LECTURE 2

CHAPTER 4: CLASSICAL METHODS OF ANALYTICAL CHEMISTRY: GRAVIMETRICMETHODS OF ANALYSIS

CO3: ABILITY TO CLASSIFY SEPARATION TECHNIQUES AND TO USE GRAVIMETRIC METHODS FOR MASS DETERMINATION

Gravimetric methods of analysis

• All techniques that measure mass or a change in a mass.

Direct analysis

• Mass of substances – solid form

- solution form

• If solid- directly weight

exp. I kg of apple

suspended solid in seawage

Collect pass through preweight fiter

Dry weight

•If solution

Exp. Analyte of aq ion; Pb2+, Cu2+, S,

• Chemically converting it to a solid form

using Pt electrode.

Indirect AnalysisExp. To determine moisture in a sample.

Easier to remove the analyte and use a change in a mass as analytical signal.

• Determine analyte without ever being weighed.

• We take advantage of analyte’s chemistry

- We measure Hg2Cl2 mass even though actually we want to measure PO3

3-, since they have similar molarity.

See example in equation pg 233

TYPES OF GRAVIMETRIC METHODS

1. Precipitation Gravimetry- The analyte is separated from a solution of the

sample as a precipitate and is converted to a compound of known composition that can be weighed.

2. Volatilization Gravimetry

- The analyte is separated from other constituents of a sample by conversion to a gas of known composition whose mass serves as a measure of the analyte concentration.

TYPES OF GRAVIMETRIC METHODS

3. Electrogravimetry

- The analyte is separated by deposition on an electrod by an electrical current and the mass is measure.

4. Particulate gravimetry

The analyte is remove from the sample matrix by filtration or extraction.

Conservation of mass

• Rules in gravimetric analysis:

1. Analyte should participate in only one set of reactions. Relation between the precipitate’s mass to the analyte’s mass.

Exp. Moles Pb2+ = moles PbO2

2. The removal must be complete. Therefore we can use:

Filter’s final mass - filter’s initial mass = g suspended solid

PRECIPITATION GRAVIMETRY

1) Terminology: precipitant?2) Definition3) Steps in precipitation gravimetry- Precipitating (solubility, impurities, particle size)- Filtering the precipitate- Rinsing the precipitate- Drying the precipitate- Calculation (Composition)Explain each step involve in precipitation

gravimetry.Group 4

Quantitative Application

• Less use of this method with the presence of modern method such spectrophotometry.

• However still reliable for assessing the accuracy of other method

• For verify the composition of standard reference materials.

Inorganic analysis using gravimetric method

• Objective- to measure inorganic cations, anions

• Sample – water and waste water.

Organic and analysis using gravimetric method

• Objective- to measure organic functional groups, heteroatoms (O, Cl, S, F)

• Calculation can be simplified by applying the principle of conservation of mass.

• C4H10(g) + O2 CO2(g) + H2O(g)

• 4 x moles C4H10 = 1 x moles CO2

Example 8.1

- Solve using your basic knowledge in chemistry.

- Not necessarily follow solution in text book.

1) Relation between Fe3O4 and Fe2O3

- We know from stoichiometry;

3 x moles Fe2O3 = 2 x moles Fe3O4 So,

Fe2O3 = 2/3 Fe3O4

• In 1.5419 g ore sample, contain 0.8525 g Fe2O3

• In 1 mol,

160 g 2/3 x 232 g

0.8525 2/3 x 232 x 0.8240

160

= 0.8240 g

g Fe3O4 x 100 = 0.8240 g x 100 = 53.44 %

g sample 1.5419 g

Solve the problems

1. A 0.5962 g iron ore sample was dissolves in hot perchloric acid and then filtered. The iron content in the sample was oxidized to the ferric state, Fe3+. The solution was made basic with ammonium hydroxide, NH4OH, and the iron subsequently precipitated as ferric hydroxide. The precipitate is filtered, rinsed with dilute NH4OH, ignited, cooled in a dessicators and weighed. The resulting ferric oxide, Fe2O3 weighed 0.321 g. Calculate the weight percent iron in the ore.

Tutorial 5

• Solve problems no. 11, 14 and 18 in Chapter 8, page 268.

• Group 5&9 present the solution