During experiments with magnesium corrosion in aqueous electrolytes, an important circumstance must be kept in mind: since the nominal reaction is Mg + 2H₂O → Mg(OH)₂ + H₂, and since Mg(OH)₂ is slightly soluble in water, the pH of the system slowly drifts to more alkaline values and may eventually reach values as high as 11. Because of that the system (a) achieves a steady state more slowly than it could and (b) is changing during the measurement.
A way to cope with this problem is to saturate, in advance, your electrolyte with Mg(OH)₂. To do so:
- The solubility of Mg(OH)₂ is 6.53 mg/L at 25°C. Take double that amount to ensure the electrolyte will be saturated.
- Calculate the reagents necessary to synthesize the required mass of Mg(OH)₂ (i.e., double the solubility amount) via reaction between MgSO₄ or MgCl₂ and NaOH or KOH.
- Add the calculated masses of the reagents to the desired volume of distilled water. Add the required masses of the other electrolyte components. Dissolve the reagents (a magnetic stirrer for 10–15 minutes works well). If dissolution is not complete, leave the precipitate as is and transfer all the content to the electrolyte storage bottle.
After this preparation, proceed as usual.