Equivalent Weight - Use in Gravimetric Analysis

Use in Gravimetric Analysis

The term "equivalent weight" had a distinct sense in gravimetric analysis: it was the mass of precipitate which corresponds to one gram of analyte (the species of interest). The different definitions came from the practice of quoting gravimetric results as mass fractions of the analyte, often expressed as a percentage. A related term was the equivalence factor, one gram divided by equivalent weight, which was the numerical factor by which the mass of precipitate had to be multiplied to obtain the mass of analyte.

For example, in the gravimetric determination of nickel, the molar mass of the precipitate bis(dimethylglyoximato)nickel is 288.915(7) g mol−1, while the molar mass of nickel is 58.6934(2) g mol−1: hence 288.915(7)/58.6934(2) = 4.9224(1) grams of precipitate is equivalent to one gram of nickel and the equivalence factor is 0.203151(5). For example, 215.3±0.1 mg of precipitate is equivalent to (215.3±0.1 mg) × 0.203151(5) = 43.74±0.2 mg of nickel: if the original sample size was 5.346±0.001 g, the nickel content in the original sample would be 0.8182±0.0004%.

Gravimetric analysis is one of the most precise of the common methods of chemical analysis, but it is time-consuming and labour-intensive. It has been largely superseded by other techniques such as atomic absorption spectroscopy, in which the mass of analyte is read off from a calibration curve.