RESIDUE ON IGNITION

1. INTRODUCTION:                     
2. The ash content of a drug is the inorganic residue remaining after incineration, which is converted to the sulphated form.
3.  The substance is ignited with concentrated sulphuric acid which decomposes and oxidises organic matter, leaving a residue of inorganic sulphates.
4. PROCEDURE :                                                                                                        
5. (1)Take a suitable crucible, (silica, platinum, quartz or porcelain) ignite for 30 minutes at 600±50°C cool in a desiccator for about 30 minutes (Silica gel or other suitable desiccant) and accurately weigh (W₁).         
6. (2) Transfer accurately about 1 to 2 g of the sample or the amount specified in the individual monograph into the crucible. Take the weight accurately (W₂).            
7. (3)Moisten the sample with a small amount (usually 1ml) of sulfuric acid. Heat, gently at first, at a temperature as low as practicable until the substance is thoroughly charred, cool, then unless otherwise directed in the individual monograph, moisten the residue with a small amount (usally 1 ml) of sulphuric acid.      
8. (4)Heat gently until white fumes no longer are evolved, and ignite at 600 ± 50 °C, in a muffle furnace unless another temperature is specified in the individual monograph, until the carbon is consumed. Ensure that the flames are not produced at any time during the procedure.                
9. (5)Cool in a desiccator for about 30 minutes (Silica gel or other suitable desiccant), weigh accurately (W₃), and calculate the percentage of residue.              
10.                                                                  (W₃ - W₁) x 100
11.                        % Residue on Ignition  =  ------------------------          
12.                                                                             (W₂ - W₁)                                           
13.  (6)    If the amount of residue so obtained exceeds the limit specified in the individual monograph, again moisten the residue with 1 mL of sulfuric acid, heat and ignite as before until constant weight is attained or until the percentage of residue complies with the limit in the individual monograph.                  
14. (7) Continue the ignition  until  constant  weight ( two consecutive weighings  do not differ by more than 0.50 mg per g of substance taken, the second weighing following an additional 15 minute ignition period) is attained, unless otherwise specified in the individual monograph.
15. Note :
16. (1)Conduct the ignition in a well-ventilated hood, but protected from air currents and at as low a temperature as is possible to effect the complete combustion of the carbon.
17. (2)A muffle furnace may be used, if desired, and its use is recommended for the final ignition at 600 ± 50°.

Residue, ignition, general testing procedure, sulphated