In volumetric analysis, the volumes of the various solutions should be measured accurately. The apparatus required is as follows:
Graduated-burette, pipette, measuring flasks, measuring cylinders, titration flasks, beakers, tile, glass-rod, funnel and wash bottle.
It is a long, cylindrical tube of uniform bore fused at the lower end with a stopcock (Fig. 23.6). k is graduated in cm3 from 0 to 50. Each division is further subdivided into ten equal parts. Therefore, each subdivision reads 0.1 cm3
Before a burette is filled with the solution, it is thoroughly washed with water so that no greasy matter is sticking inside or outside the burette. Take a small volume of solution (to be taken in it as titrant), in the burette and hold in horizontal position as shown in Fig. 23.8. Rotate the burette so as to wet the inner walls of the burette. Reject this solution through the stopcock. This process is known as rinsing. Then the burette is filled through a funnel inserted in the top Fig. 23.7. The funnel must then be taken out after filling the burette.
Care must be taken that no air bubbles remain in the narrow bottom tip of the burette. To remove this air, the stopcock is opened and the liquid is allowed to run out rapidly into the beaker or flask.
Burette reading forms the most important aspect of the experiment, therefore, burette should be read very carefully, after removing parallax.
To read the burette, hold behind the level of the liquid and in contact with the burette a piece of white paper to illuminate the surface of the liquid. This paper, called antiparallax card, eliminates errors in reading due to parallax. In order to prepare an antiparallax card take a rectangular piece of paper and fold it half. Give two cuts as shown in Fig. 23.9. Open the fold and mount it on the burette.
It is to be remembered that in case of colourless solutions lower meniscus is read while in case of coloured solutions, level is read from the upper meniscus. This is due to the reason that in case of coloured solutions lower meniscus is not visible clearly.
Placing your eye exactly in front of meniscus (Fig. 23.11 ) of the solution take initial reading and then final reading at the end. Find out the difference between two readings to obtain the volume of solution consumed.
This apparatus is used for accurate measurements of definite volume of solution. It consists of a long narrow tube with cylindrical bulb in the middle and a jet at its lower end. On the upper part of the stem, there is an etched circular mark. On the bulb is marked the volume which the pipette can deliver when filled up to the circular mark (Fig. 23.12 (a )l. Pipettes of different capacities such as 1, 2, 5, 10. 20, 25, 50 and 100 cm3 are available.
Before a pipette is filled with the solution, it is washed thoroughly and rinsed. The upper part of pipette is then held by the thumb and middle finger of the right hand, the lower end is dipped into the liquid and the solution is sucked into the pipette until the liquid level is about 2 cm above the mark. The open end of pipette is then closed with index finger. The liquid is allowed to nm slowly until the lower edge of meniscus just touches the mark. The solution is then allowed to run freely out of the pipette [Fig. 23.12 (d)]. When no more of the liquid flows out, touch the tip of the pipette with the bottom of the flask [Fig. 23.12 (e)]. Some liquid will still remain in the pipette. Do not remove it by blowing because the pipette is calibrated taking this liquid into account.