Copper Sulfate Assay

• The assay of copper sulfate involves cupric sulfate (CuSO4.5H2O) with a molecular weight of 249.68 gm.

Properties and Solubility

• Cupric sulfate appears as a blue, fine prism or blue crystalline powder.
• It is soluble at 20°C in 3 parts of water and glycerol.
• Cupric sulfate is almost insoluble in 95% alcohol.
• The copper sulfate should contain not less than 98.5% and not more than 101% w/w CuSO4.5H2O.
• Copper sulfate is used as a fungicide.

Titration Procedure

• Titration is performed in a stoppered flask to prevent oxidation.
• During titration, the solution changes color from brown to faint yellow.
• The procedure involves adding 1 ml of starch and 1 gm of KSCN, then completing the titration.
• The titration uses ≈0.1N Na2S2O3 .
• 10 ml of unknown solution, 30 ml of D.W., 1.5 gm of KI, and 5 ml of acetic acid are also used.

Chemical Principles

• Copper sulfate (CuSO4) is treated with excess KI in an acidic medium.
• 2CuSO4 + 4KI results in 2CuI2 + 2K2SO4.
• Cupric iodide (CuI2) formed in the reaction is unstable.
• CuI2 breaks down into cuprous iodide (CuI or Cu2I2) and I2, expressed as 2CuI2 yielding 2CuI(Cu2I2)↓ + I2, with CuI(Cu2I2) being a yellow precipitate.
• Resulting I2 is titrated with standard Na2S2O3 solution, where I2 + 2Na2S2O3 results in 2NaI + Na2S4O6.

Key Notes

• KI is added to liberate I2.
• Excess KI is added to dissolve I2.
• HAC is added to make the medium slightly acidic.
• In acidic conditions, the liberated I2 is adsorbed by the precipitated CuI.
• KSCN (potassium thiocyanate) is added near the endpoint of the titration.
• KSCN reacts with CuI to form CuSCN and KI, with the released KI reacting with CuSO4 to regenerate I2.

Calculations

• 1 ml of 0.1N Na2S2O3 is equivalent to 0.02497 gm of CuSO4.5H2O.
• The calculations involve correcting the volume of Na2S2O3, denoted as V’.
• V’ multiplied by the Chemical Factor gives the gram weight of CuSO4.5H2O in 10 ml.
• (Gm/10) X 100 calculates the %w/v of CuSO4.5H2O in the unknown sample.