Determine calcium content:

Problem:

The calcium content of a water sample was determined by adding an excess of HgNH₃Y^2- solution to a 25 cm³ sample.  An anion of EDTA was then generated at a mercury cathode.  The constant current of 20 mA was needed to reach an endpoint after 3 minutes. Compute the milligrams of CaCO₃ per dm³ of sample. 

Answer:

Calcium content of a water sample

Q   = I × t = 0.02 A × l80   = 3.6 coulombs

2 Faraday would deposit 100 grams of CaCO₃

3.6 coulombs would deposit = 3.6/96500 × 100/2

= 0.00l86 gm moles of CaCO₃

25 ml of water sample contains 0.00l86 g. of CaCO₃

1 dm³ would contain 0.0744 g of CaCO₃

Amount of CaCO₃   = 74.4 mg/ dm³ or74.4 ppm. In water sample