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MECHANISM OF PRECIPITATION

The purity and the filterability of a precipitate greatly depend upon the particle size

of the precipitate. The particle size is determined by the relative rates of the two

processes namely, nucleation which is the production of extremely small particles

(nuclei) capable of spontaneous growth and particle growth (crystal growth) which

is the growth of the nuclei.

Once the nucleation process starts, it proceeds ultimately to form the precipitate.

This can be represented with the help of actual particle size increase as given

below:

Ions (10-8 cm) → Nucleation clusters (10-8 to 10-7 cm) → Colloidal particles

(10-7 to 10-4 cm) → Precipitate (> 10-4 cm)

This shows that the nucleation clusters pass through the stage of colloidal particle

size prior to precipitation. Problems which arise with certain precipitates include

coagulation or flocculation of a colloidal dispersion of a finely divided solid to

permit its filtration and to prevent its repeptisation upon washing the precipitate.

Hence, the effect of colloidal state on the process of precipitation is also important.

The effect of the rate of precipitation on the particle size has been studied by von

Weimarn (10). He found that faster the precipitation, smaller is the particle size.

He also found that the rate of precipitation is dependent on the relative super-

saturation.

According to von Weimarn,

Where, Q-S = relative supersaturation,

Q = molar concentration of the mixed reagents before precipitation,

S = molar solubility of the precipitate at equilibrium,

K = a constant.

Applications of the above conceptions are to be found in the following recognized

procedures in gravimetric analysis:

1) Precipitation is usually carried out in hot solutions, since the solubility generally

increases with rise in temperature. 2) Precipitation is effected in dilute solution and

the reagent is added slowly and with thorough stirring. The slow addition results in

the first particles size precipitated acting as nuclei which grow as further material

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precipitates. 3) A procedure which is commonly employed to prevent

supersaturation from occurring is that of precipitation from homogeneous solution

(11). This is achieved by forming the precipitating agent within the solution, by

means of a homogeneous reaction at a similar rate to that required for precipitation

of the species.

FACTORS AFFECTING PRECIPITATION

1. Choice of precipitant: The precipitant should be such that it produces a

precipitate which is completely insoluble i.e. solubility product should not exceed

10-6 mol. The structure of the precipitate formed should be such so as to allow

rapid filtration and washing. Organic reagents have a special place in inorganic

analysis (generally termed as organic precipitants) because of the following

advantages offered by them .

i. Many of the chelate compounds are very insoluble in water, so that metal ions

may be quantitatively precipitated.

ii. The organic precipitant often has a high molecular weight. Thus a small amount

of metal may yield a large weight of precipitate, minimizing weighing errors.

iii. Some of the organic reagents are fairly selective, yielding precipitates with only

a limited number of cations. By controlling factors such as pH and the

concentration of masking agents, the selectivity of an organic reagent can often

be greatly enhanced.

iv. The precipitates obtained with organic reagents are often coarse and bulky and

hence can be easily handled.

v. Further, metal chelates are mostly anhydrous. Hence, the precipitates dry

quickly.

This can be accelerated by washing the precipitate with alcohol.

2. Amount of precipitant: The amount of precipitant added is also of great

importance. If a large excess of precipitant is added, the precipitate formed

redissolves as it raises the solubility of the precipitate and if just enough amount

is added then complete precipitation might not take place as some amount is

required to reach the solubility product value. Hence, in precipitating a substance,

a reasonable, excess of precipitant is invariably added to ensure completeness of

precipitation. The excess precipitant provides excess of common ions and the

solubility of precipitate is decreased. For analogous reasons, the precipitate is

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washed with a solution containing common ions.

3. Effect of temperature: The solubility product of a substance is constant only

when its temperature is unaltered. Usually the solubility increases with the increase

in temperature. When the precipitation is carried out at higher temperature, the

precipitate formed is of high purity due to better crystal structure. Hence, wherever

possible, precipitation which is carried out at higher

temperature is most advantageous but then it should be cooled before filtration.

4. Effect of pH: The solubility of the precipitate with the change in pH of the

solution is inevitable. The effect depends on the type of precipitate. Generally,

the precipitate of metal hydroxides and those of sparingly soluble salts of weak

acids are precipitated only in alkaline or neutral pH ranges. Smaller the dissolution

constant for the acid, higher is the pH required for practically complete

precipitation of its salt . The selectivity of organic reagents can always be

improved by the control of pH.

5. Effect of complex formation: In the presence of certain ions, the desired

component is likely to form complex ions having higher dissociation constants and

this will lead to incomplete precipitation. So the unwanted ions should be

prevented from getting precipitated out by masking them. Masking is the

procedure of forming soluble complexes with the unwanted ions and thus keeping

them in solution

WASHING OF PRECIPITATE

The wash liquid is normally water, sometimes containing an electrolyte. The

choice of the wash liquid depends on the following aspects:

1) Higher solubility for the impurities and lower solubility for the precipitate.

2) If the precipitate is a flocculated colloid, a suitable electrolyte is added to wash

the liquid to prevent peptization of the precipitate.

3) If the ions of the adsorbed impurities are of non-volatile nature, an electrolyte

which can exchange its ions with the impurities to form a volatile adsorbate is

added. These volatile ions may be removed during drying and ignition. In addition

to the choice of the suitable wash liquid, the mode of washing is also equally

important. The precipitate on the filter paper should be thoroughly stirred using a

jet of wash liquid. This should be followed by washing the edges of the filter paper

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with the jet of wash liquid since the precipitate might spread out during washing. A

large number of washes with small volume of wash liquid is more efficient to

remove the impurities than a small number of washes with large volume of wash

liquid.

DRYING AND IGNITION OF PRECIPITATES

The precipitate which has been collected by filtration and washing is dried and/or

ignited to a compound of known composition. This is then cooled under proper

condition to be weighed accurately. Use of ash-free filter papers has greatly

simplified the ignition step. The drying and/or ignition, and finally the weighing

are repeated till constant weight of the residue is obtained to ensure the completion

of these two processes.

The temperature at which the precipitate should be ignited depends on the

following factors: The precipitate should be ignited at such a temperature range at

which it is converted into a new compound of known and definite composition.

Ignition at higher than the optimum temperature should be avoided, as it may cause

loss of the precipitate due to volatilization, sublimation or decomposition. Most of

the precipitates are dried in an oven at about 373-423 K to remove water if it is

only loosely held, and not strongly adsorbed or occluded. Drying should be done at

a temperature at which antipeptization electrolyte associated with the precipitate is

completely volatilized. In such cases the precipitate will have a known and definite

composition and can be weighed.

Ignited residue must be cooled inside a desiccator containing a dehydrating agent

to remove moisture that might have been adsorbed by the residue when exposed to

the atmosphere during initial cooling. Sometimes, a carbon dioxide free

atmosphere might have to be maintained inside the desiccator if the residue is

capable of absorbing carbon dioxide.

Precipitation from Homogeneous Solution Precipitation from homogeneous solution is a technique in which a precipitating

agent is generated in a solution of the analyte by a slow chemical reaction. Local

reagent excesses do not occur because the precipitating agent appears gradually

and homogeneously throughout the solution and reacts immediately with the

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analyte. As a result, the relative supersaturation is kept low during the entire

precipitation. In general, homogeneously formed precipitates, both colloidal and

crystalline, are better suited for analysis than a solid formed by direct addition of a

precipitating reagent.

PURITY OF THE ANALYTICAL PRECIPITATE

When a precipitate separates from a solution, it is not always perfectly pure. It may

contain varying amounts of impurities due to:

1. Co-precipitation: If a precipitate is contaminated by substances which are normally soluble in the

solution under the condition of precipitation, then co-precipitation is said to have

taken place. Co-precipitation occurs by the adsorption or occlusion.

Precipitates tend to carry down from the solution other constituents that are

normally soluble, causing the precipitate to become contaminated. This process is

called coprecipitation. In other wards, coprecipitation is a phenomenon in which

otherwise soluble compounds are removed from solution during precipitate

formation.

There are four types of coprecipitation:

1.surface adsorption,

2.mixed-crystal formation,

3.occlusion,

4. mechanical entrapment.

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Types of coprecipitation: A: surface adsorption B: inclusion-isomorphic carrying (Mixed-crystal formation) C: occlusion D: mechanical entrapment in colloidal.

1- Surface adsorption

Adsorption is a common source of coprecipitation and is likely to cause significant

contamination of precipitates with large specific surface areas, that is, coagulated

colloids.

Although adsorption does occur in crystalline solids, its effects on purity are

usually imdetectable because of the relatively small specific surface area of these

solids.

The net effect of surface adsorption is therefore the carrying down of an otherwise

soluble compound as a surface contaminant. In order to minimizing adsorbed

impurities on colloids:

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· Using digestion process to improve the purity.

· Washing a coagulated colloid with a solution containing a volatile electrolyte.

The adsorbed layers can often be removed by washing.

· Reprecipitation is effective way to minimize the effects of adsorption. 2- Mixed-crystal formation Mixed-crystal formation ,one of the ions in the crystal lattice of a solid is replaced

by an ion of another element. For this exchange to occur, it is necessary that the

two ions have the same charge and that their sizes differ by no more than about

5%. This problem occur with both colloidal suspensions and crystalline

precipitates. Ex (Pb ion replace some of the barium ion). In order to minimizing

this type of coprecipitation:

· the interfering ion may have to be separated before the final precipitation step.

· a different precipitating reagent that does not give mixed crystals with the ions

interested may be used. 3- Occlusion Occlusion is a type of coprecipitation in which a compound is trapped within a

pocket formed during rapid crystal growth. , material that is not part of the crystal

structure is trapped within a crystal. For example, water may be trapped in pockets

when AgN03 crystals are formed.

Occluded impurities are difficult to remove. Digestion may help some but is not

completely effective. The impurities cannot be removed by washing.

Reprecipitation that go on at the elevated temperature of digestion open up the

pockets and allow the impurities to escape into the solution.

4- Mechanical entrapment Mechanical entrapment occurs when crystals lie close together during growth.

Here, several crystals grow together and in so doing trap a portion of the solution

in a tiny pocket.

Mechanical entrapment can be minimumize when the rate of precipitate formation

is low—that is under conditions of low supersaturation. In addition, digestion is

often remarkably helpful in reducing this types of coprecipitation.

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2. Post-precipitation: The process by which an impurity is deposited after precipitation of the desired

substance is termed as post-precipitation. When there is a possibility that post

precipitation may occur, directions call for filtration to be made shortly after the

desired precipitate is formed

Oxime

An oxime is a chemical compound belonging to the imines, with the general formula

R1R2C=N O H, where R1 is an organic side chain and R2 may be hydrogen, forming

an aldoxime, or another organic group, forming aketoxime. O-substituted oximes

form a closely related family of compounds. Amidoximes are oximes of amides with

general structure RC(=NOH)(NRR').

Oximes are usually generated by the reaction

of hydroxylamine and aldehydes or ketones. The term oxime dates back to the 19th

century, a portmanteau of the words oxygen and imine.

Structure and properties

Oximes exist as two geometric stereoisomers: a syn isomer and an anti isomer.

Aldoximes, except for aromatic aldoximes which exist only as anti isomers, and

ketoximes can be separated almost completely and obtained as a syn isomer and

an anti isomer.

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Oximes have three characteristic bands in the infrared spectrum, at wavenumbers

3600 (O-H), 1665 (C=N) and 945 (N-O).

In aqueous solution, aliphatic oximes are 102- to 10

3-fold more resistant to

hydrolysis than analogous hydrazones.

Preparation

Oximes can be synthesized by condensation of an aldehyde or a ketone

with hydroxylamine. The condensation of aldehydes with hydroxylamine gives

aldoxime, and ketoxime is produced from ketones and hydroxylamine. Generally,

oximes exist as colorless crystals and are poorly soluble in water. Therefore,

oximes can be used for the identification of ketone or aldehyde.

Oximes can also be obtained from reaction of nitrites such as isoamyl nitrite with

compounds containing an acidic hydrogen atom. Examples are the reaction of ethyl

acetoacetate and sodium nitrite in acetic acid,[4][5]

the reaction of methyl ethyl

ketone with ethyl nitrite in hydrochloric acid.[6]

and a similar reaction

with propiophenone,[7]

the reaction of phenacyl chloride,[8]

the reaction

of malononitrile with sodium nitrite in acetic acid

A conceptually related reaction is the Japp–Klingemann reaction.

Uses

In their largest application, an oxime is an intermediate in the industrial production

of caprolactam, a precursor to Nylon 6. About half of the world's supply

of cyclohexanone, more than a billion kilograms annually, is converted to the

oxime. In the presence of sulfuric acid catalyst, the oxime undergoes

the Beckmann rearrangement to give the cyclic amide caprolactam:

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Other applications

Dimethylglyoxime (dmgH2) is a reagent for the analysis of nickel and a

popular ligand in its own right. Typically a metal reacts with two equivalents

of dmgH2 concomitant with ionization of one proton.

Figure. Structure of Ni(dmgH)2.

Oxime compounds are used as antidotes for nerve agents. A nerve agent

inactivates acetylcholinesterase molecules by phosphorylation of the

molecule. Oxime compounds can reactivate acetylcholinesterate by

attaching to the phosphorus atom and forming an oxime-phosphonate which

then splits away from the acetylcholinesterase molecule. The most effective

oxime nerve-agent antidotes are pralidoxime (also known as 2-

PAM), obidoxime, methoxime, HI-6, Hlo-7, and TMB-4. The effectiveness

of the oxime treatment depends on the particular nerve agent used.[15]

Perillartine, the oxime of perillaldehyde is used as an artificial sweetener in

Japan, as it is 2000 times sweeter than sucrose.

Salicylaldoxime is a chelator[citation needed]

.

Glyoxime, produced via the condensation

of glyoxal with hydroxylamine, forms highly energetic copper, lead and

silver salts (copper, lead and silver glyoximate respectively). However these

compounds are too unstable to be of any commercial value.[citation needed]

Diaminoglyoxime, a glyoxime derivative, is a key synthetic precursor, used

to prepare various compounds, containing the highly reactive furazan ring.

Methyl ethyl ketoxime is a skin-preventing additive in many oil-based

paints.

Some amidoximes like polyacrylamidoxime can be used to capture trace

amounts of uranium from sea water[18][19]

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Dimethylglyoxime

Dimethylglyoxime is a chemical compound described by the formula

CH3C(NOH)C(NOH)CH3. This colourless solid is the dioxime derivative of the

diketone diacetyl (also known as 2,3-butanedione). DmgH2 is used in the analysis

of palladium or nickel. Its coordination complexes are of theoretical interest as

models for enzymes and as catalysts. Many related ligands can be prepared from

other diketones, e.g. benzil.

Structure

IUPAC name: 2,3-Butanedione Dioxime

Preparation

Dimethylglyoxime can be prepared from butanone first by reaction with ethyl

nitrite followed by sodium hydroxylamine monosulfonate:[1]

Ni(dmgH)2

Dimethylglyoxime is used as a chelating agent in the gravimetric analysis of

nickel:

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The use of DMG as a reagent to detect nickel was discovered by L. A. Chugaev in

1905.[2]

For qualitative analysis, dmgH2 is often used as a solution in ethanol. It is

the conjugate base, not dmgH2 itself, that forms the complexes. Furthermore, a pair

of dmgH- ligands are joined through hydrogen bonds to give a macrocyclic ligand.

The most famous complex is the bright red Ni(dmgH)2, formed by treatment of

Ni(II) sources with dmgH2. This planar complex is very poorly soluble and

so precipitates from solution. This method is used for the gravimetric

determination of nickel, e.g. in ores.

A sample of Ni(dmgH)2