Potassium dichromate, (KCrO7) Potassium dichromate is one of the most important compound of chromium, and also among dichromates. In this compound Cr is in the hexavalent (+6) state. Preparation : It can be prepared by any of the following methods, (i) From potassium chromate : Potassium dichromate can be obtained by adding a calculated amount of sulphuric acid to a saturated solution of potassium chromate. K CrO H SO K CrO K SO H O potassium dichromate yellow orange 4 4 7 4 potassium chromate KCrO7 Crystals can be obtained by concentrating the solution and crystallisation. (ii) Manufacture from chromite ore : KCrO7 is generally manufactured from chromite ore (FeCrO4). The process involves the following steps. (a) Preparation of sodium chromate. Finely powdered chromite ore is mixed with soda ash and quicklime. The mixture is then roasted in a reverberatory furnace in the presence of air. Yellow mass due to the formation of sodium chromate is obtained. 4FeCrO 4 O FeO3 4CrO 3 4CrO 3 8NaCO3 6O 8NaCrO4 8CO (g) 4FeCr O 8Na CO 7O Fe O 8CO g 8Na CrO 4 3 3 4 sodium chromate Page1
The yellow mass is extracted with water, and filtered. The filtrate contains sodium chromate. The reaction may also be carried out by using NaOH instead of NaCO3.The reaction in that case is, 4FeCrO 4 16NaOH 7O 8NaCrO4 FeO3 8HO (b) Conversion of chromate into dichromate. Sodium chromate solution obtained in step (a) is treated with concentrated sulphuric acid when it is converted into sodium dichromate. Na CrO H SO Na Cr O Na SO H O 4 4 7 4 sodium chromate sodium dichromate On concentration, the less soluble sodium sulphate, NaSO4.10HO crystallizes out. This is filtered hot and allowed to cool when sodium dichromate, NaCrO7.HO, separates out on standing. (c) Concentration of sodium dichromate to potassium dichromate. Hot concentrated solution of sodium dichromate is treated with a calculated amount of potassium chloride. When potassium dichromate being less soluble crystallizes out on cooling. Na CrO KCl K Cr O NaCl 7 7 sod.dichromate pot.dichromate Physical properties (i) Potassium dichromate forms orange-red coloured crystals. (ii) It melts at 699 K. (iii) It is very stable in air (near room temperature) and is generally, used as a primary standard in the volumetric analysis. Page
(iv)it is soluble in water though the solubility is limited. Chemical properties (i) Action of heat : Potassium dichromate when heated strongly. decomposes to give oxygen. (ii) Action of acids 4K Cr O s 4K CrO (s) Cr O (s) 3O 7 4 3 (a) In cold, with concentrated HSO4, red crystals of chromium trioxide separate out. K Cr O (aq) conc.h SO KHSO (aq) CrO s H O 7 4 4 3 On heating a dichromate-sulphuric acid mixture, oxygen gas is given out. KCrO 7 8HSO4 KSO4 Cr (SO 4 ) 3 8HO 3O (b) With HCl, on heating chromic chloride is formed and Cl is liberated. K Cr O 14HCl aq CrCl KCl aq 7H O 3Cl g 7 aq 3 aq (iii) Action of alkalies : With alkalies, it gives chromates. For example, with KOH, K Cr O KOH K CrO H O 4 4 orange yellow On acidifying, the colour again changes to orange-red owing to the formation of dichromate. KCrO4 HSO4 KCrO 7 KSO4 HO Actually, in dichromate solution, the CrO 7 ions are in equilibrium with CrO 4 ions. Cr O 7 H O CrO H 4 Page3
(iv) Oxidising nature : In neutral or in acidic solution, potassium dichromate acts as an excellent oxidising agent, and CrO 7 gets reduced to Cr 3+. The standard electrode potential for the reaction, Cr O 14H 6e Cr 7H O is +1.31 V. This indicates that 3 7 dichromate ion is a fairly strong oxidising agent, especially in strongly acidic solutions. That is why potassium dichromate is widely used as an oxidising agent, for quantitative estimation of the reducing agents such as, Fe +. It oxidises, (a) Ferrous salts to ferric salts K CrO 4H SO K SO Cr SO 4H O 3 O 7 4 4 4 3 FeSO H SO O Fe SO H O 3 4 4 4 3 K Cr O 6FeSO 7H SO K SO Cr SO 3Fe SO 7H O 7 4 4 4 4 3 4 3 3 3 Ionic equation: Cr O 14H 6Fe Cr 6Fe 7H O 7 (b) Sulphites to sulphates and arsenites to arsenates. K Cr O 4H SO K SO Cr SO 4H O 3 O 7 4 4 4 3 NaSO 3 [O] NaSO 4 ] 3 K Cr O 4H SO 3Na SO K SO Cr SO 3Na SO 4H O 7 4 3 4 4 3 4 3 Ionic equation: Cr O 8H 3SO Cr 3SO 4H O 7 3 4 Similarly, arsenites are oxidised to arsenates. Cr O 8H 3AsO Cr 3AsO 4H O 3 3 3 7 3 4 (c) Hydrogen halides to halogens. K Cr O 4H SO K SO Cr SO 4H O 3 O 7 4 4 4 3 HX O HO X ] 3 Page4
K Cr O 4H SO 6HX K SO Cr SO 7H O 3X 7 4 4 4 3 where, X may be Cl, Br, I. 3 Ionic equation : Cr O 8H 6HX Cr 3X 7H O (d) Iodides to iodine 7 K Cr O H SO K SO Cr SO 4H O 3 O 7 4 4 4 3 KI HO O KOH I ] 3 KOH HSO4 KSO4 HO] 3 K Cr O 7H SO 6KI 4K SO Cr SO 3I 7H O 7 4 4 4 3 3 Ionic equation : Cr O 14H 6I Cr 7H O 3I 7 Thus, when KI is added to an acidified solution of KCrO7 iodine gets liberated. (e) It oxidises HS to S. KCrO 7 4HSO4 KSO4 Cr (SO 4 ) 3 4HO 3 O HS O HO S] 3 K Cr O 4H SO 3H S K SO Cr SO 7H O 3S 7 4 4 4 3 3 Ionic equation : Cr O 8H 3H S Cr 3S 7H O 7 (v) Formation of insoluble chromates : With soluble salts of lead, barium etc., potassium dichromate gives insoluble chromates. Lead chromate is an important yellow pigment. Pb NO K Cr O H O PbCrO KNO HNO 3 7 4 3 3 Page5
(vi) Chromyl chloride test : When potassium dichromate is heated with conc. HSO4 in the presence of a soluble chloride salt, the orange-red vapours of chromyl chloride (CrOCl) are formed. K Cr O 4NaCl 6H SO KHSO 4NaHSO CrO Cl heat 7 4 4 4 chromyl chloride orangered vapours Chromyl chloride vapours when passed through water give yellowcoloured solution containing chromic acid. CrO Cl H O HCl H CrO 4 Chromic acid. ( yellow solution) Chromyl chloride test can be used for the detection of chloride ion is any mixture. Uses : Potassium dichromate is used as, (i) (ii) An oxidising agent In chrome tanning (iii) The raw meterial for preparing large number of chromium compounds (iv) Primary standard in the volumetric analysis. Structures of Chromate and Dichromate Ions Chromates and dichromates are the salts of chromic acid (HCrO4). In solution, these ions exist in equilibrium with each other. Chromate ion has four oxygen atoms arranged tetrahedrally around Cr atom. (see Fig). Dichromate ion involves a Cr O Cr bond as shown in Fig. Page6