PREPARATION OF CIS-DIOXALATO DIAQUO CHROMATE (III) ION

 PREPARATION OF CIS-DIOXALATO DIAQUO CHROMATE (III) ION

Theory:

The cis isomer of this complex is obtained when a dry mixture of potassium dichromate and (crystalline) oxalic acid is gently warmed in a china dish in the presence of a small amount of water. A strong reaction took place and a thick-colored liquid was obtained. On trituration of the liquid with ethyl alcohol, a solid with purple-black granular crystals is obtained.

Chemical Equations: 

K₂Cr₂O7 + 7(COOH)₂ .2H₂O    →    2K{Cr(C₂O₄)₂ (H₂O)₂} 2H₂O + 6CO₂ + 13H₂O

Requirement:

1. Oxalic acid crystals     = 6.0 gms.

2. Potassium dichromate (solid) = 2.0 gms.

3. Alcohol  = 50.0 ml

Procedure :

(i)Take a dry mortar and add 6 g of oxalic acid crystals and 2 g of potassium dichromate crystals.

(ii) Gently grind the mixture with a pestle until the powder mixes well.

(iii) Take a China dish (10 cm in diameter) and moisten it with water. Drain excess water.

(iv)Introduce the mixture to the center of the china dish with a compact heap.

(v) Cover the china dish with a clock glass.

(vi) Gently heat the contents of the China dish over low heat (or a small microscale burner).

(vii) After a few minutes, a vigorous reaction with foaming due to the generation of carbon dioxide and water vapor begins.

 (viii) Immediately the reaction spreads throughout the mixture, resulting in a dark-colored liquid.

(ix)Pour about 20 ml of ethanol into this liquid without waiting for the thick liquid to cool.

(x) Use a metallic spatula to grind the contents until a solid is formed.

(xi) If it does not solidify completely, decant the liquid and add another 20 ml of ethanol.

(xii) Warm and resume grinding until the product is in the form of granular crystal. 

(xiii) Crystals appear black in diffuse daylight and deep purple in artificial light.

(xiv) Weigh and record the yield.

Result:

Colour = Black in diffused daylight and deep purple in artificial light. Yield =..........

Theoretical yield =...........

%age yield =.................

Precautions :

(i) Use only dry mortar and pestle.

(ii) Heat gently otherwise reaction will go out of control.

(iii) Avoid naked flame.

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