Cobalt occurs in nutrient solutions primarily as the cobalt(II) ion (Co²⁺) . Required by rhizobia, it is important for the nodulation of legumes. A non-essential micronutrient.

There are various methods for determining cobalt:

• Atomic absorption spectroscopy (AAS): High-precision determination of cobalt.
• Spectrophotometry with nitroso-R salt: formation of a colored cobalt complex.
• Complexometric titration with EDTA: Formation of a stable cobalt-EDTA complex.

Detailed titration of cobalt with EDTA

1. Principle of the method

Cobalt ions (Co²⁺) react with ethylenediaminetetraacetic acid (EDTA, C₁₀H₁₆N₂O₈) to form a stable complex:

The endpoint of the titration is detected using Eriochrome Black-T (ErioT) as an indicator. The color change occurs from **pink to blue**.

2. Chemicals

• 0.01 mol/L EDTA solution (C₁₀H₁₆N₂O₈)
• Buffer solution (pH 10, NH₃/NH₄⁺ buffer)
• Eriochrome Black-T (indicator)

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 buffer solution (pH 10).
3. Add 2-3 drops of Eriochrome Black-T indicator.
4. Titrate with 0.01 mol/L EDTA until the color changes from pink to blue.

5. Calculation of the cobalt concentration

The concentration of Co is calculated using the formula:

6. Example calculation:

• EDTA concentration: 0.01 mol/L
• Consumed volume: 9.2 mL (0.0092 L)
• Sample volume: 50 mL (0.050 L)

Conclusion

Complexometric titration with EDTA is a precise method for the quantitative determination of cobalt in nutrient solutions.