SOP: Determination of Magnesium
GIFSQ — Global Institute for Food Safety & Quality
Principle
In an alkaline solution from which calcium and iron have been removed, magnesium is precipitated as magnesium ammonium phosphate. The precipitate is dissolved in acid and the phosphorus it contains is determined colorimetrically as molybdenum blue; magnesium is then calculated from the phosphorus (1 ml of standard = 0.1 mg of P = 0.0784 mg of Mg).
Apparatus required
- Graduated centrifuge tubes, 15 ml
- Centrifuge
- Colorimeter with No. 66 red filter
Reagents
a. Saturated ammonium oxalate [(NH₄)₂C₂O₄·H₂O] solution
b. Methyl red indicator: dissolve 0.5 g methyl red in 100 ml of ethyl alcohol
c. 2% diammonium phosphate [(NH₄)₂HPO₄] solution
d. 10% ammonium hydroxide (NH₄OH) (v/v) solution
e. 0.1 N hydrochloric acid
f. Molybdic acid solution: dissolve 25 g ammonium molybdate in 300 ml of water without heating; dilute 37 ml H₂SO₄ to 200 ml with water and add to the ammonium molybdate solution; store in a brown bottle
g. 2% hydroquinone solution: add 1 drop of H₂SO₄ for each 100 ml; discard when the solution starts to become brown
h. 10% sodium sulphite (Na₂SO₃) solution: prepare fresh weekly
i. Potassium dihydrogen phosphate (KH₂PO₄)
Note: when reagents f, g and h are mixed, no blue or green colour should appear.
Sample preparation
Destroy the organic matter of the sample by dry ashing and prepare the ash solution as described in SOP: Destruction of Organic Matter.
Procedure
Measure 10 ml of ash solution into a 15-ml graduated centrifuge tube; add 1 drop of methyl red indicator
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Neutralize with NH₄OH; add 1 ml of ammonium oxalate and make the volume to 13 ml with water
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Mix and allow to stand overnight; centrifuge for 10 min and discard the precipitate (removes calcium)
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Measure 1 ml of the supernatant liquid into a 15-ml centrifuge tube; add 3 ml of water, 1 ml of ammonium phosphate and 2 ml of NH₄OH
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Mix and allow to stand overnight; centrifuge for 7 min and discard the supernatant liquid
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Mix the precipitate with 5 ml of dilute NH₄OH, centrifuge for 7 min and discard the supernatant liquid; dry the precipitate by placing the tube in a container of hot water
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Add 1 ml of dilute HCl and 5 ml of water to dissolve the precipitate; add 1 ml of molybdic acid solution, 0.5 ml of hydroquinone and 0.5 ml of sodium sulphite solution
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Mix and allow to stand for 30 min; transfer to a colorimeter tube and read the absorbance using a No. 66 red filter, setting the instrument scale at zero with water
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Prepare a standard curve: dissolve 0.4389 g potassium dihydrogen phosphate in water and make to 1 litre (1 ml = 0.1 mg of P = 0.0784 mg of Mg); treat aliquots of 0.1 to 0.5 ml as directed for the sample (add dilute HCl, water, molybdic acid, hydroquinone and sodium sulphite, stand 30 min and read)
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Read the phosphorus in the sample from the curve and convert to magnesium: mg of Mg = mg of P × 0.784; scale to the sample using the dilution (10 ml ash solution taken, 1 ml of the 13-ml calcium-free volume carried forward) and the total ash solution volume.
Calculation
Magnesium, mg/100 g = (mg of Mg in the measured aliquot × Total volume of ash solution × 100) ÷ (ml of ash solution represented by the aliquot × Wt of sample taken for ashing)
Where,
- mg of Mg in the measured aliquot = mg of P read from the standard curve × 0.784
- ml of ash solution represented by the aliquot = 10 × (1/13), since 1 ml of the 13-ml calcium-free solution derives from the 10 ml of ash solution taken
- Total volume of ash solution = volume to which the ash solution was made up (ml)
- Wt of sample taken for ashing = weight of sample ashed (g)
References
- Holzapfel, C.R., Onderstepoort J. Vet. Sci. Anim. Ind., 2, 115 (1934).
- McCance, R.A. & H.L. Shipp, The Chemistry of Fresh Foods and their Losses on Cooking, Med. Res. Council Spec. Rept. Series No. 187 (1933).
- Ward, G.M. & F.B. Johnston, Chemical Methods of Plant Analysis, Canada Department of Agriculture, Publication 1064, p. 28 (1962).