Chemistry · Ch 4 — Transition and Inner Transition Elements
Chemical Properties of Potassium Dichromate
Chemical Properties of Potassium Dichromate
Potassium dichromate acts as a powerful oxidising agent specifically in ACIDIC medium (its oxidising power in neutral or alkaline medium is comparatively much weaker, in contrast with potassium permanganate, whose oxidising chemistry is explored across all three media in section 4.4.3.4). Its oxidising action in the presence of H⁺ ions is captured by the half-reaction Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O -- note carefully that the change in the oxidation state of chromium here is from Cr⁶⁺ (in dichromate) down to Cr³⁺, a substantial three-unit reduction involving the transfer of six electrons per dichromate ion (i.e. three electrons transferred per chromium centre).
This general oxidising half-reaction underlies a wide range of specific worked oxidation examples given in the textbook, each pairing acidified dichromate's reduction half-reaction with the oxidation of a different reducing substrate: (i) dichromate oxidises ferrous (Fe²⁺) salts to ferric (Fe³⁺) salts -- the classic basis of a dichromate-based volumetric estimation of iron; (ii) dichromate oxidises iodide ion (I⁻) to molecular iodine (I₂) -- the basis of an iodometric determination of dichromate concentration, where the liberated iodine is subsequently titrated against standard thiosulphate; (iii) dichromate oxidises sulphide ion (S²⁻) to elemental sulphur, visible as a milky/pale turbidity, e.g. when H₂S gas is passed into an acidified dichromate solution; (iv) dichromate oxidises dissolved sulphur dioxide (SO₂) gas to sulphate ion (SO₄²⁻); (v) dichromate oxidises stannous (Sn²⁺) salts to stannic (Sn⁴⁺) salts; and (vi) dichromate, used in excess with concentrated sulphuric acid present, oxidises ethanol all the way through to acetic acid (CH₃COOH) -- a strong-oxidant, complete oxidation outcome, in contrast to the milder partial oxidation to acetaldehyde that a weaker oxidant or more controlled conditions would instead give.
A second, entirely distinct piece of chemistry associated with potassium dichromate is the CHROMYL CHLORIDE TEST, an important qualitative-analysis confirmatory test for the presence of chloride ion. When potassium dichromate is heated together with any solid chloride salt in the presence of concentrated sulphuric acid, distinctive orange-red vapours of chromyl chloride (CrO₂Cl₂) are evolved: K₂Cr₂O₇ + 4NaCl + 6H₂SO₄ → 2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂ + 3H₂O. Crucially, this test is SPECIFIC to chloride -- bromide and iodide salts do not give the same orange-red vapour under identical conditions -- which is precisely why the test is diagnostically useful for confirming chloride ion specifically in inorganic qualitative analysis, distinguishing it from the other halides. …
Worked out. Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O -- dichromate oxidises Fe²⁺ (ferrous) to Fe³⁺ (ferric) in acidic solution, itself being reduced from Cr⁶⁺ to Cr³⁺. This reaction is the basis of the classic dichromate-based volumetric estimation of iron content in an ore or solution, since the colour change from orange (Cr₂O₇²⁻) toward green (Cr³⁺) can be followed with an internal …
Worked out. Cr₂O₇²⁻ + 6I⁻ + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O -- dichromate oxidises iodide ion to molecular iodine in acidic solution. This is the basis of an iodometric determination of dichromate concentration: the liberated iodine is titrated against standard sodium thiosulphate solution using starch as the indicator, giving the classic blue-to-colour …
Worked out. Cr₂O₇²⁻ + 3S²⁻ + 14H⁺ → 2Cr³⁺ + 3S + 7H₂O -- dichromate oxidises sulphide ion (e.g. from H₂S passed into the acidified solution) to elemental sulphur, observed as a milky/pale yellow turbidity, while the chromium is reduced from Cr⁶⁺ to Cr³⁺; this colour-and-turbidity change is used as a qualitative test for hydrogen sulp …
Worked out. Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO₄²⁻ + H₂O -- acidified dichromate oxidises dissolved sulphur dioxide gas to sulphate ion in solution, itself being reduced from Cr⁶⁺ to Cr³⁺; the colour change from orange dichromate to green Cr³⁺ is a visible confirmation of the reaction, and this half-reaction pairs with the general oxidation half-reaction Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O given earlier in …
Worked out. Cr₂O₇²⁻ + 3Sn²⁺ + 14H⁺ → 2Cr³⁺ + 3Sn⁴⁺ + 7H₂O -- dichromate oxidises Sn²⁺ (stannous) to Sn⁴⁺ (stannic) in acidic solution, with chromium going from +6 to +3. This reaction (and its reverse direction using SnCl₂ as a reducing agent) is a standard example used to illustrate the interconversion between the two common tin oxidation states in inorganic qualitative …
Worked out. 2K₂Cr₂O₇ + 8H₂SO₄ + 3CH₃CH₂OH → 2K₂SO₄ + 2Cr₂(SO₄)₃ + 3CH₃COOH + 11H₂O -- acidified potassium dichromate oxidises ethanol all the way through to acetic acid (a strong-oxidant, excess-reagent outcome, distinguishing it from milder partial oxidation to the aldehyd …
Worked out. When potassium dichromate is heated with any solid chloride salt in the presence of concentrated H₂SO₄, orange-red vapours of chromyl chloride (CrO₂Cl₂) are evolved: K₂Cr₂O₇ + 4NaCl + 6H₂SO₄ → 2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂ + 3H₂O. This distinctive vapour confirms the presence of chloride (as opposed to bromide or iodide, which do not give this reaction) in inorganic qualitative analysis. The vapour is then dissolved in NaOH, acidified with acetic acid and treated with lead acetate to give a yellow precipitate of lead chromate: CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O, followed by Na₂CrO₄ + Pb(CH₃COO)₂ → …