Test for anions (copy)

The eight anions

Thiosulfate, sulfate, sulfite, carbonate, hydrogen carbonate, nitrate, chloride, phosphate

Reagent of test for thiosulfate

Dilute HCl (Hydrochloric acid)

Precipitate of test for thiosulfate

Sulfur - a yellow precipitate

Products of test for thiosulfate

4 products:

NaCl

H2O

S (yellow precipitate)

SO2 (Has a distinct smell and is quite pungent)

How to identify sulfur dioxide

By putting a spot of potassium permanganate on filter paper and putting it reaction. The purple colour disappears as sulfur dioxide acts as a reducing agent reducing Mn (+7) which is purple to Mn (+2) which is colourless

Reagent of test for sulphates

Barium chloride

Precipitate of test for sulphates

Barium sulfate (a white precipitate)

Reagent of test for sulfites

Barium chloride

Precipitate of test for sulfites

Barium sulfite (a white precipitate)

If sulphate is present when distinguishing between sulphate and sulphite

After adding dilute HCl to distinguish, no reaction occurred. Precipitate remains as barium sulphate is insoluble in dilute acid.

If sulphite is present when distinguishing between sulphate and sulphite

After adding dilute HCl to distinguish, a reaction occurs. Precipitate disappears as barium sulphite is soluble in dilute acid and sulfur dioxide is given off.

Reagent to test of presence of carbonate or hydrogen carbonate ions in aqueous solution

Dilute acid

When carbonate or hydrogen carbonate is present

Then carbon dioxide will be given off on the addition of dilute acid. Note effervescence due to release of carbon dioxide

To test for carbon dioxide gas

Bubble the carbon dioxide gas through a colourless solution of limewater. The limewater turns milky white. Note - limewater is an aqueous solution of calcium hydroxide.

Reagent to distinguish between a carbonate and hydrogen carbonate ion

Magnesium sulfate

When carbonate is present

After adding magnesium sulfate, a white precipitate forms - (magnesium carbonate)

When hydrogen carbonate is present

After adding magnesium sulfate, no precipitate forms as magnesium hydrogen carbonate is soluble in solution

If clear hydrogen carbonate solution is boiled

White precipitate is formed as hydrogen carbonates decompose in heating

Reagent(s) of test for nitrates

1- Freshly made iron (II) sulfate

2- Concentrated sulfuric acid

Test for nitrates is also known as the

Brown ring test

Method to test for nitrates

-Add sodium nitrate to test tube

-Add fresh iron (II) sulfate solution

-Add concentrated sulfuric acid (H2SO4) solution carefully down inside the test tube

-Concentrated sulfuric acid falls down the bottom, as it is more dense

-If a brown ring forms at the junction, then this shows the presence of a nitrate ion in aqueous solution

Reagent(s) of test for chlorides

-Silver nitrate solution

- (and conc. nitric acid)

Positive test for chlorides

After adding reagent(s), immediate white precipitate is formed (ionic reaction)

Reagent(s) of test for phosphates

Ammonium molybdate (and conc. nitric acid)

Condition - Heat gently

Positive test for phosphates

Yellow precipitate formed - Ammonium phosphomolybdate