Residue on Ignition Calculation Formula

To measure the residue on ignition (Sulfated Ash), a weighed sample is ignited, thoroughly charred and placed in a high temperature oven (600°C/900°C). After the carbon is completely consumed, the residue is cooled, weighed and the percent inorganic impurities in the organic sample is calculated.

To calculate the percentage Residue on ignition as per BP, the formula to use is as follows.

[(w1– w2) / (w3 – w2)] × 100


w1 = weight in g of the ignited crucible and residue

w2 = weight in g of the crucible

w3 = weight in g of the crucible and sample

Care must be taken to avoid moisture pick-up from the atmosphere. The absorption of moisture will alter the weight of the initial sample and the residue, thus influencing the percent residue on ignition.

Common instrument required for measuring Residue on ignition

Analytical balance capable of ±0.0001 gram accuracy

  • Muffle furnace, controlled at 600 ± 50°C
  • Iron wire triangle with clay pipe stems
  • Desiccator with silica gel or other suitable desiccant
  • Silica, porcelain, platinum or quartz crucible, of suitable size
  • Ring stand with ring
  • Gas Bunsen burner or hot plate
  • Liquid dropper
  • Crucible cover 
  • Tongs

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