SOP: Determination of Total Solids in Water

GIFSQ — Global Institute for Food Safety & Quality

Principle

Total solids include water-soluble as well as insoluble matter. A measured volume of the well-shaken sample is evaporated in a tared platinum dish and the residue is dried in an oven and weighed. The temperature at which the sample was dried is stated with the result.

Apparatus required

• Tared platinum dish
• Water bath
• Oven (105°C)
• Desiccator

Procedure

Evaporate a suitable quantity (100–500 ml) of the well-shaken sample in a tared platinum dish on a water bath

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Dry the residue in the oven at 105°C for an hour

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Cool in a desiccator and weigh

Notes: Some workers dry at 103°C, some at 120°C and others at 180°C. In all cases some water of hydration is included, but at 180°C there is little except what can be calculated. When sulphates of calcium or magnesium are present along with other saline matter, one molecule of water always remains attached to the magnesium sulphate at 180°C, and it is also difficult to drive off the whole of the water from the calcium sulphate. If calcium or magnesium chlorides or nitrates are present, they are so hygroscopic that it is practically impossible to obtain an accurate weighing; in these rare cases, add sodium carbonate and deduct the weight added from the total weight of the residue. Even with this correction, the results may be slightly erroneous, as any magnesium present is precipitated as a basic carbonate.

Calculation

Total solids, ppm = (Weight of residue, mg × 1,000,000) / (ml of sample taken)

Note: express the results to the nearest 5 mg/litre.