Residues on Ignition USP and Includes Ash EP, and Sulphated Ash EP

This method covers Residues on Ignition USP and includes Ash EP, Sulphated Ash EP and determining silica from residue on ignition. In the USP Ignition is referred to as Sulphated Ash, whereas in the BP & EP, the terms used are Ash & Sulphated Ash and are two separate tests.

A - Ash Determination EP 

B - Sulphated Ash EP 

C - Residue on Ignition USP 

D - Silica Content using residue on ignition 24055, 24222, 23927


EP 1997 European Pharmacopoeia

USP 1997 USP 23

Operations Manual for Furnace

Health & Safety Datasheet for Chemicals used in this method.

Definitions and Principals of Residues on Ignition

4A - Ash EP Sample dried at 100°C and ignited to constant weight at 600°C. Calculated

4B - Sulphated Ash EP Sample moistened with dilute sulphuric acid, dried and ignited at 600°C. Ammonium carbonate is added and the residue ignited to constant weight at 600°C

4C - Residue on Ignition USP Sample moistened with dilute sulphuric acid, heated gently until charred, then moistened with sulphuric acid and ignited at 800°C to constant weight.

6D - Silica Content The ash as determined in B may be used to find the silica content by treating it with Sulphuric Acid and Hydrofluoric acid where silica forms silicon tetrafluoride which is volatile leaving behind the non-silica residues. The difference in weight is equal to the weight of silica.


6.1 Sulphuric Acid 96% Analytical grade

6B.1 Ammonium Carbonate Solution - 5g Ammonium Carbonate Analytical Grade dissolved in a mixture of 5M Ammonia and 50 ml of distilled water dilute to 100 ml with water and filter, if necessary.

6B.2 Dilute Sulphuric Acid - 10%w/v H2SO4 in distilled water. Use 96% Sulphuric acid reagent quality to prepare this reagent (6.1)

6D.1 Sulphuric Acid 96% Analytical grade

6D.2 40% w/w Hydrofluoric Acid - Analytical Reagent

6D.3 98%v/v Ethanol - Analytical Grade


7.1 Platinum or Silica Crucible or suitable crucible.

7.2 Desiccator

7.3 Hot plate for drying sample at 105°C

7.4 Balance weighing to 0.1 mg (Uncertainty of measurement ± 0.3 mg)

7.5 10 ml measuring cylinder (For sulphated ashes)

7.6 Ashless filter paper and filter funnel withstand and beaker to catch filtrate. (Ash only)

7A.1 EP methods - Muffle furnace set at 600°C and calibrated to hold 600°C ± 25°C (Checked with a calibrated temperature probe)

7A.2 Bunsen burner.

7C.1 USP method - Muffle furnace set at 800°C and calibrated to hold 800° ± 25°C (Checked with a calibrated temperature probe)

7C.2 For determining silica contents a platinum crucible of a dish is necessary.

7D.1 Hotplate set at 95 - 105°C surface temperature.

7D.2 Suitable gloves with no holes in them.

7D.3 10 ml plastic measuring cylinder.


The residues may be reduced if the sample bursts into flame so care is required to char the sample first at a lower temperature before placing it in the muffle.

Residues on Ignition USP and includes Ash EP, Sulphated Ash EP

The USP excepts the results from the EP sulphated ash unless there is a note under the substance being analysed specifying that different results are obtained if the EP is used.


Care is required when sampling to achieve a representative sample. Avoid taking samples from the surface if the material has been exposed to the air where it may pick up or lose surface moisture. Where the sample is heterogeneous it may be necessary to sample from several points and mix the samples before commencing the analysis.



Read safety data sheets on chemicals, and the Operation Manual for the Furnace & Local Health and Safety practices before carrying out this analysis. All relevant Health and Safety precautions shall be observed.

Ash Content Method

Heat a silica or platinum crucible to redness for 30 minutes, allow it to cool in a desiccator and weigh to the nearest 0.1 mg)

Weigh into 1 ± 0.1 g to the nearest 0.1 mg of sample and place in a crucible.

Dry for 1 hour at 105°C

Heat residue in a muffle furnace at 600°C ± 25°C (Do not let the sample burst into flame - use a lid if necessary.)

If the residue contains charred material after prolonged ignition stir with hot water and filter residue through an ashless filter paper and ignite the residue and filter paper. Combine the filtrate and ash, carefully evaporate to dryness and ignite to constant weight.

Calculate the residue retained in the crucible.

Sulphate Ash EP Method

10B.1 Heat a silica or platinum crucible to redness for 30 minutes, allow it to cool in a desiccator and weigh it to the nearest 0.1mg)

10B.2 Weigh out 1 ± 0.1 g to the nearest 0.1 mg of a sample or the stated quantity in the crucible.

10B.3 Add 2 ml dilute sulphuric acid.

10B.6 Heat on a hotplate at 105°C cautiously over a flame then progressively to 600°C.

10B.7 Continue the incineration until all black particles have disappeared and allow the crucible to cool.

10B.8 Add a few drops of dilute sulphuric acid, heat and incinerate as before and allow to cool.

10B.9 Add a few drops of ammonium carbonate solution. Evaporate and incinerate carefully, allow to cool, and weigh.

10B.10 Repeat the ignition period for 15 minutes until constant weight is achieved.

Residue on Ignition USP Method

10C.1 Weigh accurately 1- 2 g to the nearest 0.1 mg or the amount specified in the specification into a crucible previously ignited cooled and weighed.

10C.2 Heat gently at first, until the substance is thoroughly charred, and cool, then, unless otherwise directed in the specification, moisten the residue with 1 ml of 98% sulphuric acid (6.1).

10C.3 Heat gently until fumes are no longer evolved, and ignite at 800°C ± 25°C unless another temperature is specified in the specification until the carbon is consumed.

10C.4 Cool in a desiccator, weigh and calculate the percentage of residue.

10C.5 If the residue exceeds that quoted in the specification, again moisten the residue with 1 ml of sulphuric acid, heat and ignite as before, and again calculate the percentage of residue. Continue the ignition until constant weight is attained unless otherwise specified.

10C.6 Conduct the ignition in a well-ventilated hood, but protected from air currents, and at as low a temperature as is possible to affect the complete combustion of the carbon. A muffle furnace may be used, if desired and its use is recommended for the final ignition at 800 ± 25°C

SILICA - using Sulphated Ash

Precautions: Read health & Safety Data Sheets for Hydrofluoric Acid before handling the reagent

10D.1 Using a Platinum dish determine the loss on ignition method C. Then proceed as follows:

10D.2 Add 0.2 ml 98 % w/w Sulphuric acid and sufficient 96% Alcohol to moisten the residue completely.

10D.3 Wearing impervious gloves add 6 ml (plastic cylinder) 40% Hydrofluoric Acid and evaporate in a fume cabinet with extraction to dryness carefully to avoid spluttering, on a hot plate at 95 - 105°C.

10D.4 Heat at 900°C for 30 minutes, allow to cool in a desiccator and weigh.


%w/w Ash = Weight of residue x 100/Weight of sample

11D Residue on Ignition minus Residue after HF treatment = Loss in weight due to the formation of SiF4 % Silica present = (Weight of dried material - weight after HF treatment) x 100 weight of raw material taken


For results under 0.1% report as under 0.1%. The accuracy of the method will depend on the sampling and heterogeneous materials may give variable results. For uniform materials, the accuracy depends on the crucible retaining its weight throughout the analysis and prevention of mechanical losses from the crucible. If these factors are not significant then at 0.1% a ± 3% variation may be achieved. For Silica contents of approximately 100%, a variation of ±1% is achievable.


The results should be reported for A Ash EP

Liebermann-Burchard Test  Procedure 


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