SOP: Determination of Water-Soluble Calcium and Magnesium in Salt
GIFSQ — Global Institute for Food Safety & Quality
Principle
Water-soluble calcium and magnesium are determined by complexometric titration with standard EDTA solution. The combined calcium and magnesium are titrated at pH 10 with Eriochrome Black T indicator (wine-red to pure blue end point); calcium alone is titrated in alkaline medium with murexide or calcein indicator, and magnesium is obtained by difference. The EDTA solution is first standardized against a standard calcium solution.
Note: the book’s opening sentence of this section is garbled in the extraction (“the principle is essentially the same as that used in the determination of water …”); the printed titrimetric scheme is complete and is given below.
Apparatus required
- 250-ml conical flask
- 100-ml pipette (for the two filtrate aliquots)
- Burette (for the EDTA titration)
Reagents
a. Buffer solution: dissolve 67.5 g of ammonium chloride, NH₄Cl, in a mixture of 570 ml of ammonia (sp. gr. 0.90) and 250 ml of water. Separately, dissolve 0.931 g of the disodium salt of ethylene diamine tetra acetic acid (EDTA) (dihydrate) and 0.616 g of magnesium sulphate (MgSO₄·7H₂O) in about 50 ml of water. Mix the two solutions and make up to one litre.
b. Standard calcium solution: dry calcium carbonate at 120°C, cool, weigh 1.0 g, dissolve in the minimum quantity of dilute HCl, and make up to 1 litre. One ml of this solution is equivalent to 0.4008 mg of calcium (as Ca).
c. Standard EDTA solution: dissolve 3.72 g of the disodium salt of EDTA in water and make up to 1000 ml. Standardize against standard calcium carbonate as described in the procedure.
d. Eriochrome Black T indicator: dissolve 0.1 g of the dye in 20 ml of rectified spirit. Prepare fresh every week.
e. 10% NaOH solution.
f. Murexide indicator: grind 0.2 g of murexide with 10 g of sodium chloride until the mixture is homogeneous.
g. Calcein indicator: mix 0.1 g of calcein and 0.06 g of thymolphthalein with 10 g of KCl.
Sample preparation
Use the filtrate from the determination of water-insoluble matter, made up to 1000 ml.
Procedure
- Standardization of EDTA solution: take 25 ml of standard calcium solution in a conical flask; add 25 ml of water, 10 ml of buffer solution and 5 drops of the Eriochrome Black T indicator; titrate against EDTA to a blue end point
↓
Titrate 25 ml of the buffer solution against the standard calcium solution using the Eriochrome Black T indicator (usually it will be 0.1 ml); find the corresponding value for 10 ml
↓
Subtract the blank titre value of the buffer from the titre value found using EDTA solution; calculate the calcium equivalent of 1 ml of EDTA solution — let this be A
Pipette 100 ml out of the 1000 ml of filtrate into a 250-ml conical flask; add 10 ml of buffer solution and 5 drops of Eriochrome Black T indicator; titrate against standard EDTA solution — if calcium and magnesium salts are present, a wine-red colour is formed on adding the indicator, which changes to pure blue at the end point; note the volume of EDTA solution required — let this be B
To another 100 ml of the filtrate, add 5 ml of dilute NaOH and 0.2 g of murexide or 100 mg of mixed calcein indicator; titrate against EDTA solution until the wine-red colour changes to blue with murexide, or green to purple with mixed calcein indicator; note the titre value — let this be C
Calculation
% Calcium as Ca = (C × A × 100) ÷ (W × 1000)
% Magnesium as Mg = ((B − C) × A × 0.6068 × 100) ÷ (W × 1000)
Where,
- A = calcium equivalent of 1 ml of EDTA solution, from step 1 of the procedure, in mg of Ca per ml
- B = ml of EDTA solution used in step 2 of the procedure (calcium + magnesium)
- C = ml of EDTA solution used in step 3 of the procedure (calcium alone)
- 0.6068 = conversion factor from calcium equivalent to magnesium equivalent, as printed
- W = weight of salt in 100 ml of the filtrate, in g
- × 1000 = unit conversion, since A is in mg per ml while W is in g
Note: the fraction lines of the book’s printed calculation are illegible in the extraction; the formulae above follow the printed variables (A, B, C, W) and the printed factor 0.6068 in the standard form of the EDTA scheme. With W declared in grams, the × 1000 unit conversion is required — without it the result would be 1000 times too high (e.g. a compliant dairy salt at 0.01% Ca would read as 10%).
Precautions
- Prepare the Eriochrome Black T indicator fresh every week.
- Subtract the buffer blank titre before calculating the calcium equivalent A.
- Note the colour changes exactly as described for each indicator.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.