Sodium concentration in serum

Sodium concentration in serum may be determined using the technique of sample addition.

Equipment Required                                          '

  1. Ion analyzer (recommended) or pH meter with millivolt scale.
  2. Sodium ion selective electrode


  1. Sodium 100 mmol/l-1 stock standard
  2. 1 Molar calcium chloride (ISAB and outer reference electrode filling solution)

Standard Preparation

Dissolve 5.844g of NaCI (analar) in a 1 liter volumetric flask with deionized water and dilute to the mark.

This is 100 mmol/l-1 Na stock standard.

By serial dilution of this standard prepare also a 10 and a 1 mmol/l1-  sodium standard.

Slope Determination

  1. Fill the outer reference electrode compartment with 1 Molar CaCl2.
  2. To 20 ml of 10 and 1 mmol/l-1 Na+ solutions add 2 ml of 1 Molar CaCl2.
  3. Measure the potential of the 1 mmol/l-1 standard (mV1).
  4. Rinse the electrodes with deionized water and read the potential of the 10 mmol/l-1 standard (mV2).

Electrode slope = mV2 – mV1 (mV/decade)


  1. To 5 ml of the 1 mmol/l-1 Na+ standard in a 10 ml volumetric flask add 1 ml 1 Molar CaCl2. Dilute to the mark with deionized water.
  2. Pour this solution into a 25 ml beaker. Immerse the electrodes and record the potential (mV1).
  3. Add 0.5 ml serum, mix well and record the new stable potential (mV2).
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