Gravimetry Lectures

8/16/2019 Gravimetry Lectures

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Gravimetric analysis

It is the process of isolation of element in pure form and weighing of the final product of

reaction.

The final product must be

pure

stable

of definite chemical structure

Isolation

1. Precipitation method.

2. Volatilization or evolution method.

3. Electroanalytical method.

4. Extraction and chromatographic method.

Requirements for precipitation method

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1. The substance to be analyzed must be precipitated quantitatively (low Ksp)

2. The substance must be precipitated in pure form (i.e. not contaminated).

3. Suitable physical form rapid filtration and washing.

(i.e. ppt of large crystals have small surface area to decrease adsorption of impurities andconsequently washed easily).

Advantages

1. precise

2. It has wide range of applications Disadvantage Time consuming


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Steps:

I. Precipitation

II. Digestion

III. Washing

IV. Filtration

V. Drying

VI. Ignition

VII. Weighing

I. Precipitation

• Choice of the precipitant

1. Precipitant should be specific

• i.e. Precipitate single substance thus preventing interference.

• Usually available precipitant are mostly selective, i.e. precipitate a group of substances• we have to improve selectivity of precipitant by:

a. adjusting pH

b. adjusting oxidation no. of element.

c. the use of masking agent.

d. removal of interfering ions.2


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2. Precipitant should precipitate the substance to be analyzed completely

(quantitatively) in pure form.

Example:

During precipitation of SO42-

we have to use Ba2+ (not Pb2+ or Ca2+ or Sr2+) as BaSO4 has the lowest KSP.

BaCl2 is better than Ba(NO3)2 as BaSO4 precipitate is contaminated with Ba(NO3)2.

Types of precipitates

1-Colloidal PrecipitateThe particle size of this precipitate is small enough to pass through ordinary filter paper, i.e. it

behaves like true solution.

Electric double layer Similar charge in the particles no coagulation.

Coagulation of colloids

Addition of an electrolyte neutralizes ions forming the double electric layer coagulation

N.B.: The coagulation effect of electrolyte increases with increasing the valence of its ions and

vice-versa.3


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a. The solid is dispersed in the form of aggregates of molecules or ions

(while in true solution, solid is dispersed as individual molecules or ions).

b. The solid present in a colloidal suspension has a negligible effect, F.P., B.P and

osmotic pressure.

c. The colloidal suspension has the tendency to scatter visible radiation.

Properties of colloids:

d. The ratio of surface/wt is large due to small particle size.

e. The colloid is stable without coagulation due to the presence of similar electric

charges on its particles (double electric layer).


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Peptization

Upon washing a Precipitate with pure water or (distilled water) part of electrolyte (neutralizing

the double electric layer) is removed and falls below the minimum amount necessary for

coagulation leading to recharging of the precipitate again and pass into colloidal state.

2- Amorphous precipitate:

Where addition of precipitant causes rapid formation of numerous no. of very minute nuclei

which grow by joining not by deposition.

Amorphous precipitate may be formed in case of:a. substances of very low Ksp

e.g. MS (metal sulphildes) and MOH (metal hydroxides) which form amorphous precipitates as

their Ksp is greatly exceeded by addition of small amount of precipitant.

b. if the condition of precipitation reduces the solubilitye.g. precipitation of BaSO4 in 30-60% alcohol.

Disadvantages of amorphous precipitates:

a. Large surface area adsorbs impurities.

b. Weak bonds between the individual crystal nuclei in aggregates broken to colloidal ppt.5


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3. Crystalline Precipitate:

Conditions of formation of crystalline precipitate:

a. Both of the sample and precipitant must be dilute.b. Sample and precipitant must be hot as solubility rises on hot.

c. Slow addition of precipitant especially at the beginning of precipitation.

d. Stirring continuously to avoid local super saturation.

To overcome this problem we have to carry out homogenous precipitation.

• Homogenous precipitation:

Generation of the precipitating ion gradually throughout the solution by suitable chemical rxn.

Examples:

a. Urea: It is used for homogenous precipitation of Al

3+

& Fe

3+

as Al(OH)3 & Fe(OH)3 . By heatingat temperature below the B.P., urea is decomposed producing OH - gradually as represented by

the following equation:

(H2N)2CO + 3H2O 2 NH4OH + CO2

b. Dimethyl sulphate [(CH3)2SO4]: It is used for homogenous precipitation of Ba2+ as BaSO4.

Dimethyl sulphate is hydrolysed in water producing SO42- as represented by the following

equation:(CH3)2 SO4 + 2 H2O 2 CH3OH + H2SO4

minimizes contamination by adsorption

of impurities

Definite crystal shape and large particle size(small surface area)

easily filtered and washed.

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Contamination of precipitate during precipitation:

a. Coprecipitation

If the solution of the sample and precipitant are concentrated, the precipitate is formed rapidly

and becomes contaminated with the other soluble products of reaction and excess precipitant

present in the mother liquor.

Coprecipitation is classified into:

1. Adsorption coprecipitation

.e. adsorption of impurities on the surface of precipitate as in case of formation of gelatinous

precipitate, the salts present in the mother liquor are adsorbed to the surface of precipitate.

To overcome this type of contamination:

1. We have to avoid the formation of colloidial and amorphous precipitates.

2. Washing of the precipitate.

3. Drying or ignition remove volatile contaminants.

4. Choose precipitation conditions to exclude ions which are liable to be adsorbed.

5. Recrystallisation.

N.B.: Increasing the temperature decreases adsorption of impurities.

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2. Occlusion coprecipitation

In this type of contamination, impurities are coprecipitated within the precipitated particles.

Therefore it cannot be removed by washing.

There are two types of occlusion coprecipitation:

a. Mechanical occlusion

It is the simple mechanical enclosure of a portion of the mother liquor within the small hollows

or pockets formed during aggregation of amorphous particles or rapid crystal growth.

b. Adsorptive occlusion

Adsorptive occlusion is the trapping of initially adsorbed impurities within the growing crystals.

N.B. During the growing of crystals impurities are displaced by ions constituting the lattice of

the formed precipitate, however this process is incomplete.

To overcome the problem of occlusion coprecipitation, we have to carry out recrystallisationand/ or ageing.

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3. Isomorphous replacement (or formation of mixed crystals):

• Ions having the same radius as any of the ions constituting a precipitate will have priority in

taking position in the primary or secondary adsorption layer around the precipitate.

Example:

• If we precipitate Ba2+ as BaSO4, the presence of K+ in the medium causes displacement of

Ba2+ by K+ in BaSO4 crystals, while Na+ cannot displace Ba2+.

• (as K+ has radius of 1.33 A° close to Ba2+ = l.35A° while Na+ has radius of 0.95 A”).

N.B.:

Pb2+ is isomorphous to Ba2+.

CrO42- is isomorphous to SO4

2-.

B. Contamination by true precipitation

•

As precipitants are usually selective (not specific), contamination by true precipitation ofimpurity (or interfering ion) present in the medium takes place.

• This type of contamination is described as simultaneous precipitation. In this type of

contamination, the Ksp of both of the sample and interfering ion (impurity) are exceeded at

one and the same time. 9


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• In this type of contamination, the Ksp of both of the sample and interfering ion (impurity) are

exceeded at one and the same time.

Examples:

• In case of precipitation of Ba2+ as BaSO4 if Pb2+ is present it will be precipitated as PbSO4

together with BaSO4.

• In case of precipitation of Fe3+ as Fe(OH)3, if Al3+ & Cr3+ are present, they are precipitated as

Al(OH)3 & Cr(OH)3, together with Fe(OH)3.

Treatment of contamination by simultaneous precipitation:

1. The use of specific reagent to the desired component only.2. Improvement of selectivity of precipitant by suitable adjustment of pH, oxidation number of

element or the use of masking and demasking agents.

e.g.: Removal ofPb2 + (interfering ion) from the medium as PbS before precipitation of Ba2+ as

BaSO4.

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II. Digestion (ageing or ripening)

• During which the precipitate is allowed to stand with the mother liquor for 12-24 h at room

temperature or by sometimes in contact with the mother liquor.

• During standing of the precipitate with the mother liquor, the very fine crystals will go into

solution and redeposit on large crystals.

• Fine crystals which are formed during precipitation disappear, i.e., crystals tend to perfect

themselves.

• After digestion, precipitate becomes of large crystals (i.e. easily filtered) and of low surface

area (i.e. of low adsorption power) leading to minimum contamination by adsorption

coprecipitation.

• N.B.: Rise of temperature accelerate ripening, as it increases the rate of dissolution of fine

crystals.

Contamination of precipitate during digestion:

During digestion contaminants may be precipitated on the surface of the precipitate of the

sample, this phenomenon is known as contamination by post precipitation. 11


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Example:

During precipitation of Ca2+ as CaC2O4 in Ca2+/Mg2+ mixture, CaC2O4 is precipitated first (being of

lower Ksp), but if it is allowed to stand in contact with the mother liquor, MgC2O4 will be

precipitated on the surface (being of higher Ksp) after complete precipitation of CaC2O4.

Treatment of post precipitation:

1. Avoid digestion.

2. Filter rapidly after short digestion.

Post precipitation

It is the precipitation of impurity on the surface of pure precipitate of the sample to be analyzed

after its precipitation

(i.e. Ksp of pure precipitate is exceeded first then Ksp of impurity is exceeded during digestion).

Usually occurrs with sparingly soluble substances which form supersaturated solution containing

ion in common with the precipitated sample.

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• After removal of the mother liquor, final purification of precipitate is carried out by washing

to remove any adherent mother liquor together with any other soluble products of reaction

and excess precipitant present in the medium which may be adsorbed on the precipitate.

III. Washing

• N.B. We have to avoid washing with pure water to avoid peptization.

Requirements of washing liquid:

1. It should be electrolytic solution to avoid peptization.

2. It should contain electrolyte having ion in common with the precipitate if possible to

minimize solubility of precipitate during washing.

Example:

Washing of CaC2O4 with dilute solution of (NH4)2C2O4.

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3. Washing liquid should contain volatile electrolyte (e.g.NH4+ Salt, ammonia, dilute acids) to be

easily removed at temperature of drying leaving pure precipitate.

4. It should dissolve impurities without any solvent action or dispersive action on the precipitate.

5. It shouldn’t form volatile or insoluble product with the precipitate.

N.B.: NH4+ salts are the most commonly used as they are volatile on ignition.

Washing must be continued until a portion of the washing liquid (filtrate) gives negative test for

some ions which are known to be present in the original solution, sample or the mother liquor

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IV. Filtration

• It is the process of separation of the precipitate from the mother liquor.

• The filtering medium used is usually governed by the nature of precipitate and the

temperature at which drying or ignitions is carried out:

1-Ashless filter paper is used when the precipitates are dried at 100-110oC & then incinerated

(ignited).

2. If the filter paper would carbonize at the temperature of drying (>110oC), filtration must be

done on porcelain or silica filtering crucible.

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V. Drying and ignition

• Drying is usually done at temperature < 250°C, usually at (110-135°C)

• Ignition is carried out at temperature more than 250°C.

• Drying or ignition are carried out to convert the precipitate to a suitable weighed

form of definite chemical composition.

N.B.:

1. The precipitate is only dried if it has definite chemical composition and is heated toremove its water of hydration.

2. Ignition is usually done in order to convert the precipitate to a substance of definite

composition (if drying is not efficient).

3. Fe(OH)3 which contains a variable no. of water of hydration is ignited at 1200°C to

remove elements of water leaving pure Fe2O3.

Fe(OH)3 is first heated at 100-105°C for 1-2h (for drying) to remove adsorbed water

where Fe2

O3

. X H2

O is produced, then ignited to remove elements of water leaving pure

Fe2O3.16


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4. According to the nature of precipitate it may be incinerated with or apart from thefilter paper.

a. Incineration apart from filter paper:

This is done in case of precipitate which are easily reduced by the action of carbon (C°

)produced on burning the filter paper.

In this case, after drying the precipitate at 100-105°C, the precipitate is separated from

filter paper by friction, then the filter paper is incinerated in a crucible until its C° is

completely burnt. Transfer the precipitate into a crucible and incineration is completed

until the crucible acquires constant weight.

b. Incineration together with the filter paper.

Some precipitates are very stable and are not reduced by the carbon of burnt filter

paper e.g. Al2O3, in this case the precipitate is incinerated together with the filter paper.17


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Documents

Gravimetry Lectures