Residue on Ignition / Sulfated Ash Test Methods

Residue on Ignition; The Residue on Ignition (RoI), also known as Sulfated Ash test, is a commonly used analytical procedure in the pharmaceutical and chemical industries. It measures the amount of inorganic residue remaining after the sample is heated to high temperatures in the presence of sulfuric acid. The purpose of this test is to assess the quantity of inorganic substances or impurities in a sample, which could include metals, salts, or other non-volatile elements. This test is typically performed on organic substances to quantify the residual inorganic material that does not volatilize when ignited. The procedure for performing this test can vary slightly depending on the specific pharmacopoeia being followed. The following outlines the methods for the Residue on Ignition/Sulfated Ash test, as per various pharmacopoeias, including the British Pharmacopoeia (BP), United States Pharmacopeia (USP), and Indian Pharmacopoeia (IP).

Sulfated Ash Test – British Pharmacopoeia (BP) Method: Residue on Ignition

The Sulfated Ash Test as per the BP uses the following procedure:

1. Preparation of the Platinum Dish:
   • Heat a platinum dish to redness for 10 minutes to remove any residues.
   • Allow the dish to cool in a desiccator and weigh it (this weight is recorded as W1).
2. Weighing of Sample:
   • Weigh about 1.0 g of the sample to be tested and place it into the cooled platinum dish. Record this weight as W2.
3. Moistening and Ignition:
   • Moisten the sample with a small amount of sulfuric acid.
   • Ignite gently and then repeat the process of moistening with sulfuric acid, followed by gentle ignition at approximately 800°C until all organic material is incinerated.
4. Cooling and Weighing:
   • After ignition, allow the residue to cool.
   • Weigh the dish and residue again. Record this weight as W3.
5. Final Ignition:
   • If necessary, continue ignition for 15-minute intervals until two successive weighings do not differ by more than 0.5 mg.
6. Calculation of Ash Content:
   • The percentage of residue or ash can be calculated using the formula:

Percentage of Ash=W3−W1W2−W1×100\text{Percentage of Ash} = \frac{W3 – W1}{W2 – W1} \times 100Percentage of Ash=W2−W1W3−W1​×100

Sulfated Ash Test – United States Pharmacopeia (USP) Method

The Residue on Ignition or Sulfated Ash test, as per the USP (281), follows a very similar procedure, with minor differences:

1. Crucible Preparation:
   • Heat a suitable crucible, such as silica, platinum, quartz, or porcelain, at 600°C ± 50°C for 30 minutes.
   • Allow the crucible to cool in a desiccator over silica gel and weigh it. This weight is recorded as W1.
2. Sample Preparation:
   • Weigh approximately 1.0 g of the sample (or as specified in the individual monograph) and place it in the same crucible. Record this weight as W2.
3. Moistening and Ignition:
   • Moisten the sample with 1 ml of sulfuric acid and gently heat the sample until it is thoroughly charred.
   • After cooling, moisten the residue with another 1 ml of sulfuric acid, heat gently until white fumes cease, and ignite the residue at 600°C ± 50°C until it is completely incinerated.
4. Cooling and Weighing:
   • After ignition, allow the crucible to cool in a desiccator over silica gel.
   • Weigh the crucible and the residue. This weight is recorded as W3.
5. Repeat Ignition:
   • If the amount of residue exceeds the prescribed limit, repeat the moistening and ignition for 30-minute intervals (noting the weight as W4) until two consecutive weighings do not differ by more than 0.5 mg or until the percentage of residue meets the prescribed limit.
6. Calculation of Ash Content:
   • The percentage of residue or ash is calculated using the formula:

Percentage of Ash=W3−W1W2−W1×100\text{Percentage of Ash} = \frac{W3 – W1}{W2 – W1} \times 100Percentage of Ash=W2−W1W3−W1​×100

Sulfated Ash Test – Indian Pharmacopoeia (IP) Method

The Sulfated Ash Test as per the Indian Pharmacopoeia (IP) follows a process very similar to the BP and USP methods:

1. Crucible Preparation:
   • Heat a silica or platinum crucible to redness for 10 minutes.
   • Allow it to cool in a desiccator and weigh it. This weight is recorded as W1.
2. Sample Preparation:
   • Weigh about 1.0 g of the sample (or as specified in the individual monograph) and place it in the crucible. Record this weight as W2.
3. Moistening and Ignition:
   • Moisten the sample with 1 ml of sulfuric acid and ignite gently at first to char the sample thoroughly.
   • After cooling, moisten the residue with another 1 ml of sulfuric acid and heat gently until the white fumes stop evolving.
   • Ignite the residue at 800°C ± 25°C until no black particles remain.
4. Cooling and Weighing:
   • Allow the crucible and residue to cool and weigh again. This weight is recorded as W3.
5. Repeat Ignition:
   • If necessary, repeat the moistening with sulfuric acid and ignition for 15-minute intervals (noting the weight as W4) until two consecutive weighings do not differ by more than 0.5 mg.
6. Calculation of Ash Content:
   • The percentage of residue or ash is calculated using the formula:

Percentage of Ash=W4−W1W2−W1×100\text{Percentage of Ash} = \frac{W4 – W1}{W2 – W1} \times 100Percentage of Ash=W2−W1W4−W1​×100

Summary Comparison of Methods

Conclusion

The Residue on Ignition or Sulfated Ash test is a critical method for determining the inorganic content in a sample. It is important to follow the specific guidelines outlined in the relevant pharmacopoeia (BP, USP, or IP) to ensure accurate and reliable results. This test is essential in ensuring the quality, safety, and compliance of pharmaceutical products, especially for substances that require strict quality control measures to assess their purity and composition.