Test for sulfide ion | Qualitative Analysis of S^2- Anion

Sulfide ion forms compounds with cations. Some of them are soluble in water in water and metal sulfide compounds form precipitates. Specially, d block metal sulfides have colours which is used to identify the cation in such cases. However, alkali metal sulfides and alkali earth metal sulfides are very much soluble in water. But 3d metal sulfides are insoluble in water and show colours when they form precipitates in the water. In this tutorial, we will discuss followings in detail, how to identify sulfide anion.

Identify s block metal sulfides

There are two groups as alkali metals and alkali earth metals in s block. It's important to learn sulfides of those metal cations when identify sulfide compounds.

S block metal sulfides in water

Most of s block metal sulfides (except Strontium sulfide) are soluble in water and hydrolysis to alkaline aqueous colourless solutions releasing Hydrogen sulfide gas. Also, these s block metal sulfide emits Hydrogen sulfide gas when metal sulfides are hydrated. Therefore, you are unable to identify two s block metal sulfides by dissolving in water. As an example, you cannot see a colour when you dissolve sodium sulfide and potassium sulfide.

s block metal sulfides at solid state

Here we are going to learn some s block sulfide compounds their characteristics.

Sulfides of alkali metal: colour, physical state and water solubility

• Lithium sulfide (Li₂S) : yellow-white deliquescent powder and readily hydrolysis to hydrogen sulfide.
• Sodium sulfide (Na₂S) : Has a rotten eggs smell. yellow-pink or white crystals
• Potassium sulfide (K₂S): Colourless, because of K₂S readily reacts with water, it is rarely encounted in nature.
• Rubidium sulfide (Rb₂S): White solid

Sulfides of alkali earth metals: colour, physical state and water solubility

• Berrylium sulfide (Bes): White solid, in the presence of water, it decomposes.
• Magnesium sulfide (MgS): White (in pure form) to reddish brown powder, MgS decomposes to Hydrogen sulfide gas and Magnesium hydroxide (Mg(OH₂).
• Calcium sulfide (CaS): White solid and hydrolysis to Hydrogen sulfide gas.
• Strontium sulfide (SrS): White solid, slightly soluble in water
• Barium sulfide (BaS): White solid and hydrolysis in water

Calcium sulfide (CaS) : White solid

Test for sulfide ion: Silver nitrate and aqueous sulfide solution

Add silver nitrate to aqueous sulfide solution. It gives black Silver sulfide precipitate which is not soluble in dilute nitric acid. But it will dissolve in concentrated nitric acid.

Test for sulfide ion: With sodium nitroprusside solution

Add little bit of Sodium hydroxide to aqueous sulfide solution to make the solution alkaline. Then add sodium nitroprusside ( Na₂[Fe(CN)₅(NO)] ) solution. You can see the purple colour.

Test for sulfide ion: with aqueous barium chloride solution | BaCl₂

Add aqueous Barium chloride to the aqueous sulfide ion solution. Barium sulfide (BaS) is a colourless solution and not a precipitate like Barium sulfate (BaSO₄).

This reaction can be used to separate sulfide from and sulfate ions.

Test for Sulfide ion: Solid metal sulfide and a dilute acid

Add a dilute acid to a solid sulfide compound. Hydrogen sulfide acid (H₂S) is produced which is an weak dibasic acid.

Insoluble sulfides of p block and 3d metals

Most of 3d metal sulfide compounds and also some other p block metal sulfide compounds are insoluble in water and show different colours. Characteristics of such metal sulfides are explained in detail here.

3d and p block insoluble metal sulfide compounds

Following sulfide are precipitates and have specific colours. You have to remember those colours of precipitates when answering inorganic chemistry examinations.

Name of metal sulfide	Chemical Formula	Colour
Mercuric sulfide	HgS	Black
Antimony trisulfide	Sb2S3	red-orange
Cadmium sulfide	CdS	yellow
Ferrous sulfide / Iron(II) sulfide	FeS	Black
Stannous sulfide / Tin(II) sulfide	SnS	Yellow brown or dark brown
Tin(IV) sulfide	SnS2	yellow
Copper sulfide	CuS	Black
Copper sulfide	CuS	Black
Zinc sulfide	ZnS	White
Manganese sulfide	MnS	Pink
Nickel sulfide	NiS	Black
Cobalt sulfide	CoS	Black

Hydrogen sulfide | H₂S

Because many metal sulfide compounds has the capability of releasing Hydrogen sulfide gas in a contact of water or moisture, it is importnat to know the several characteristics of Hydrogen sulfide gas.

• H₂S has a characteristic smell of rotten eggs.
• H₂S can be identified by blackening of paper moistened with a solution of lead acetate Pb(CH₃COO)₂
• A very toxic gas. Inhaling in very low concentrations is enough to death.
• Largely produced in petroleum refining as a impurity. But can be used to manufacture sulfuric acid.

Commom questions and answers in test of sulfide ion

Here, we are going to see how metal sulfide compounds are tested and identify them from other compounds.

How to identify copper sulfide and nickel sulfide? | CuS and NiS

You may have observed that both Copper sulfide (CuS) and nickel sulfide (NiS) are black precipitates. Cations are different in compound. but anion, sulfide is common for both compounds. So we have to do the testing for cations to identify.

• Add dilute HCl to both precipitates. You can see a gas is emitted in both cases because when a dilute acid is added to a metal sulfide hydrogen sulfide is released. Also in the aqueous solutions, copper chloride and nickel chloride compounds are produced.
• Aqueous copper(ii) chloride solution is blue colour and aqueous nickel chloride is a green colour solution.
• Accoring to the colour different, copper ion and nickel ion solutions can be identified and copper sulfide and nickel sulfide black precipitates too.

Propose a method to identify sulfide ion and sulfite ion

If sulfide ion solution and sulfite ion solution are as aqueous solutions, add barium chloride solution. You can see, in one flask, a white precipitate forms which is barium sulfite. But barium sulfide is a colourless solution.

Give chemical reactions which can identify zinc sulfide and zinc carbonate

Zinc sulfide and zinc carbonate are white solids at room temperature. Also both of them form white precipitates in the water. So we cannot identify zinc sulfide and zinc carbonate from dissolving.

• Metal carbonates emit carbon dioxide when add a dilute acid. Metal sulfide emit hydrogen sulfide when add a dilute acid. Then we can test these two gases.
• Send hydrogen sulfide and carbon dioxide gas to the two separate lead nitrate solutions.
• With hydrogen sulfide, lead sulfide (a black precipitate) is given. With carbon dioxide, lead carbonate (a white precipitate) is given.
• From that two different colour precipitates, we can identify two different gases and that also tells us which precipitates are zinc sulfide and zinc carbonate

Identify lead sulfide and lead sulfate

We are going to consider about insoluble compounds of sulfide and sulfate ions such as PbS or PbSO₄. Add a dilute acid to the precipitates of lead sulfide and lead sulfate.

• You can see, both precipitate will remain same. But in one solution, you can see a gas is emitted.
• Sulfide will reacts with dilute HCl and produce hydrogen sulfide gas. You can do a test to identify hydrogen sulfide. The solution which emits the hydrogen sulfide gas is related to the lead sulfide.
• Precipitates of sulfate ion does not dissolve in dilute acids.

chemical formula for nickel(ii) sulfide

NiS. Oxidation number of nickel (+2) is mentioned inside brackets.

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