Laboratory Techniques For Qualitative Analysis of Salt
To identify an anion and a cation in a salt, some techniques have to be used.
1. Heating of a salt or solution in a test tube
(i) While heating a salt or a solution in a test tube, hold the test tube in such a manner that the mouth of the test tube is away from yourself or any other person working in adjacent place.
(ii) Heat the test tube gently by placing its one side in the outermost zone of the flame. While heating, shake the test tube occassionally to avoid any spurtting.
(iii) Always heat the top layer of liquid in a test tube, so that it boils quickly. Never apply flame on the bottom of test tube, otherwise bumping will start. You may use a piece of procelein to avoid bumping.
(iv) While heating for a long time, use a test tube holder. Hold it between your thumb and the fingers if the volume in the test tube is less then half and you have to only warm the contents.
2. Use of a Reagent Bottle
Take out the desired reagent bottle from shelf. Remove the stopper and hold it in right hand. Hold the test tube between the thumb and first two fingers of the left hand. Now pour the required quantity of the reagent along the side of the test tube. Put the bottle back on the shelf and stopper it immediately. Never put the stopper on the table.
3. Testing of a gas evolved
On adding a reagent to a salt if effervescence are given out in cold or on heating it indicates the evolution of a gas. The gas evolved can be tested in the following manner:
(i) Note the colour of gas evolved.
(ii) Smell the evolved gas by puff of hand. Never inhale a gas by placing the nose directly into the vapour.
(iii) By bringing a filter paper strip or a glass rod with its end dipped into the testing reagent near the mouth of the test tube.
(iv) By passing the gas evolved for few seconds through a reagent taken in a test tube, shake the test tube well and observe the change.
4. Precipitation
Insoluble compound formed by addition of a reagent to a solution under the test or passing a gas through the solution under the test is called a precipitate and the process is called precipitation. As soon as a clear solution turns turbid, it is an indication that precipitate is formed.
In some cases the precipitate may dissolve in excess of precipitating reagent, in that case two observations should be taken, one on addition of precipitating reagent drop wise and second on adding excess of precipitating reagent.
5. Filtration
It is a process by which an insoluble compound is separated from a reaction mixture. To carry out filtration, first the filter paper cone is made. It is then opened in such a way that the three layers of paper are on one side, while the fourth layer is on the other side. By doing so a hollow cone of filter paper is obtained.
The cone is then placed in funnel and by wetting with water, it is fixed properly but gently in funnel. The liquid mixture is then poured over the filter paper funnel along the side of a rod. Never fill the filter paper cone more than 2/3rd of its capacity. Note that there should be no space between filter paper cone and glass funnel. It reduces the rate of filtration.
The liquid passes through the pores of filter paper and collects in the vessel kept below the stem of the funnel. The clear liquid so collected is called Filtrate. The insoluble compound or precipitate, which remains over the filter paper is called residue.
6. Washing and collection of precipitate
It is essential to wash the precipitate before carrying out any test on it. The washing is done by slowly pouring distilled water over the precipitate while it is still in the funnel. After washing, carefully remove the filter paper from funnel open up and spread it on a dry filter paper and remove the precipitate carefully with the help of a spatula and collect it on a watch glass. Use a small portion of it each time for carrying out various tests.