Cementation experiment (for Copper)

Aim

Cemenatation of Copper from an aqueous solution of Copper Sulfate using iron metal.

Chemicals and apparatus required for the experiment.

1. Copper Sulfate (CuSO$_4$)
2. Water (H$_2$O)
3. $1$ L beaker
4. Iron metal sheet
5. Magnetic stirrer

Principle and Theory behind the experiment

Precipitation of copper from an aqueous solution of copper sulfate (CuSO$_4$) by scrap iron is known as Cementation. The precipitated metal is usually cemented on the added metal's surface. In galvanic series, the metal having the higher emf series is more reactive  which is anodic metal. The metal present lower in the series is less reactive which is the cathodic metal.

In the galvanic series, iron is present above Copper, hence iron is more reactive comparatively. So, when iron gets dissolved in the CuSO$_4$ solution, iron substitutes the copper in the solution. Thus, Iron gets oxidized while copper is reduced. Thus copper is deposited on the iron metal sheet, hence undergoing precipitation.  

The pH (i.e. the negative log of the hydrogen ion concentration) of the solution is very important factor in cementation. H$_2$SO$_4$ is added in the solution to maintain acidic pH of the solution. It is maintained to about ~2 to prevent the hydrolysis of Fe$^{+2}$ and Fe$^{+3}$ ions. Appreciable amounts of iron is dissolved in the acid present with generation of hydrogen. Ferric iron is also formed and it's presence contributes to the dissolution of more scrap iron. 

The following reactions take place at the metal-solution interface.

Cu$^{+2}$ $+$ Fe $\longrightarrow$ Fe$^{+2}$ $+$ Cu$^{0}$

Fe $+$ 2Fe$^{+3}$ $\longrightarrow$ 3Fe$^{+2}$

The presence of trace amounts of arsenic and phosphorus as impurities in scrap iron favors the formation of highly poisonous gases Arsine (AsH$_3$) and Phosphine (PH$_3$), through the following reactions

Fe$_3$As$_2$ $+$ 6H$^+$ $\longrightarrow$ 3Fe$^{+2}$ $+$ 2AsH$_3$

Fe$_3$P$_2$ $+$ 6H$^+$ $\longrightarrow$ 3Fe$^{+2}$ $+$ 2PH$_3$

The product is usually contaminated by clay, iron oxide and metallic iron. A linear powder is obtained at high Cu$^{+2}$ ion concentration.

Results and Conclusions

The reaction involved is a zero order kinetics, i.e. the rate of reaction is constant.

Precautions to be measured while performing the experiment

1. Iron sheet should be polished properly to remove metal surface before reaction.
2. H$_2$SO$_4$ should be added before the reaction.
3. Time should be noted and proper time interval of reaction should be maintained while the iron is dipped in the solution.
4. The iron metal should be completely immersed into the copper sulfate solution.