There are tests to check the presence of chloride ion in the compounds. Chloride ion can make compounds which are soluble and insoluble in water. Chloride ion is a halide ion have some different behavior from bromide and iodide ion.
In Qualitative analysis for chloride ion, colors, solubility, precipitates of chloride compounds are important and we are learn different experiments to test the presence of chloride ion.
First, we discuss which experiments can be conducted to test for chloride ion and then discuss how they are used to identify chloride ion from other anions.
Silver chloride (AgCl), lead chloride (PbCl2) and Mercurous chloride (Hg2Cl2) are all white color precipitates. Other inorganic metal chloride compounds are soluble in water.
Lead(II) chloride, white precipitate is formed when aqueous chloride solution is added to lead(II) acetate (Pb(CH3COO)2) solution or lead(II) nitrate (Pb(NO3)2) solution.
Lead(II) chloride (PbCl2), white precipitate is formed.
First, add dilute nitric acid solution to chloride ion solution. Then, Silver chloride, a white precipitate is formed when aqueous chloride solution is added to silver nitrate (AgNO3) solution.
Presence of solid metal chloride compounds can be tested from this reaction. As an example, solid NaCl is considered. Hydrogen chloride (HCl) vapor and sodium bisulfate are given as products. Produced HCl vapor can be tested by ammonia gas testing methods.
Bromide and iodide ions are oxidized to bromine and iodine respectively by concentrated sulfuric acid and that behavior can be used as a test to identify chloride ion from bromide and iodide ions.
Chloride ion is oxidized to chlorine gas by manganese dioxide (MnO2). Manganese dioxide will be reduced to manganese +2 ion. You can see, aqueous solution will become pale pink or colorless.
Add potassium dichromate (K2Cr2O7), concentrated sulfuric acid (H2SO4) to solid metal chloride compound. Then heat the mixture. A red color vapor, Chromyl chloride (CrO2Cl2) is formed.
Treat CrO2Cl2 vapor with NaOH. This will give yellow solution of chromate ion (CrO42-. Then add acetic acid (CH3COOH) and Lead(II) nitrate (Pb(CH3COO)2). A yellow color Lead(II) chromate (PbCrO4) precipitated is formed. PbCrO4(s) is soluble in dilute nitric acid.
For an example, we can consider aqueous NaBr and NaCl which gives bromide and chloride ions to the water.
Both NaCl and NaBr are soluble in water their aqueous solutions are colorless. So we cannot identify chloride ion from their colors of solutions..
We use silver nitrate solution to identify chloride ion and bromide ion and we have learned this as an experiment above in this tutorial.
Due to color difference of two precipitates in two solutions, you can identify in which solution NaCl was.
As silver nitrate solution, we can use lead(II) nitrate solution to identify chloride ion and bromide ion and we have learned this as an experiment above in this tutorial.
Due to color difference of PbCl2 or PbBr2 precipitates formed in two solutions, you can identify in which solution NaCl was.
Questions asked by students
You may hear that, bromine is reduced to bromide ion when iodide ion is added to bromine ion solution because bromine is located below the iodine in electrochemistry series. As like that, we can expect, fluorine gas can be used to oxidize chloride ions to chlorine gas while fluorine is reduced to fluoride ion.
But, reality is different from what we expected. When fluorine gas is sent to aqueous chloride ion solution, fluorine gas reacts with water violently instead of oxidizing chloride ion to chlorine. Therefore, that reaction is not possible to test chloride ion.
When ammonium chloride reacts with aqueous NaOH, ammonia gas is produced. This happen due to when ammonium salts react with alkalis to emit ammonia gas. That is not depend on chloride ion. Whether this ion is ammonium chloride or ammonium sulfate or other, ammonia gas will be released. Therefore, you cannot identify presence of chloride ion from this reaction.
When chloride, bromide and iodide ions meets concentrated sulfuric acid, they give HCl, HBr and HI respectvely. From these three, HBr and HI are not stable to heat and decompose. HBr and HI will give brown color Br2 and purple I2 vapors respectively. But HCl is stable and does not decompose. Therefore concentrated sulfuric acid can be used as a testing for chloride ion.