Magnesium is an essential mineral found in many foods, including milk. It plays a vital role in numerous bodily functions, such as muscle and nerve function, blood sugar control, and bone health. The magnesium content in milk contributes to its nutritional value, making milk a beneficial part of a balanced diet.

The amount of magnesium in milk can vary depending on factors such as the type of milk (whole, skim, or low-fat) and the diet of the cows producing the milk. On average, cow’s milk contains about 24-27 milligrams of magnesium per 8-ounce (240-milliliter) serving. While milk is not the highest source of magnesium when compared to foods like nuts, seeds, leafy greens, and whole grains, it still contributes to the daily intake of this important mineral.

Including milk in your diet can help you reach the recommended daily intake of magnesium, which varies depending on age, sex, and other factors. For adults, the recommended dietary allowance (RDA) for magnesium is 310-420 mg per day, according to the National Institutes of Health (NIH). Consuming a variety of magnesium-rich foods, including milk, can help meet these needs.

Requirements

• Citric Acid Solution: A 1 molar (M) solution of citric acid is employed to chelate calcium and other interfering ions, thereby facilitating the selective determination of magnesium.
• Ammonium Hydroxide (NH₄OH): This reagent, with a specific gravity of 0.88, is utilized in varying dilutions (1:9 and 1:10 volume/volume) to adjust the pH of the solution, which is critical for the precise measurement of magnesium.
• Phosphate Buffer: A solution prepared by dissolving 100 grams of ammonium hydrogen phosphate (NH₄H₂PO₄) in hot water and diluting to 1 liter, with an addition of 5 milliliters of chloroform, serves as a stabilizing buffer. This buffer ensures the maintenance of a consistent pH level during the analysis.
• Ethanol (95%): Ethanol is used as a solvent and for precipitation purposes in the process of isolating magnesium from the milk matrix.
• Dilute Nitric Acid (HNO₃) Solution: A diluted solution of nitric acid, prepared in a 1:4 volume/volume ratio, is used for sample preparation and to facilitate the dissolution of precipitates.
• Dilute Ammonium Nitrate Solution: This solution, prepared by saturating an ammoniacal solution with ammonium nitrate, is utilized for its role in the precipitation and separation of magnesium from the sample.

Procedure

1. Sample Preparation:
   • Begin by adding 2 ml of a citric acid solution to the sample. This step is crucial for chelating calcium and other interfering ions, which might affect the accuracy of magnesium determination.
   • Introduce 100 ml of ammonium hydroxide (with a specific gravity of 0.88) to adjust the pH, optimizing the conditions for magnesium precipitation.
   • Add 50 ml of ethanol to the mixture. Ethanol aids in the precipitation process by dehydrating the solution and facilitating the aggregation of magnesium ions.
2. Precipitation:
   • With constant stirring, incorporate 25 ml of a phosphate solution into the mixture. This addition aids in the formation of magnesium phosphate complexes, which are essential for the subsequent filtration step.
   • Allow the solution to rest for 12-24 hours, ensuring complete precipitation.
3. Filtration and Washing:
   • Carefully filter the solution to separate the precipitated magnesium phosphate. Ensure that the filtration is thorough to collect all solid material.
   • Wash the collected precipitate twice with a diluted ammonium hydroxide solution (1:9 v/v), removing any impurities or unreacted reagents.
4. Dissolution of Precipitate:
   • Dissolve the washed precipitate in dilute nitric acid. This step is crucial for converting the magnesium phosphate into a soluble form for further analysis.
   • Transfer the dissolved solution back into the original beaker, adjusting the volume to between 100-150 ml with additional dilute nitric acid if necessary.
5. Final Precipitation:
   • Add a small amount of ammonium hydroxide to the solution, approximately 1/10th of the solution’s volume, to fine-tune the pH for optimal magnesium precipitation.
   • Introduce 2 drops of the phosphate solution to the beaker, stirring vigorously to ensure uniform distribution.
   • Allow the solution to stand for 3-4 minutes, facilitating the formation of the final magnesium precipitate.
6. Second Filtration and Washing:
   • Filter the solution once again to collect the final magnesium precipitate. This step is critical for isolating the magnesium for quantification.
   • Wash the precipitate with a slightly more diluted ammonium hydroxide solution (1:10 v/v) to remove any remaining impurities.
7. Preparation for Quantification:
   • Soak the filter paper containing the magnesium precipitate in a dilute ammonium nitrate solution. This step prepares the precipitate for the final conversion to magnesium pyrophosphate.
   • Ignite the dried precipitate, converting it to magnesium pyrophosphate, a form suitable for weighing and quantification.
8. Quantification:
   • Weigh the resultant magnesium pyrophosphate. This weight is used to calculate the magnesium content in the original milk sample, providing valuable nutritional information.

Results – Formula for Calculating Magnesium Percentage

The formula to calculate the magnesium percentage in the milk sample is given by:

Magnesium (%)=(109.24×w)/W​

Where:

• w represents the weight in grams (g) of the magnesium pyrophosphate obtained after the completion of the analytical process.
• W denotes the weight in grams (g) of the milk sample that was initially taken for analysis, for instance, 10 grams.

Understanding the Calculation

1. Weight of Magnesium Pyrophosphate (w): This is the final weight measured after igniting the precipitate obtained during the analytical process. This weight is critical as it directly influences the calculation of magnesium content. The precision in obtaining this weight determines the accuracy of the magnesium percentage in the final result.
2. Weight of Milk Sample (W): The initial weight of the milk sample is crucial for standardizing the results. By knowing the exact amount of milk analyzed, the results can be expressed in terms of percentage, making it easier to compare across different samples or against standard nutritional values.
3. Constant (109.24): This constant is derived from the molecular weights of magnesium and pyrophosphate ions and the stoichiometry of their reaction. It is used to convert the weight of magnesium pyrophosphate to the equivalent weight of elemental magnesium in the sample.

Interpreting the Results

Once the magnesium percentage is calculated using the formula, the result provides a direct measure of the magnesium content in the milk sample. This percentage is indicative of the nutritional value of the milk concerning its magnesium content. Higher percentages denote a richer source of magnesium, which is essential for various bodily functions, including muscle and nerve function, blood glucose control, and bone health.