Estimation of Calcium and Magnesium

In this article we will discuss about the complexometric method for estimation of calcium and magnesium in plants.

Principle:

Many complex-forming ions or molecules lead to a stepwise formation of complexes. Each complex is characterized by its own stability constant. This property of stepwise formation of complex can be used in titrimetry and, thus, one can be able to estimate the concentration of the respective ions.

Usually EDTA (ethylene diamino-tetra acetic acid), an amino poly-carboxylic acid, a good complexing agent, is used for such complexometric estimation. In this titration, metalochromic indicators (Solichrome dark blue for Calcium and Eriochrome black T for Magnesium) are often used.

The reactions are:

Calcium or Magnesium salt + Buffer soln. (pH 10)

↓

Free ions of Calcium or Magnesium + 2e^–

↓

Ca⁺⁺ or Mg⁺⁺

Indicator dye (Solochrome or Erichrome) + 2e^– → Indicator — Metal complex

(Blue colour) (Red-coloured complex)

Indicator—Metal complex + EDTA Soln. → Metal EDTA complex + Indicator (free)

(Red-coloured complex) (Chelate Complex) (Blue colour)

Requirements:

(a) Reagents:

1. EDTA solution — 0.01 M

2. Indicator—

(a) Solochrome dark blue for Calcium

(b) Eriochrome black T for Magnesium

3. Buffer solution of pH 10

[142 ml of NH₄OH with 17 gms of NH₄CI|

(b) Glass-wares:

1. Pipettes, Burettes, Conical flasks, Beakers etc.

(c) Salt Solution:

Calcium or Magnesium salt soln. of desirable concentration is made for estima­tion of the respective metal content.

Test Procedure:

1. 5 ml of salt solution is taken in a conical flask. Then 2 ml of buffer solution and 4-5 drops of indicator are added.

2. The mixture turns red immediately.

3. Then the content is titrated against 0.01 M EDTA Solution, until the colour changes from red to blue. Disappearance of the last tinge of red colour indicates the end-point.

4. By repeating the process several readings are taken.

5. The metal content of the solution is then calculated by using the known EDTA metal- complexing data:

1 ml of 0.01 M EDTA Ξ Complexes with 0.4008 mg of Calcium.

1 ml of 0.01 M EDTA Ξ Complexes with 0.2432 mg of Magnesium.