B.KIRUTHIGA LECTURER DEPT OF … Overview • Measure tools for gravimetric analysis filter crucibles, an oven, balance • Calculate stoichiometry • Report values % composition,

Gravimetric Analysis

B.KIRUTHIGALECTURER

DEPT OF PHARMACEUTICAL CHEMISTRY

The Process

• Separate - by phases– volatilization methods– precipitation methods

selective ppt'naccount for interferencesassure completeness (solubility equilibrium)

Precipitation:

• Dissolve sample• Add ppt’ing

reagent• Filter• Dry• Weigh

precipitating agent

sample

dissolvedcomponents

Calculating Amount of Precipitation Reagent

• How many mL of 1% potassium chloride would be needed to precipitate all of the silver in a 0.5 g ore sample that contains 1.5 parts per thousand silver? Allow for a 15% excess of the chloride solution.

Gravimetric Overview

• Measuretools for gravimetric analysis

filter crucibles, an oven, balance• Calculate

stoichiometry• Report

values% composition, ppt, ppm, ppb

uncertaintyaverage standard deviation, confidence interval

The Precipitation Process(from solution)

• Particle Size / Filterability– produce particles large enough to be 'caught‘– ideally, produce crystals– avoid colloidal suspension

particle size = 1 - 100 nm

Precipitate Formation

• crystallization– nucleation:

particles join to produce aggregates– crystal growth

aggregate grows and 'fall out' of solution

– We want a few big chunks of precipitate!supersaturation:

more solute than should be present in solutionrelative supersaturation:

a measure of supersaturation, (Q-S)/SQ = actual solute concentrationS = equilibrium solute concentration

Controlling Precipitation

• Increase S– Increase temperature

• Decrease Q– Dilute solution– Well mixed (stirring)

pH control of precipitation

Ca2+ + C2O42- CaC2O4 (s)

H2C2O4 2 H+ + C2O42-

Feeder Reaction:

Homogeneous Precipitation

• Fe3+ + 3 HCO2- Fe(HCO2)3⋅nH2O

(NH2)CO + 3 H2O + heat

HCOOH + OH- + CO2 + 2 NH4+

Feeder Reaction:

High Electrolyte Concentrationto Aid Precipitation

Excess charge on colloid creates ionic atmosphere around particle

D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686

Composition by Gravimetric Analysis

Ni2+ (aq) + H2DMG Ni(DMG)2 + 2 H+

• A 0.8234 g ore sample produced 0.1397 g of bis(dimethylglyoximate) nickel (II) (FW = 288.91 g/mol). Find the nickel content.

• Explain how to create a large, filterable precipitate.

Combustion Analysis

Absorbs water

Absorbs CO2

D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p691

Find the empirical formula for a 13.72 mg organic sample that produced 6.97 mg of water and 28.44 mg of carbon dioxide

Gravimetric Interferences

• Coprecipitated impurities– Surface adsorption

• Contaminants bound to the surface• Dry the sample

– Mixed crystals• Replacements in the crystal lattice• Reprecipitation

– Occlusions/mechanical entrapment• Impurity pockets• Slow digestion/masking agents

Gravimetric Overview

• Simple• Cheap

– Glassware– Reagents– ovens, etc.– Balances

• Specific• Timely• Accurate• Precise• Sensitive

THANK YOU