Lesson 5: Gravimetric Analysis

Gravimetric analysis

• is a quantitative method to determine an analyte concentration by weighing a pure, solid form of the analyte.

• measurement of mass

Solid

The ____ form is obtained by adding a precipitating reagent to a solution containing the analyte

Theodore W. Richards

Gravimetric analysis was developed by

Analyte

a substance whose chemical constituents are being identified and measured.

Precipitant

also known as the precipitating agent refers to the chemical that is used to cause precipitation

Precipitate

the insoluble substance formed in the gravimetric method

Size

Particle retention depends on the ____ of the filter’s pores.

Suspended solids

• suppose you are to determine the total suspended solids in water released from a sewage-treatment facility.

• _____ are just that solid matter that has yet to settle out of its solution matrix. The analysis is easy.

• You collect a sample and pass it through a pre-weighed filter that retains the suspended solids. After drying to remove any residual moisture, you weigh the filter. The difference between the filter’s original mass and final mass gives the mass of _____.

Cellulose fiber filters 

commonly referred to as filter paper, range in pore size from 30 µm to 2–3 µm.

Glass fiber filters

constructed from chemically inert borosilicate glass, range in pore size from 2.5 µm to 0.3 µm.

Membrane filters

which are made from a variety of materials, including cellulose nitrate and PTFE, are available with pore sizes from 5.0 µm to 0.1 µm.

Recrystallization 

• If the analyte is an aqueous ion, use

• In this case we cannot isolate the analyte by filtration because the ion (Pb2+ ion) is dissolved in the solution’s matrix. We can still measure the analyte’s mass by chemically converting it to a solid form.

• With use of Pt electrodes, The Pb2+ ion in solution oxidizes to PbO2 and deposits on the Pt electrode serving as the anode. If we weigh the Pt before and after, the difference in the two measurements gives the mass of PbO2 (calculated using reaction’s stoichiometry).

Precipitation

• easiest way using the precipitation process (the reaction that creates an insoluble solid product from the reaction of two soluble solid products)

• Analyte is converted to sparingly soluble precipitate.

Solution preparation, precipitate formation, filtering, washing, heating, weighing

Key steps in precipitation

Volatilization

• using thermal or chemical energy to determine their masses

• ____ gases (that can be easily evaporative), like carbon dioxide, chlorine, etc., can be separated with the help of _____ gravimetry.

• Example: In determining the moisture content of food, thermal energy vaporizes the H2O.

Larger particles, supersaturated

Particle size and filterability of precipitates

Selective, specific, readily filtered, low solubility, known composition

Properties of precipitant (precipitating agent)

Errors

There are different causes of ___ in gravimetric analysis:

• Inaccurate weighing

• Incomplete and imperfect precipitation

• Use of substandard reagent and apparatus.

• Haste and impatience