Precautions in Gravimetric & Volumetric Analysis Presented by: Dar Jaffer Yousuf AEM-MA

Precautions taken during volumetric analysis Volumetric analysis: It is a quantitative analytical method in which the amount of a substance is determined by measuring the volume that it occupies and the volume of a second substance that combines with the first in known proportions

Continued All equipments like burette, beaker, pipette & volumetric flask should be washed properly with distilled water A burette should be as uniformly cylindrical as possible and the divisions etched shall correspond closely to actual contents The process of filling the pipette should be accurate to avoid excess addition of solutions Do not blow through the pipette to expel the last drop of solution from it

Continued Index finger should be used for pipetting the solution The flask should be shaking well after adding the indicator and also with addition of each drop of solution from burette The indicator should not be used in excess Read burette readings at meniscus level Put a plain paper underneath the volumetric flask to easily note the colour change The addition of acid should be drop wise

Continued Towards the end, add the solution drop by drop and swirl to avoid errors The flask should be removed as the indicator changes color Avoid formation of air bubbles in the nozzle of the burette Keep eyes in the level of the liquid surface during the time of taking the burette reading or measuring flask and pipette etc. Lower meniscus and upper meniscus are always read in case of color less and colored solutions respectively

Gravimetric Analysis Method of quantitative analysis in which any element or compound gets precipitated from its solution by using suitable reagent The precipitate can be collected by filtration, is washed and dried to remove moisture and other impurities and weighed The mass of the precipitate gives the amount of analyte in the original sample

Methods of Gravimetric Analysis Precipitation methods Volatilisation methods Electrogravimetry Thermogravimetry

Steps in a Gravimetric Analysis  Preparation of the Solution  Precipitation  Digestion of the Precipitate  Washing and Filtering the Precipitate

– Principal Reagent + Analyte Solid Product (collect and measure mass) – Desired Properties of Solid Product  Should be very insoluble  Easily filterable (i.e., large crystals)  Very Pure  Known and constant composition

Properties of Precipitates and Precipitating Reagents 1.Precipitating agent should react specifically and selectively with the analyte 2.Readily filtered and washed free of contaminants 3.Should be very insoluble 4.Unreactive with constituents of the atmosphere 5.Of known composition after it is dried or, if necessary, ignited

Particle Size of Precipitates Precipitates of large particles are generally desirable in gravimetric work – large particles are easy to filter and wash free of impurities – precipitates are usually purer than are precipitates made up of fine particles

Controlling Particle Size Elevated temperatures to increase S Dilute solutions to minimize Q Slow addition of the precipitating agent with good stirring Larger particles can also be obtained by pH control, provided the solubility of the precipitate depends on Ph Increase solubility by precipitation from hot solution

Precautions During Precipitation Precipitation must be practically complete Precipitate must be free from contamination with other substances Precipitate must be of known composition As a rule, a slight excess of the reagent must be added

Continued Precipitates should be obtained in a granular or Crystalline form For obtaining a granular precipitate a)The solution must be dilute b)It must contain a little hydrochloric acid c)It must be heated to the boiling point d)The reagent (barium chloride) must be hot and dilute and must be added slowly

Complications associated with Precipitation Coprecipitation Surface adsorption Peptization AgCl(colloidal) AgCl(s)

Precautions During Filtration Size of the filter paper Filter paper when folded must be somewhat smaller than the funnel If possible, liquids should be filtered hot Filter paper can be chemically attacked by some solutions (such as conc. acid or base), and may tear during the filtration of large volumes of solution For precipitates like Al or chromic hydroxides or ZnS filter pump shall be used

Precautions during washing of precipitates Wash with hot water, if there is no objection to its use If the precipitate settles rapidly, wash it several times by "decantation" Transfer the precipitate to the filter by means of a jet of water If any cracks or channels form in a bulky precipitate close them carefully with a jet of water Avoid over-washing as no precipitate is quite insoluble In case of colloidal precipitates dilute nitric acid, ammonium nitrate or dilute acetic acid may be used

Use of Crucibles They should not undergo any permanent change during the ignition Platinum crucibles can be employed at high temperatures but possibility of reduction with metals Silica crucibles are less likely to crack on heating than porcelain crucibles ~ smaller coefficient of expansion Ignition of substances not requiring too high a temperature may be made in porcelain or silica crucibles Fe, Ni or Ag crucibles are used in special cases

Operation of Crucibles Crucibles after ignition are allowed to cool in a desiccator before weighing Crucibles when hot should be allowed to cool in air until when held near the skin little heat is noticeable Constant heating of platinum causes a slight crystallization of its surface ~ polishing

General Precautions Number of transfers of liquids should be as small as possible to avoid the risk of slight losses Adjustment of balance Grinding of sample Calibration of glass measuring device

Methods of cleaning glass apparatus Wash with NaOH (to remove grease), then successively with water, dilute nitric acid and finally several times with water If the glass is very greasy, alcoholic NaOH or a mixture of NaOH and alcohol is more efficient than the aqueous solution Grease may be removed by prolonged treatment with chromic acid solution (a mixture of K 2 Cr 2 O 7 solution and conc. H 2 SO 4 ) A pipette or burette is clean if there is no formation of drops on the surface of the glass after the liquid is run out

References  Douglas, A.S. and Donald, M.W., Fundamentals of Analytical Chemistry. USA,804pp  Henry, P.T., An Introductory Course of Quantitative Chemical Analysis with Explanatory. Project Gutenberg, 1213pp  Welch, P.S., Limnological Methods. Narendra Publishing House, New Delhi, 1999pp  Greenfield, H., Food Composition Data, Production Management and Use. FAO

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