Important General Chemical Tests.
1. Brown – Ring Test :
It is used for chemical analysis of nitrates in which the solution to be tested is mixed with iron sulphate solution in a test tube and concentrated H2SO4 (sulphuric acid) is carefully poured along the side of the test tube.
In nitrate containing substances a brown ring is formed where the layer of acid touches the solution (FeNO)SO4.
2. Flame Test :
It is used to identify certain elements in which a clean platinum wire is dipped into the mixture to be tested and the wire is heated using a busen flame.
The presence of certain elements can be detected by the change in the colour of flame.
For example, a brilliant organe – yellow will indicate sodium; crimson, strontium; and apple green barium.
3. Beilstein’s Test :
It is used for the detection of halogen in an organic compound in which a clean copper wire is heated in an oxidizing flam till the flame is no longer green.
The wire is then dipped in a solution of the substance to be analyzed and heated again. If CI, Br or I is present the flame turns a bright green.
4. Fehling’s Test :
It is used to detect sugars and aldehydes in a solution. Equal amounts of solution of copper sulphate (Fehling A) and sodium tartrate (Fehling B) are mixed in a test tube, On boiling this with a given solution a red precipitate forms is sugar or aldehyde is present.
5. Kjedahl Method :
It is used to measure nitrogen in an organic compound. The compound boiled with concentrated sulphuric acid and copper sulphate (catalyst) to convert nitrogen to ammonium sulphate.
An alkali is added to the mixture and boiled to distill of ammonia which is passed into a standard acid solution and estimated by titrating the solution.
6. Molish’s Test :
It is used to detect carbohydrates in a solution. The solution to be tested is mixed with a small quantity of alcoholic alphanaphthol and concentrated sulphuric acid is slowly poured down the side of the test tube.
When the two liquids meet the formation of deep violet rings indicates presence of carbohydrate.
7. Rast’s Method :
It is used to determine molecular weight by measuring the depression of freezing point of a camphor by a known weight of the solute.
8. Schiffs Test :
It is used to distinguish between aldehydes and ketones. An aqueous solution of resoniline and sulphurous acid (Schiff’s reagent) is used to test for the presence of aldehydes, which oxidize the reduced from of the dye rosaniline back to its original magenta colour.
The aldehydes restore the colour immediately whereas ketones, restore the colour slowly.
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