Principles of Gravimetric Analysis

Introduction

Gravimetric analysis measures mass rather than titrant volume or instrumental light response. The analyte may be isolated directly, converted into a sparingly soluble solid, or inferred from a controlled mass loss. The method can be accurate when conversion, separation and weighing are quantitative, but a precipitate that traps impurities or retains variable water gives a precise mass for the wrong chemical composition.

Core explanation

In precipitation gravimetry, a reagent converts dissolved analyte into a solid of known composition. To determine chloride, Ag⁺ can precipitate AgCl. After suitable digestion, filtration, washing and drying, the mass of AgCl reveals chloride moles because one mole of AgCl contains one mole of Cl⁻. If m(AgCl) is measured, n(Cl⁻) = m(AgCl)/M(AgCl), and chloride mass is that amount times M(Cl). The original sample amount follows after any dilution or aliquot factors.

A suitable precipitate should have low enough solubility that analyte loss to the liquid is small, form particles that can be retained by filtration, contain minimal coprecipitated impurities, and be convertible to a stable weighing form with a definite formula. The precipitated form and weighing form need not always be identical. Some precipitates are ignited to a stable oxide before weighing. The conversion must be complete and its stoichiometry known.

Mass can also be measured through volatilisation. If a sample loses only water during a validated heating step, before–after mass difference estimates water content. If other volatile substances leave or a solid decomposes, the difference no longer uniquely represents water. Combustion analysis can collect known gaseous products to infer carbon or hydrogen content, another use of mass and stoichiometry.

Gravimetry shifts measurement challenges from a calibrated solution concentration to chemical recovery and solid handling. Fine particles may pass through a filter; excessive washing may dissolve precipitate; inadequate washing can leave soluble salts; insufficient drying leaves moisture. A stable mass after repeated heat–cool–weigh cycles is useful evidence, though the target chemical form must still be correct. A blank can detect mass contributed by reagents or filter material.

The gravimetric factor converts weighing-form mass into target analyte mass. It is a ratio of molar masses multiplied by stoichiometric coefficients. For AgCl to Cl, factor = M(Cl)/M(AgCl). It is not a universal constant for “chloride gravimetry”; it depends on the exact weighing form and analyte expression. If reporting chloride concentration in water, divide recovered chloride mass by the original water volume, not by the volume of a later digest.

Step-by-step reasoning

1. Define the analyte and original sample basis. 2. Choose a conversion yielding a separable, stable product of known composition. 3. Ensure precipitation or volatilisation is complete and interferences are controlled. 4. Isolate, wash and condition the product to the specified weighing form. 5. Convert product mass by stoichiometry and reverse any aliquot factors.

Visual explanation

Draw a flow path from solution containing Cl⁻ to AgCl solid, then filter, wash, dry and weigh. Under the mass box write m(AgCl) → n(AgCl) → n(Cl⁻) → m(Cl⁻). Add small warning symbols beside filtrate, washing and drying to indicate loss or extra mass at those steps.

Real-world analogy

To count small objects, one could pack each into an identical known-weight carrier, weigh the filled carriers and infer object count. Gravimetry similarly transforms a hard-to-weigh dissolved analyte into a bulk product with a defined composition. If the carriers retain water or pick up unrelated objects, mass no longer gives the right count.

Real-world example

A laboratory measures sulfate by forming barium sulfate, BaSO₄, then isolating and weighing it under a validated procedure. One mole of BaSO₄ contains one mole sulfate. The laboratory checks whether other ions coprecipitate and whether the solid is sufficiently dry, because either issue can raise measured mass and overstate sulfate.

Why?

Why is a stable weighing form essential? A balance can determine mass precisely, but it cannot identify how much of that mass is analyte, retained water or impurity. A definite formula makes the mass-to-moles conversion valid. Without stable composition, improved balance resolution adds digits without improving the chemical result.

Common misconception

“All insoluble products are good gravimetric precipitates” ignores particle size, purity and filterability. Another mistake is assuming a mass gain must mean more analyte; trapped salt or moisture can make a contaminated precipitate heavier.

Worked example

An isolated AgCl precipitate has dry mass 0.2870 g. With M(AgCl) ≈ 143.32 g mol⁻¹, n(AgCl) ≈ 0.002002 mol. The same moles of chloride correspond to about 0.0710 g Cl using 35.45 g mol⁻¹. If this came from 100.0 mL original water, the concentration is about 710 mg L⁻¹. The answer assumes AgCl was fully recovered and contained no other mass.

Quick check

1. Why can a precipitate weighed before complete drying give a falsely high analyte result? Answer: Retained water adds mass unrelated to the analyte. Stoichiometric conversion treats all measured mass as the specified weighing form and therefore overestimates analyte amount.

Exam focus

Write the analyte-to-weighing-form mole ratio before calculating masses. Name the product's formula and ensure the mass belongs to that product after conditioning. Distinguish incomplete precipitation or filtration loss, which often biases low, from contamination or residual moisture, which often biases high.

Advanced insight

Low solubility does not mean zero solubility. Precipitation loss may be estimated from a solubility product under specified conditions, while common-ion effects can reduce it. Yet adding too much precipitant can increase impurity adsorption or create complex ions in some systems, so the procedure balances recovery against precipitate purity.

Summary

Gravimetry relates an isolated mass or controlled mass change to analyte amount through chemical stoichiometry. Precipitation methods need a low-solubility, filterable, pure and stable weighing form. Quantitative conversion and original-sample bookkeeping are as important as balance precision.

Practice questions

1. How many moles sulfate correspond to 0.0100 mol pure BaSO₄? Answer: 0.0100 mol sulfate, because each BaSO₄ formula unit contains one sulfate ion.

2. Name one cause of low and one cause of high result in precipitation gravimetry. Answer: Loss of fine precipitate through a filter can bias low; retained moisture or coprecipitated impurity can bias high.

3. Why might an analyst ignite a precipitate before weighing it? Answer: Controlled ignition can convert it into a stable weighing form of definite composition, removing variable water or decomposing a precursor according to known stoichiometry.