to 0.1 mM/L.
  4. For concentrations above this range, the BaCl2 concentration must be increased accordingly.
  5. For sulphate concentrations below 0.1 mM/L the volume of sample must be increased - but be sure that this will not cause the final Ba concentration (after reacting with the sample) to fall below the lower limit of the linear range of the Barium electrode (about 0.01 mM/L).

Apparatus Required

Ion-Selective Electrode for Barium ion (ELIT 8081 – PVC membrane)

Reference electrode: single junction silver chloride (ELIT 001)

Dual electrode head (ELIT 201 - with BNC connector)

Standard solution: BaCl2

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

Calibration

Before use, the electrodes must be calibrated to find an accurate value for the slope under the current operating conditions. This can done by measuring 1 mM/L and 0.1 mM/L Barium standard solutions and finding the difference between the two measured voltages. Normally the slope is expected to be about 26±3 mV per decade of concentration for divalent ions.

Sample Measurement

  1. Pipette 100ml of 0.1mM/L BaCl2 reaction standard into a beaker, immerse the electrodes, swirl the solution, and record the stable voltage in still solution after a few minutes of stabilization (E1).
  2. Add 10 ml of sample, slowly, with stirring, and wait for a stable voltage when precipitation is complete, then stop stirring and wait for voltage to fall to new stable value (E2).

Results

The unknown sulphate concentration (in moles) can be calculated from the following equation:

Cu = Cs x {(Vu + Vs) / Vu} x {(10^((E2-E1)/S))) - (Vs / (Vs + Vu))}

Where: Cu and Vu are the concentration and volume of the unknown sulphate sample,

Cs and Vs are the concentration and volume of the standard barium solution,

E2-E1 is the change in electrode potential

S is the electrode slope

Interference

The following ions cause interference to the Barium measurement (selectivity coefficients in brackets): Strontium (0.085), Potassium (0.015), Sodium (0.015), Magnesium (0.006), Ammonium (0.003), Calcium (0.002), Lithium (0.0016).

Strontium has the highest interference but is unlikely to be present in significant concentrations in most samples. Any Potassium or Sodium ions present will cause a significant positive error if they have concentrations of greater than ten times that of the Barium. For example, 1 mM of K is equivalent to about 0.015 mM of Ba so, if K/Ba = 10 then the Ba result will be approximately 15% too high and hence the SO4 will be approx. 15% too low. Similarly, Magnesium can be tolerated up to about twenty times the Barium, and Calcium & Lithium up to about fifty times.
But note that these ratios are of the concentrations in the final solution, after precipitation of BaSO4, when the interfering ions in the sample will have been diluted more than ten times and some of the barium will have been used up.