JNTU Pharm. D I Year PHARMACEUTICAL INORGANIC CHEMISTRY Previous Question Paper (October - 2020 - R08)

In pharmaceutical analysis, errors represent the difference between the measured value and the true value of a substance. Understanding these errors is critical because pharmaceutical products must meet strict purity and potency standards; even minor deviations can compromise patient safety or therapeutic efficacy. Errors are classified as determinate (systematic), which have a known cause and can be corrected, and indeterminate (random), which arise from unpredictable fluctuations and are handled using statistical methods.

Significant figures are the digits in a measurement that are known with certainty plus the first uncertain digit. They indicate the precision of an analytical measurement. Rules for significant figures include: all non-zero digits are significant; zeros between non-zero digits are significant; leading zeros are not significant; and trailing zeros in a number containing a decimal point are significant. Proper use of significant figures ensures that the reported result of an analysis does not imply a level of precision that the measuring equipment cannot support.

Back titration is an indirect analytical method used when a direct titration is not feasible. This occurs when the analyte reacts too slowly with the titrant, the reaction is incomplete, or the analyte is insoluble in water. In this method, a known excess of a standard reagent is added to the analyte, allowing the reaction to go to completion. The remaining unreacted portion of the reagent is then titrated against a second standard solution.

For example, the determination of calcium carbonate in an antacid tablet involves back titration. Calcium carbonate is poorly soluble and reacts slowly with acids. A weighed amount of the sample is treated with an excess of standard hydrochloric acid (HCl):

The excess HCl is then titrated with a standard sodium hydroxide (NaOH) solution using an indicator like methyl orange:

By calculating the amount of NaOH used, the amount of excess HCl is determined, which is then subtracted from the total initial HCl to find the exact amount that reacted with the calcium carbonate.