Potassium dichromate primary standard

The standardisation of thiosulphate solutions may be effected with potassium iodate, potassium dichromate, copper and iodine as primary standards, or with potassium permanganate or cerium)IV) sulphate as secondary standards. Owing to the volatility of iodine and the difficulty of preparation of perfectly pure iodine, this method is not a suitable one for beginners. If, however, a standard solution of iodine (see Sections 10.112 and 10.113) is available, this maybe used for the standardisation of thiosulphate solutions. 

Note Potassium dichromate can be obtained as a primary standard reagent and hence, standard solutions may be prepared determinately and stored for long periods of time. 

Procedure Transfer 20 ml of the primary standard solution (Mohr s salt) to the titration flask and add 20 ml of 2 N sulphuric acid. Take the potassium dichromate solution in the burette. Put drops of freshly... 

Potassium dichromate, K2Cr207, is a primary standard. Its solutions are stable, and it is cheap. Because Cr20 is orange and complexes of Cr3+ range from green to violet, indicators with distinctive color changes, such as diphenylamine sulfonic acid or diphenylbenzi-dine sulfonic acid, are used to find a dichromate end point. Alternatively, reactions can be monitored with Pt and calomel electrodes. 

Among the other oxidizing agents, potassium dichromate and potassium bromate may be used as primary standards without any standardization. The solids should be dried at 150°C before weighing. Potassium metaperiodate K5I06 may also be used as a primary standard. It is stable if prepared in alkaline solution. 

Potassium iodate (KI03), potassium hydrogen iodate (KHI03), potassium dichromate (K2Cr207), and potassium ferricyanide (K3Fe(CN)6) are some of the primary standards commonly used to standardize sodium thiosulfate titrant. 

As mentioned, other oxidizing primary standards are equally efficient. Thus, the molecular reaction of potassium dichromate with potassium iodide in the presence of a strong acid produces a stoichiometric amount of iodine as follows ... 

Potassium Dichromate Solution (0.1 N, primary standard) Transfer 4.9032 g of K2Cr207 (National Institute of Standards and Technology No. 136) to a 1-L volumetric flask dissolve in and dilute to volume with water. 

Sodium Thiosulfate, 0.1 N (15.81 g Na2S203 per 1000 mL) Dissolve about 26 g of sodium thiosulfate (Na2S203-5H20) and 200 mg of sodium carbonate (Na2C03) in 1000 mL of recently boiled and cooled water. Standardize the solution as follows Weigh accurately about 210 mg of primary standard potassium dichromate, previously pulverized and dried at 120° for 4 h, and dissolve in 100 mL of water in a 500-mL glass-stoppered flask. Swirl to dissolve the sample, remove the stopper, and quickly add 2 g of sodium bicarbonate, 3 g of potassium iodide, and 5 mL of hydrochloric acid. Stopper the flask, swirl to mix, and let stand in the dark for 10 min. Rinse the stopper and inner walls of the flask with water, and titrate the liberated iodine with the sodium thiosulfate solution until... 

Standard solution Potassium dichromate can be prepared as a primary-standard chemical by recrystallization from water and drying at 150°C. Its standard solutions, usually prepared by direct weight, are extraordinarily stable. Carey found that a 0.017 M solution did not change appreciably in titer in 24 years. Dichromate reacts less readily with organic matter than does permanganate and does not react with chloride in acid solutions in the cold at concentrations less than 3 M hydrochloric acid. 

Other primary standards for sodium thiosulfate are potassium dichromate, potassium bromate, potassium hydrogen iodate, potassium hexacyanoferrate(III), and metallic copper. All these compounds liberate stoichiometric amounts of iodine when treated with excess potassium iodide. 

Potassium dichromate solutions are indefinitely stable, can be boiled without decomposition, and do not react with hydrochloric acid. Moreover, primary-standard reagent is available commercially and at a modest cost. The disadvantages of potassium dichromate compared with cerium(IV) and permanganate ion are its lower electrode potential and the slowness of its reaction with certain reducing agents. 

Potassium dichromate(VI) can be obtained in a high state of purity, and its aqueous solutions are stable. It is used as a primary standard, The colour change when chroinium(VI) changes to chromium(lll) in the reaction... 

S,Oj -(aq) -t- Ij(aq) 21 (aq) + S40t (aq) Sodium thiosulphate, NajSjOj SH]0, is not used as a primary standard as the water concent of the c stals is variable. A solution of sodium thiosulphate can be standardised against a solution of iodine, or a solution of potassium iodate(V) or potassium dichromate or potassium mangana (VlI). 

Solutions of potassium dichromate(vi) can be used as primary standards. 

As iodine and tri-iodide ions solutions are not primary standard solutions, the standardization of thiosulfate solutions may be achieved with the help of iodine solutions extemporaneously prepared from primary standards. Iodine is usually prepared by the iodate/iodide reaction or by the oxidation of iodides with potassium dichromate, but other possibilities exist. 

Potassium dichromate is a primary standard. Of course, its concentration must be the limiting factor. Moreover, we must add more than 6 moles of iodides and Mmoles of protons for 1 mole, weighed exactly, of potassium bichromate. Figure 18.5 summarizes these considerations. However, this reaction exhibits some drawbacks The iodine formation is not instantaneous and iodide ions are easily oxidized with air dioxygen in the presence of chromic ions. 

Hence iron (II) solutions can be titrated against standard potassium dichromate solution. Because of the high purity of the purest solid, it can be considered as a primary standard. Solutions are made up by weighing the finely ground solid and making up in a volumetric flask. A redox indicator is necessary for dichronate titrations. 

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