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

Reagents

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/l^1-  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)

Method                                                   

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).