Aim: To determine unknown concentration of nickel chloride from the given solution.
Reference: Svehla, G. Vogel’s Qualitative Inorganic Analysis, 7th ed. Pearson Education India. 2008. p. 143.
Requirements: Nickle chloride powder, Concentrated ammonia solution, distilled water, pipette, volumetric flask etc.
Principle: Nickle on reaction with aqueous ammonia or other hydroxides, produces green coloured precipitates of Ni(OH)2.
Ni+2 + 2OH– → Ni(OH)2↓
The precipitate is insoluble in other hydroxides, but in presence of excess ammonia precipitates gets dissolved completely.
Ni(OH)2↓ + 6NH3 → [Ni(NH3)6]+2 + 2OH–
Ni(OH)2↓ + 6NH4+ + 4OH– → [Ni(NH3)6]+2 + 6H2O
The resulting solution have clear blue colour due to formation of a complex [Ni(NH3)6]+2.
Procedure:
- Weigh accurately 2 g of nickel chloride powder and dissolve it in distilled water up to 100 mL (2% w/v solution of nickel chloride). This solution is considered as stock solution of nickel chloride.
- From, the stock solution of nickel chloride, pipette out various aliquots to have final concentration, 2, 4, 6, 8 and 10 mg/mL nickel chloride. The final volume of solution to be prepared is 10 mL.
- Use the following table to prepare different dilutions for the purpose of standard curve of nickel chloride.
| Flask No. | Solution | Volume of stock solution taken | Volume of ammonia added | Volume of distilled water added | Total volume |
| 1 | 2 mg/ mL Std. NiCl2 | 1 mL | 2 mL | 7 mL | 10 mL |
| 2 | 4 mg/ mL Std. NiCl2 | 2 mL | 2 mL | 6 mL | 10 mL |
| 3 | 6 mg/ mL Std. NiCl2 | 3 mL | 2 mL | 5 mL | 10 mL |
| 4 | 8 mg/ mL Std. NiCl2 | 4 mL | 2 mL | 4 mL | 10 mL |
| 5 | 10 mg/ mL Std. NiCl2 | 5 mL | 2 mL | 3 mL | 10 mL |
| Unknown | Unknown | 3 mL unknown | 2 mL | 5 mL | 10 mL |
- Scan the 6 mg/mL std. solution between 200-800 nm, to find out the absorbance maxima by taking mixture of 2 mL ammonia and 8 mL distilled water as a blank.
- Measure the absorbance of the remaining solutions including unknown at the absorption maxima.
- Plot the graph of absorbance Vs. concentration. Find out the equation of straight line.
- Find out the unknown concentration of nickel chloride directly from the graph as well as using equation of straight line.
Result: The given unknown solution contains _______ % w/v of nickel chloride.
Pharmaceutical Analysis III 