Test for Cations and Anions

Experiment 9

https://drive.google.com/file/d/17TYYDZrk7qxvBBw2-Vmxo44lhdc7wTJn/view?usp=drivesdk

 

Topic: Qualitative Analysis

Purpose: To determine the anions in inorganic salts.

Procedures:

(a) You are required to plan and carry out tests to identify the anions in a solid mixture KA1.

In all the tests, the reagent should be added gradually until no further change is observed. Record your observations and deductions you make from them in the spaces provided.

Deduce what you can about the anions in KA1. Observations should include details of colour changes, precipitates and tests on gases evolved, and you should indicate clearly at which stage in a test a change occurs.

Test:

1. KA1 is heated gently then strongly heated. Gas evolved is test with:

a. damp blue/ red litmus paper and

b. a glowing wooden splinter/ a lighted splint held near test tube’s mouth.

Observation: …………………………..
Deduction: …………………………..

2. KA1 is then added with a few drop of conc. H2SO4.

Observation: …………………………..
Deduction: …………………………..

3. KA1 is dissolved using distilled water and was separate into 5 portion for further testing.

i. Add 2 mL of conc. FeSO4 into KA1, followed by 2 drop of conc H2SO4.

Observation: …………………………..
Deduction: …………………………..

ii. Add 2 g of Devarda alloy to KA1 solution.

Observation: …………………………..
Deduction: …………………………..

iii. Add 2 mL of AgNO3 into KA1

Observation: …………………………..
Deduction: …………………………..

iv. Add 2 mL of Pb(NO3)2 into KA1.

Observation: …………………………..
Deduction: …………………………..

Identify of anions present in KA1:

………………………………………………………

Devarda ‘s alloy is used as reducing agent in analytical chemistry for the determination of nitrates after their reduction to ammonia under alkaline conditions.
It was often used in the quantitative or qualitative analysis of nitrates in agriculture and soil science before the development of ion chromatography, the predominant analysis method largely adopted worldwide today.

Devarda’s Alloy Powder.
Zn : 50%
Al : 45%
Cu : 5%

Test for Cations and Anions

Cation test.

What makes a white precipitate?

When the reaction occurs in a liquid solution, the solid formed is called the ‘precipitate’. The chemical that causes the solid to form is called the ‘precipitant’. If silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs forming a white precipitate of silver chloride.

What reacts with sodium hydroxide to form a blue precipitate?

Copper sulphate reacts with sodium hydroxide to form a blue precipitate of copper hydroxide and sodium sulphate.

Reaction Mg ion with NH3?

Magnesium ion rarely forms complex ions. All salts are white; most are soluble in water.

Aqueous ammonia precipitates white gelatinous Mg(OH)2 :

Mg2+ (aq) + 2NH3 (aq) + 2H2O (l)<=> Mg(OH)2 (s) + 2NH+4 (aq)

Ammonium salts dissolve Mg(OH)2 or prevent its precipitation, when added to aqueous ammonia. This is a buffer effect and shifts the pH to a lower value, causing a shift of the precipitation equilibrium in the above equation to the left.

Anion test.

What happens when sodium thiosulphate Na2S2O3 reacts with hydrochloric acid?

Reaction between Sodium thiosulphate and Hydrochloric Acid Sodium thiosulphate reacts with dilute acid to produce sulphur dioxide, sulphur and water. However, as the reaction proceeds, sulphur precipitates that make the solution more opaque and eventually the mark gets completely masked.

What does the positive KMnO4 test indicate?

When a purple solution of the oxidizing agent KMnO4 is added to an alkene, the alkene is oxidized to a diol and the KMnO4 is converted to brown MnO2. Thus, if the purple color changes to brown in this reaction, it is a positive reaction. Alkanes and aromatic compounds do not react with potassium permanganate.

How do we test sulphate, SO4 2- and sulphide, SO3 2-?

By adding barium chloride to the solution you are testing, a white precipitate will form. If you add hydrochloric acid, the sulphate (SO42-) WILL NOT dissolve and the sulphite (SO32-) will. An acrid choking  (unpleasantly bitter or pungent) sulfur dioxide gas formed. Identify any sulphite salt + hydrochloric acid ==> chloride salt + sulphur dioxide.

How to test decolourise KMnO4 with SO2?

Acidified KMnO4 can be decolorized by SO2. Potassium permanganate (KMnO4) has a purple colour. Potassium permanganate is an oxidizing agent. When sulphur dioxide reacts with potassium permanganate the solution decolourizes.

What indicates of test any gas SO2 evolved with fresh potassium dichromate(VI) paper?

The dichromate paper turns from orange to green. The sulphur dioxide reduces the dichromate(VI) to chromium(III). Note: sulphites do not give ppt. with acidified barium chloride/nitrate because sulphites dissolve in acids.

What happens when you add NH4Cl to NH3?

The addition of NH4Cl to a solution of ammonia shows the effect of adding a common ion to a system at equilibrium. Addition of NH4Cl adds NH4+, driving the reaction to the left and consuming OH-.

 

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