Chemistry > Salt Analysis > 2.0 Classification of Anions
Salt Analysis
1.0 Basic Information
1.1 List of different Coloured Salts (with colourless anions)
1.2 Action of Heat (Colour of Residue)
1.3 Gases
1.4 Flame Test
2.0 Classification of Anions
3.0 Classification of Cations
4.0 Identification of Basic Radicals
2.2 Class B
1.2 Action of Heat (Colour of Residue)
1.3 Gases
1.4 Flame Test
1. Sulphates ($SO_4^{2-}$)
All sulphates except those of $Ba,\ Pb,\ Sr$ are soluble in water. Sulphates of Calcium & Mercury (II) are slightly soluble. The following tests are performed with the salt solution.
(a) Barium Chloride Solution: White precipitate of Barium Sulphate $BaS{O_4}$ insoluble in warm dil. Hydrochloric Acid & in dil. Nitric Acid, but moderately soluble in boiling, conc. Hydrochloric Acid.
$$SO_4^{2 - } + B{a^{2 + }} \to BaS{O_4} \downarrow $$
(b) Mercury (II) Nitrate Solution: It gives yellow precipitate of basic Mercury (II) Sulphate.
$$SO_4^{2 - } + 3H{g^{2 + }} + 2{H_2}O \to HgS{O_4}.2HgO \downarrow + 4{H^ + }$$
2. Chromate ($CrO_4^{2 - }$) and Dichromate ($C{r_2}O_7^{2 - }$)
Metallic chromate gives yelllow solution when dissolved in water. In the presence of ${H^+}$ chromates are converted into dichromates (orange-red solution).
$$\begin{equation} \begin{aligned} 2CrO_4^{2 - } + 2{H^ + } \rightleftharpoons C{r_2}O_7^{2 - } + {H_2}O \\ C{r_2}O_7^{2 - } + 2O{H^ - } \rightleftharpoons 2CrO_4^{2 - } + {H_2}O \\\end{aligned} \end{equation} $$
It may also be expressed as:
$$2CrO_4^{2 - } + 2{H^ + } \rightleftharpoons 2HCrO_4^ - \rightleftharpoons C{r_2}O_7^{2 - } + {H_2}O$$
- Barium Chloride Solution: Pale - yellow precipitate of Barium Chromate soluble in dil. mineral acids but insoluble in water and acetic acid. $$CrO_4^{2 - } + B{a^{2 + }} \to BaCr{O_4} \downarrow $$ Dichromate ion $\left( {C{r_2}O_7^{2 - }} \right)$ also gives the same precipitate but due to the formation of strong acid, precipitation is partial. $$C{r_2}O_7^{2 - } + 2B{a^{2 + }} + {H_2}O \rightleftharpoons 2BaCr{O_4} \downarrow + 2{H^ + }$$ If sodium hydroxide or sodium acetate is added, precipitation becomes quantitative.
- Silver Nitrate Solution: Brownish - red precipitate of Silver Chromate $A{g_2}Cr{O_4}$ which is soluble in dil. Nitric Acid & in Ammonia solution, but is insoluble in acetic Acid. $$\begin{equation} \begin{aligned} CrO_4^{2 - } + 2A{g^ + } \to A{g_2}Cr{O_4} \downarrow \\ 2A{g_2}Cr{O_4} + 2{H^ + } \to 4A{g^ + } + C{r_2}O_7^{2 - } + {H_2}O \\ A{g_2}Cr{O_4} \downarrow + 4N{H_3} \to 2{[Ag{(N{H_3})_2}]^ + } + CrO_4^{2 - } \\ A{g_2}Cr{O_4} \downarrow + 2C{l^ - } \to 2AgCl + CrO_4^{2 - } \\\end{aligned} \end{equation} $$ A reddish brown precipitate of Silver Dichromate$A{g_2}C{r_2}{O_7}$ is formed with a conc. solution of a dichromate. $$C{r_2}O_7^{2 - } + 2A{g^ + } \to A{g_2}C{r_2}{O_7}$$
- Lead Acetate Solution: Yellow precipitate of Lead Chromate $PbCr{O_4}$ insoluble in Acetic Acid, but soluble in dil.Nitric Acid is formed. $$CrO_4^{2 - } + P{b^{2 + }} \to PbCr{O_4} \downarrow $$$$2PbCr{O_4} \downarrow + 2{H^ + } \rightleftharpoons 2P{b^{2 + }} + C{r_2}O_7^{2 - } + {H_2}O$$
- Hydrogen peroxide ${H_2}{O_2}$: If an acidic solution of a chromate is treated with ${H_2}{O_2}$ a deep blue solution Chromium Penta Oxide is obtained. $$CrO_4^{2 - } + 2{H^ + } + 2{H_2}{O_2} \to Cr{O_5} + 3{H_2}O$$ $Cr{O_5}$ is unstable. So, it decomposes yielding oxygen and a green solution of a $C{r^{3 + }}$ salt.
3. Permanganate ($MnO_4^ - $)
- Hydrogen Peroxide ${H_2}{O_2}$: It decolourises acidified potassium permanganate $\left( {KMn{O_4}} \right)$ solution. $$2MnO_4^ - + 5{H_2}{O_2} + 6{H^ + } \to 5{O_2} \uparrow + 2M{n^{2 + }} + 8{H_2}O$$
- Iron (II) sulphate in the presence of sulphuric acid reduces permanganate to manganese(II). The solution becomes yellow because of the formation of Iron(III) ions. $$MnO_4^ - + 5F{e^{2 + }} + 8{H^ + } \to 5F{e^{3 + }} + M{n^{2 + }} + 4{H_2}O$$
- Action of Heat: On heating, a residue of potassium manganate ${K_2}Mn{O_4}$ and black manganese dioxide remains behind. Upon extracting with water and filtering, a green solution of potassium manganate is obtained. $$2KMn{O_4} \to {K_2}Mn{O_4} + Mn{O_2} + {O_2} \uparrow $$
| Group | Acidic Radicals | Reagent |
| I | Carbonate($CO_3^{2 - }$), Sulphite($SO_3^ - $), Sulphide(${S^{2 - }}$), Nitrite($NO_2^ - $), Acetate($C{H_3}CO{O^ - }$), Thiosulphate(${S_2}O_3^{2 - }$) | Dilute ${H_2}S{O_4}$ or Dilute $HCl$ |
| II | Chloride($C{l^ - }$), Bromide($B{r^-}$), Iodide(${I^-}$), Nitrate($NO_3^ - $) , Oxalate(${C_2}O_4^{2 - }$) | Conc. ${H_2}S{O_4}$ |
| III | Sulphate($SO_4^{2-}$), Phosphate($PO_4^{3 - }$) , Borate($BO_3^{3 - }$), Fluoride(${F^ - }$) | Do not give characteristic gas with either dil. ${H_2}S{O_4}$ or conc. ${H_2}S{O_4}$ |
