Sodium occurs in nutrient solutions primarily as the sodium ion (Na⁺) . Na ⁺ can partially replace K ⁺ in some plant functions, but K ⁺ is still an essential nutrient.

There are different methods for determining sodium:

• Flame photometry: A fast and precise method for the quantitative determination of sodium.
• Atomic absorption spectroscopy (AAS): Highly precise determination at very low concentrations.
• Precipitation titration with aluminum oxinate: A chemical method for the determination of sodium.

Detailed titration of sodium using precipitation titration

1. Principle of the method

Sodium can be precipitated with aluminum oxinate (C₉H₆NO₂Al) in a slightly alkaline medium. The reaction proceeds as follows:

The sodium oxinate complex formed can be determined by back titration.

2. Chemicals

• 0.01 mol/L aluminum oxinate solution
• 0.01 mol/L perchloric acid (HClO₄) for back titration
• Phenolphthalein as an indicator
• Sodium chloride solution as standard

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Pipette (10 mL)
• Magnetic stirrer

4. Implementation

1. Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
2. Add 10 mL of aluminum oxinate solution.
3. Wait a few minutes while stirring until the precipitate is completely formed.
4. Back-titrate excess aluminum oxinate with 0.01 mol/L perchloric acid.
5. Determine the endpoint by color change from pink to colorless with phenolphthalein.

5. Calculating the sodium concentration

The concentration of Na⁺ is calculated using the formula:

6. Example calculation:

• Perchloric acid concentration: 0.01 mol/L
• Consumed volume: 7.8 mL (0.0078 L)
• Sample volume: 50 mL (0.050 L)

Precipitation titration with aluminum oxinate is a practical method for the quantitative determination of sodium in nutrient solutions, but less precise than flame photometry or AAS.