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Techniques of Volumetric Analysis

Volumetric analysis is a practical approach towards accurate measurement of concentration, molecular mass, purity percentage, formula of compounds, percentage composition of an element and stoichiometry of a chemical equation. It involves 3 important techniques. The first one is the use of apparatus like burette, pipette and volumetric flasks. These are specially made to offer the highest degree of accuracy. The second aspect is the use of balance in weighing. Most balances come with a minimum and a maximum range – both in terms of weight measured and decimal places correctly reported. The third aspect is the use of appropriate indicators.

Volumetric analysis can further be classified into 3 techniques depending on the nature of reactions:

Acid-Base titrations which involve the reaction of an acid and a base.

Redox titrations which includes redox reaction between analyte and titrant as the key reaction.

Complexometric titration which involves the formation of a coloured complex compound.

Key Points:

  1. Volumetric analysis depends on the use of accurate apparatus like burette, pipette and volumetric flasks.
  2. Accurate weighing of substances is the key to accurate results.
  3. Indicators are often required for establishing the end-point in a volumetric analysis.
  4. Acid-Base titrations, Redox titrations and Complexometric titrations are the major techniques in volumetric analysis.

 

Essential terms for volumetric analysis

In order to carry out accurate and precise volumetric analyses, one needs to know the following terms associated with it:

  1. MOLARITY: “The number of gram moles of a solute dissolved per liter of solution”.

Molarity (M) = Wsolute/Msolute × 1/Vsovent (L)

Where, Msolute = Gram molecular weight of solute.

 

  1. MOLALITY: “The number of moles of the substance dissolved in 1kg of the solvent”.

Molality (m) = Wsolute/Msolute × 1/Wsovent (Kg)

 

  1. NORMALITY: “The number of gram equivalents of the substance dissolved per liter of the solution.

Normality = Wsolute/Esolute × 1/Vsovent (L)

Where, Esolute = Gram equivalent weight of solute.

  1. TITRATION:The procedure of ascertaining the volume of one solution essential to react entirely with a definite volume of another solution of known concentration”.
  2. TITRANT:The solution of known concentration (strength)”.
  3. TITRATE:The solution whose concentration (strength) to be ascertained”.
  4. INDICATOR:The reagent which specifies the endpoint or equivalent point of the titration”.

 

Apparatus and essential reagents

Volumetric analysis is always performed with solutions made with distilled or deionized water.

Volumetric analysis requires highly accurate graduated apparatus like burette, pipette, graduated flasks etc. which should be rinsed properly with distilled or deionized water. These must not contain any contaminants and must not be rinsed with hot water. Hot solutions should not be used for measurement with burette or pipettes.

Burette is used to deliver accurate volumes of liquid within its range. Before using the burette, it must be checked against presence of any air bubble within the liquid. Burette can be read up to two decimal places and the normal convention is to read the lower meniscus for clear liquids/ solutions and upper meniscus for dark colored solutions/ liquids.

A pipette can be used to draw a definite volume of liquid and transfer it to the conical flask/ beaker etc. The liquid should be allowed to drain out from the pipette on its own and some portion remains inside it which should be drained out by touching the tip of the pipette to the mouth of the conical flask/ beaker. The most important thing to notice is that one should never blow the liquid that remains.

Graduated flask (Volumetric flask) is used to prepare solutions of known strength.

Indicators are used to determine the end points i.e. the completion of reaction. Most of the titrations require external indicators for detecting the end points which essentially means the change in pH of the system. Some of the common examples of indicators are:

Phenolphthalein for strong acid vs strong base reactions.

Methyl orange for strong acid vs weak base reactions.

Starch for titrations involving iodine and thiosulfate.

Potassium chromate and fluorescein for Silver nitrate titrations.

Many non-acid-base titrations require the maintenance of a constant pH throughout the titration and in such cases, buffer solutions may be used for the purpose.

 

Calculations

After the experimental part is done, the results are calculated using the volume of reactant consumed. The amount of analyte is determined using the formula:

Ca = Ct Vt M / Va

Where,

Ca is the concentration of analyte, expressed in molarity.

Ct is the concentration of titrant, expressed in molarity.

Vt is the consumed volume of the titrant, expressed in liters.

M is the molar ratio of the analyte versus the reactant, obtained from the balanced chemical equation.

Va is the pipetted out volume of the analyte, expressed in liters.

 

References:

  1. http://www.science.uwaterloo.ca/~cchieh/cact/c120/formula.html
  2. http://www.knockhardy.org.uk/sci_htm_files/15volan.pdf
  3. Modern Abc Chemistry by Dr. S.P. Jauhar