Gravimetric Methods

重量分析法

Analysis based on weight changes

Gravimetric methods are quantitative methods based on determining the mass of a pure compound to which the analyte(分析物 )is chemically related.

gravimetric analyses are highly accurate and precise because they use the analytical balance as measurement tool.

Gravimetric methods do not require a calibration( 标定 ) or standardization(与标准校验 )step because the results are calculated directly from the experimental data and molar masses(摩尔质量 ).

What are properties of gravimetric methods?

What are the types of gravimetric analysis?

A. Precipitation Methods （沉淀法） : The analyte(分析物 ) is converted to a sparingly soluble pre

cipitate(难溶沉淀 )and then separated from the supernatant(上层清液 ).

The precipitate is washed free of impurities(杂质 ), and converted to a product of known composition (usually by heating to obtain a more stable form).

Finally, the product is weighed and its weight is related back to the amount of analyte(分析物 ) in the original sample via a gravimetric factor (GF)(重量校正因子 ).

Equations for a gravimetric barium(钡 )analysis using precipitation

BaCl2(aq) + H2SO4(aq) −−→BaSO4(S)↓ + 2HCl(aq)

Ba2+(aq) + SO42-(aq) −−→ BaSO4(S)↓(net ionic equation)

B. Volatilization Methods( 挥发法 ): The analyte (分析物 ) or its decomposition (分解 ) p

roducts are volatilized(挥发 ) and the volatile product is collected and weighed.

Or the weight of the product can be determined indirectly by determining the weight loss of the sample.

Example 1:

Equation for the gravimetric determination of water content in barium chloride dihydrate(二水合氯化钡 )

BaCl22H2O + heat −−−−→ BaCl2 + 2H2O(g) ↑

Example 2:

NaHCO3(aq) + H2SO4 −−−−→CO2(g) + H2O(l) + NaHSO4(aq)

Particle Size and Filterability 颗粒尺寸和可滤性

Colloidal(胶体 ) suspension(悬浮液 ): diameter: 10-7--10-4 cm; not to settle from solution; nor can be filtered(过滤 ).

Crystalline(结晶的 ) suspension: diameter: tenths of a millimeter or greater; settle from solution; readily filtered.

Large crystals (but, not a chunk(厚块 )) are preferable(更好的 ) because they are readily filterable, have less surface area to react with atmosphere or adsorb impurities(杂质 ).

What factors determine particle size?

Q is the concentration of the solute(溶质 ) at any time and S is the equilibrium(平衡 ) solubility(溶解度 ).

A supersaturated solution is an unstable solution that contains more solute than a saturated solution

To increse particle size of a precipitate, minimize the relative supersaturation during precipitate formation.

When (Q-S)/S is large, the precipitate tends to be colloidal; when (Q-S)/S is small, a crystalline solid is more likely.

Relative supersaturation （相对过饱和比） =

Q S

S

How do precipitates form?

Precipitates form by nucleation( 成核作用 ) and particle growth( 晶粒生长 ).

Nucleation results in large number of fine particles,

Particle growth results in a relatively small number of large particles.

How to Control particle size?

Slow addition of precipitant with frequent stirring (搅拌 ) (decrease Q)

Use of dilute solutions of analyte and precipitant (沉淀剂 ) (decrease Q)

Elevated temperatures increase solubility (increase S)

Why are the colloidal particles relatively stable?

How to let colloidal particles precipitate?

Treatment of colloidal precipitates

Heating and stirring(搅拌 ) Increasing the electrolyte(电解质 ) concentratio

n Digestion(陈化 ) (a process in which a precipita

te is treated for an hour or more in the solution from it was formed)

Coprecipitation(共沉淀 )

Surface adsorption Mixed-crystal formation Occlusion(吸留 ) Mechanical entrapment(包裹 )

What are the characteristics of each type of coprecipitation?

How to minish the minus effect caused by coprecipitation?

Homogeneous precipitation(均相沉淀 )

A process which a precipitate is formed by slow generation of a precipitating reagent homogenously throughout a solution

Solids formed by homogenous precipitation are generally purer and more easily filtered than precipitates generated by direct addition of a reagent to the analyte solution

Why do the results obtained?

Drying and Ignition of Precipitates

Thermogravimetric curve for calcium salicylate( 水杨酸钙 )

•Heating removes the solvent and volatile species

•Igniting( 灼烧 ) decompose the precipitate and form a compound named weighting form

Calculating results from gravimetric data

The Gravimetric Factor

Calculation of weight percent of analyte

Example A 0.3516 g sample of a commercial phosphate(磷酸盐 ) detergent(洗涤剂 ) was ignited(灼烧 ) at a red heat to destroy the organic matter. The residue(残渣 ) was then taken up in hot HCl, which converted the P to H3PO4. The phosphate was precipitated as MgNH4PO4H2O by addition of Mg2+ followed by aqueous NH3.After being filtered and washed, the precipitate was converted to MgP2O7(222.57g/mol) by ignition at 1000℃.This residue weighed 0.2161g, Calculate the percent P(30.974 g/mol) in the sample.

The Gravimetric Factor

Properties of Precipitates and Precipitating Agents

The precipitating agent (precipitant) should react specifically (or at least selectively) with the analyte.

The resulting product should be: 1. readily filtered and washed free of contaminants 2. of sufficiently low solubility so that no significant loss o

f the analyte occurs during filtration and washing 3. unreactive with constituents of the atmosphere 4. of known composition after it is dried or, if necessary, i

gnited

Applications of gravimetric methods

Inorganic precipitating agents Organic precipitating agents

What are advantages by using Organic precipitating agents?

Steps in a Gravimetric Analysis

For precipitation techniques the usual steps are: 1. Find a stable, insoluble form of the analyte 2. Separate the analyte from the other matrix comp

onents in this form 3. Wash the precipitate containing the analyte free

of any impurities 4. Convert the precipitated form of the analyte to a

more reliable weighing form (usually by heating)

Representative Gravimetric Analyses

Representative Gravimetric Analyses

Gravimetric method overview

Simple Cheap

• Glassware• Reagents• ovens, etc.• Balances

Specific Accurate Precise Sensitive

Summary Colloidal and crystalline suspension Relative supersaturation (Q-S)/S Nucleation and particle growth Controlling particle size Coprecipitation: surface adsorption, mixed-cr

ystal formation, occlusion and mechanical entrapment

Homogenous precipitation Gravimetric Factor Applications