Question

We just did this lab but I'm confused on these questions and how to answer them?

Part A.) How can 6 M nitric acid be used to identify CO2-3 ions dissolved in the same solution with SO2_4 and PO 3-4 ions?

Part B.) How can you distinguish between SO2-4 and PO3-4 if they are present in the same solution?

C.) What would you do, and how would you know if a precipitate contained either AgCl, AgI, or both?

D.) Why should you test for the presence of NO-3 ions in a solution BEFORE testing for the presence of Cl- and I- ions in that solution?

E.) Why should a person NOT test for the presence of NO-3 ions in a solution that they plan to subsequently test for the presence of sulfate ions?

Answer 1

Well, here is your answer described in the following passages.

Part A

Ans. CO₃^2- comes from carbonate salt. Now this carbonate will react with concentrated nitric acid will give effervescence of CO₂. But, SO₄^2- and PO₄^3- will be remain as it was before the carbonate- nitric acid reaction. Reaction is given below:

CO₃^2- + 2HNO₃ = CO₂ + H₂O + NO₃^-

Part B: Detection of sulfate in presence of phosphate:

Ans. Both sulfate and phosphate are of similar shape and size. So, identification of one in presence of other is difficult because one interfares of other’s detection process. So, first phosphate has to be masked from the solution using meso- tartaric acid or oxalic acid. Then you have to perform the test of sulfate. To the 5-10 drops of the unknown solution, you have to add 1-2 mL of conc. HCl solution and some amount of BaCl₂ solution. Formation of the white precipitate indicates the presence of SO₄^2-.

Ba²⁺ + SO₄^2- à BaSO₄

Phosphate and carbonate do not react in the presence of HCl.

The molybdate test used for detection of phosphate in the presence of sulfate without any interference which is described below.

molybdate test:

When ammonium molybdate is added to the solution containing phosphate ions (PO₄^3-) and slightly warmed with nitric acid it forms a canary yellow precipitate of ammonium phosphomolybdate. This is a confirmatory test for the presence of phosphates.

PO₄^3- + 12(NH₄)₂MoO₄ + 21HNO₃ + 3H⁺ -> (NH₄)₃PO₄·12MoO₃↓ + 21NH₄NO₃ + 12H₂O

Part C: It is difficult to understand wheather in the mixture AgI or AgCl is present or both present because both are insoluble and present in the heterogeneous phase. Now if they are present as a single component then you can differentiate by colour of the ppt. AgI is yellow colour while AgCl is colourless or white colour. But, if your question is about the detection of Cl^- in presence of I^- then I will say put some amount of CCl4 in the aqueous solution containing both I^- and Cl^- then I^- going in the organic layer will give purple colour organic phase while Cl^- won’t give any colour or by Chromyl chloride test.

Chromyl chloride test

Chloride reacts with potassium dichromate in presence of conc. H2SO4 and libarates red fumes of chromyl chloride.

4NaCl + K₂Cr₂O₇ + 3H₂SO₄ -> 2Na₂SO₄ + K₂SO₄ + 2CrO₂Cl₂ + 3H₂O

Part D: Nothing will happen to NO₃^- test even in the presence of Cl^- but presence of I^- will interfare your test because Fe³⁺/Fe²⁺ has standard reduction potential of +o.77V whereas Cl₂/Cl^- has +1.36V but I₂/I^- has +0.54V. So, Cl^- won’t be oxidised but I^- will be oxidised to I₂. That is why you question is valid.

Part E: During test of NO₃^- you have to use FeSO4 salt which will generate SO₄^2- in your solution. So, in your solution wheather SO₄^2- is present or not, you will get positive sulfate test due to FeSO₄. For your convenience nitrate test is described below.

Brown-ring test

This test can be carried out adding a solution of FeSO₄ to the solution of nitrate salt in the test tube, followed by the slow addition of conc. sulphuric acid from another test tube through the wall of test tube containing mixture of FeSO₄ and nitrate salt. Below the aqueous solution, the sulphuric acid forms a layer. The formation of a brown ring at the junction of two layers indicates the presence of nitrate. In the reaction, reduction of nitrate ion is happened by iron (II) and itself oxidised to iron (III) and formation of a penta-aqua nitroso ferrous sulfate complex (brown ring).