GRAVIMETRIIC ANALYSIS (OR) QUANTITATIVE ANALYSIS

BY WEIGHT

introduction

The substance to be determined by converting in to pure chemical compound then weighed. For conversion of compound to pure compound to follow various technique. Eg: Precipitation methods by precipitating agent. Volatilisation methods or ignition Electro gravimetry Thermo gravimetry. The substance to be measured at the pure from without chemical change by gravimetry.

ADVANTAGE

It is accurate and precise.Possible source of error and readily checked.To involve direct measurement of substance.Need only inexpensive apparatus.

PRINCIPLEIt is process to isolating and weighing an pure compound from the element. The separation of the element or the compound by following ways: 1.Precipitation methods 2.Volatilisation - ignition or drying. 3.Electro gravimetry.

PRECIPITATION

Weighed quality of sample dissolved in suitable solvent to solution form.

Precipitated by precipitation agent

Filter –dried – weighed – calculate the % of content.

VOLATILISATION OR EVOLUATION METHOD

To remove the volatile substance. It is carried out by: 1. By simple ignition in air 2. By treating with chemical agent to get volatile substance. 3. By treating with chemical agent to get non substance. The volatilised substance absorbed by suitable medium then weighed. (or) the weight of residue after volatilisation is determined .

he substance is deposited electrolytically to suitable electrode then fi lter mostly it used to estimate.

ELECTRO GRAVIMETRY

THERMOGRAVIMETRY To keep both sample and returns them apply thermal power. The measure what are the changes occuring in both sample and reference.

VARIOUS STEPS INVOLVED IN GRAVIMETRIC ANALYSIS : To follow following steps: Precipitation, Filteration, washing of the precipitate

Drying or ignition of the precipitated.

PRECIPITATION Precipitation are usually made by using beakers.Beakers is covered by clock glass.To add precipitating reagent (by using pipette or burette) by slowly with sufficient stirring.To avoid the splashing by adding precipitating at moduate speed.The stirring rod should not touch the surface or walls of the beaker.Then allow the percipitate settle overnight or by heating the precipitate.To reduce the coarseness of the precipitate.FilterCollect the precipitate

CO-PRECIPITATION The soluble compounds from solution removed during precipitation. The soluble substance present in mother liquor in solution form.

TYPES ( 4 TYPES ):Surface AdsorptionMixed Crystal FormationOcculsionMechanical Entrapment.

SURFACE ADSORPTION

By using surface adsorption process to get the co-precipitation. eg: coagulated colloids. The reaction between AgNo3 in determination chloride ion to get Agcl as colloids and also the silver ions adsorbed with nitrate or other anions.

2.MIXED – CRYSTAL FORMATION:

It is type of co-precipitation. The contaminate ion replaced by ion containing crystal. Eg: Barium sulphate formed by adding Bacl2 to a solution containing sulphate, lead, and acetate. The co-precipitation is the lead ion replaces the barium ion in Baso4 to get lead sulphate as co-precipitation.

3.OCCUSION OR MECHANICAL ENTRAPMENT:

When crystal is growing rapidly during precipitation the formation is affected by mechanical entrapment or pressure disturbance.

Digestion process is helpful to avoid the co-precipitation process.

The precipitation and occurs on the surface of the first precipitation after its formation. Eg: Formation of calcium as calcium oxalate in the presence of magnesium, but it produce presence of magnesium oxalate gradually, but it produce error in determine. Eg: Mercury II sulphide in 0.3m Hcl in the presence of zn ions but later your get zinc sulphide precipitation.

POST PRECIPITATION:

Difference between co-precipitation and post precipitation:

1. The contamination or impurities is in post precipitate.

2. But co-precipitate 3. The contamination increases in post precipitate

by (mechanical or thermal process.) digestion but in co-precipitate-contamination is

4. The magnitude of contamination by post precipitate much be greater in co-precipitate.

FILTRATION

To separate the precipitate by using filteration process.

Filter paper, sintered crucible or gooch crucible.

FILTER PAPER:Ashless filter paperWhatmann filter paper.

FILTRATION BY APPARATUS:1.Gooch crucible either porcelaim or silicon in avaliable market.2.Sintered glass or silica crucibles.GOOCH CRUCIBLES:

Asbestos pad

Perfoated plate

Particular pore Size plate

flask

Gooch crucibles

WASHING THE PRECIPITATE:

1. Moisken the filter paper2. Keep the precipitate in central portion of mat3. Pour the liquid slowly with continious stirring4. Add about 10-20ml of liquid and allow to

stand for settling of precipitate.5. To repeat this procedure 3 or 4 times.Drying: After washing the precipitate containing

definite composition of compound and dried at specified as per description. But nowdays ovens with thermo regulator available. Eg: for formulation: amodiaquine Phenobarbitone tablet.

1.Determination of sulphate as barium sulphate:

The dilute solution of barium chloride added slowly to sulphate ion to get Baso4 as precipitate. The precipitate is filtered,washed H20.Ignite and weighed as BaSo4.

Procedure: 0.3g of substance 1.25 ml H2O + 0.3 -0.6 ml CON Hcl dilute to 225ml.

heat – dropwise add 10-12ml of BaCl2.

stir check the precipitate by adding 1-2 ml of BaCl2 keep at 1hr/hot condition filter collect dried weigh

wash

DETERMINATION OF CALCIUM AS CALCIUM OXALATE:

The calcium is precipitited as calcium oxalate by treating c hot Hcl c ammonium oxalate and slowly neutralising c ammonia. the precipitate is washed c dilute ammonium oxalate then weighed. 0.2 gm of cal +10 ml H2O + 15 ml of dil Hcl Dil to 200 ml add 2gm of ammonium oxalate and ammonium up to making alkaline filter collect the precipitate wash dried.

Now days heating crucibles also available finally to keep the substance at dessicator.THERMO ANALYTICAL METHODS: To analyse the substance according to temperature.TGA- thermo gravimetric analysis-weight lossDTG-derivative thermo gravimetric –analysisDTA-differential thermal analysis-temptDSC-differential scanning calorimetry-enery input

THERMO GRAVIMETRIC CURVE To draw the graph b/w temperature vs % wt loss to get thermo gravimetric curve.Eg: CuSo4.5H2o CuSo4 (90-150 c) CuSo4.H2O CuSo4 (200-275 c) CuSo4 Cuo + So2 + ½ O2 (700-900 c) 2Cuo Cu2o + ½ O2 (1000-1100 c)

APPLICATION

Aluminum content of various compounds of aluminum analysed by precipitation technique. Nacl can be assayed by gravimetrically by acidified c HNO3.Sulphate ion in Barium sulphate.Phosphate ion can be precipitated as magnesium, Ammonium phosphate- washed, dried to pyrophosphate.

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