Estimation of Cu in Brass -Iodometric method

Posted Date:     Total Responses: 0    Posted By: Prabha Kishore   Member Level: Gold   Points/Cash: 7   

Brass is an alloy of copper (55-80 %) and zinc (45 -20% ). Weighed quantity of brass is dissolved in Concentrated HNO3 acid. Both copper and zinc dissolve in the acid as ,their nitrates , Reddish brown nitrogen dioxide gas evolves, when the alloy dissolves.

(i)Cu + 4 HNO3----- Cu(NO3)2 +2NO2 +2H2O

Cupric nitrate, Cu(NO3)2 liberates equivalent quantity of iodine , when exess KI is added to the brass solution.

( ii ) 2Cu(NO3)2 + 4KI ------------ Cu2I2 + I2 + 4KNO3

Cuprous iodide

Iodine is sparingly soluble in water. It dissolves readily in KI solution forming tri iodide, I3 – ions. Hence exess KI solution should be added to dissolve liberated iodine. Otherwise iodine vaporizes and escapes from the solution . By estimating the amount of iodine liberated is estimated.

Iodine is estimated by titrating it with a standard sodium thiosulphate, Na2S2O3 (hypo), solution.

( iii ) I2 + 2Na2S2O3 ------- Na2S4O6 + 2NaI
Sodium tetrathionate

According to the equation (ii) :

2 molecules of Cu(NO3) = 2 atoms of copper
= 2 equivalents of iodine

Equivalent wt. of copper = its atomic wt. =63.57
The following precautions must be observed in this titration :

(i) Dissolved oxides of nitrogen (NO, NO2 etc.) must be removed from brass solution, before adding KI solution to it.Otherwise , the oxides of nitrogen also liberate I2 from KI and hence the iodine liberated will be more than the equivalent amount of Cu2+ ions present in the solution. Oxides of nitrogen are decomposed by boiling the brass solution with urea, when they are removed as H2O and N2.

(NH2)2CO + NO2 +NO -------- 2N2 + CO2 +2H2O

(ii) Strong mineral acids present in the brass solution must be neutralized with NH4OH before adding KI to it. If these acids are present in the solution, dissolved oxygen liberates iodine by oxidizing cuprous iodide. Cu2I2 to cupric iodide CuI2 which is unstable and changes to Cu2I2 and I2. Hence iodine liberated will be more than the equivalent amount of Cu2+ ions present in the solution.

(iii) The brass solution also should not be alkaline before adding KI. Cu2+ ions precipitate in alkaline medium as bluish white Cu(OH) 2 ppt which does not liberate iodine on adding KI. Hence iodine liberated will be less than the equivalent amount of Cu2+ ions present in the solution. Thus alkaline solution should be made acidic by adding a weak acid like acetic acid before adding KI soln. Best result will be obtained by maintaining the PH at 4 - 5.5.

(iv) Freshly prepared starch solution is used as indicator in this titration. Starch reacts with iodine in the presence of iodide to form intensely blue coloured complex. When enough Na2S2O3 is added to react with all iodine, including from the starch – iodine complex, the intense blue colour will be discharged and the solution becomes colourness. It appears milky white due to the presence of white Cu2I2 precipitate at the end point.

(v) The starch indicator must not be added until just before the endpoint is reached .If the starch solution is added when the iodine concentration is high some iodine may remain adsorbed even at the end-point and forms starch-iodine insoluble complex. Adsorbed I2 react slowly with Na2S2O3 solution, resulting in a higher titre valur. Hence starch indicator is to be added when the solution in the flask is faintly yellow which indicates low concentration of iodine.

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