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Gravimetric Tests

Gravimetric Test Overview

Gravimetric tests are a general category of testing in which the final product is weighed to provide the result.  For most gravimetric tests, the sample either undergoes a thermal process or a dissolution and precipitation process.

Preparation

For thermal process tests, samples are typically weighed directly into a crucible.  The crucibles may be added directly to a furnace or oven without further addition of reagents (such as Loss on Drying or Ash tests).  For other tests, reagents may be added (such as concentrated sulfuric acid in the case of a residue on ignition test, ROI). For precipitation tests, a sample portion is dissolved in a suitable solvent and a precipitating reagent is added to precipitate the analyte (such as silver nitrate for gravimetric chlorides).  The precipitate is then filtered and is weighed.

Analysis

The sample result is measured by weighing the remaining sample or residue on a suitable analytical balance.

Report

The final result may be reported as the total mass of residue found (such as in mg) or as a function of the mass of sample (wt/wt basis).  For some tests, the final result is compared to a limit and a determination of Pass/Fail is made. Common gravimetric tests performed at Galbraith:
  • Residue on Ignition USP <281>
  • Sulfated Ash
  • Total Ash
  • Acid-insoluble Ash
  • Water Soluble Ash
  • Loss on Drying by USP <731>
  • Loss on Ignition by USP <733>
  • Inerts/Solids
  • Gravimetric Chlorides
  • Gravimetric Sulfates

Gravimetry