Volumetric Analysis: General Cosiderations

GENERAL COSIDERATIONS

Volumetric apparatus invariably used in titrimetric assays, meant either to deliver a definite volume of liquid viz., burettes and pipettes, or to contain a definite volume of liquid viz., volumetric flasks and measuring cylinders, have essentially the following three cardinal general considerations, namely :

(a) Cleaning of volumetric apparatus,

(b) Calibration of volumetric apparatus, and

(c) Effect of temperature on volumetric measurement.

These three aspects will be discussed briefly hereunder :

1. Cleaning of Volumetric Apparatus

New as well as used volumetric apparatus, namely : burettes, pipettes, volumetric flasks and measuring cylinders etc., employed in carrying out most of the pharmacopoeial assays should be extremely clean. It is particularly of great importance where small volumes of liquids are measured.

A positive evidence for a dirty apparatus may be sought by observing the adherence of droplets to the walls of a burette or pipette. However, in a clean volumetric apparatus, the liquid drains down quite uniformly thereby wetting the walls so that no droplets are visible to the naked eye.

A few very effective cleaning fluids that are used in good analytical laboratories are, namely :

(i) Chromic Acid Mixture,

(ii) Synthetic Detergent Solutions (or Alkaline Cleansing Agents), and

(iii) Teepol.

1.1. Chromic Acid Mixture

Materials Required : Sodium dichromate : 200 g ; Sulphuric acid : 1500 ml.

Procedure : Weigh 200 g sodium dichromate and transfer to a 2 Litre hard-boroslicate glass beaker. Dissolve it in 100 ml of water and cool in an ice-bath to about 10-15°C. Now, add to it 1500 ml of sulphuric acid (36 N) in small bits at intervals with constant stirring. Chromic acid mixture is extremely corrosive and hygroscopic and must be stored in closed glass-stoppered bottles.

Precautions :

(i) Chromate solution should be chilled before addition of H₂SO₄,

(ii) Safety goggles should be worn during the addition of the acid,

(iii) In case, a green colour develops, discharge the mixture into a sink with continuously flowing water,

(iv) Chromic acid must not be used for cleaning calibrated containers employed for optical measure-ments,

(v) Glass apparatus washed with chromic acid mixture must be subjected to adequate prolonged rinsing because glass (silicates and borosilicates) have a tendency to absorb the chromic acid,

(vi) Hot solutions should be avoided when cleaning accurately calibrated apparatus, due to the production of a permanent change in volume caused by heat known as thermal aftereffect,

(vii) All volumetric glasswares must be finally rinsed with purified water (distilled water) before use for analytical purposes.

1.2. Synthetic Detergent Solutions (or Alkaline Cleansing Agents)

Detergents are synthetic cleansing agents used with water. The most commonly used anionic surfactants containing carboxylate ions are known as soaps which are generally prepared by the saponification of natural fatty acid glycerides in alkaline solution. Usually a 2 to 5% (w/v) solution of a good detergent powder in water serves as a reasonably effective cleansing agent.

1.3. Teepol (or Gardinol Type Detergents)

It is a mixture of the sodium salts of sulphated fatty alcohols made by reducing the mixed fatty acids of coconut oil or cottonseed oil, and fish oils. Sometimes natural waxes such as spermaceti, wool fat and bees wax are sulphated directly.

A 1 to 3% (w/v) solution of Teepol in water may also serve as a good cleansing agent for the removal of stubborn deposits and stains present in glass apparatus.