Step 1: A 12.70mL sample of cyanide solution was mixed with 25.00mL of a solution of nickel ions at an appropriate pH to form the Ni-cyanide complex. The resulting solution was titrated cs 0.0130M EDTA and required 10.10mL to reach the end point.

Step 2: Then 30.00mL of the same nickel solution appropriatly titrated separately vs the same EDTA and required 39.30mL of EDTA
a) Suggest an appropriate pH for the step 1 (acid, base, neutral) and give your reason for eliminating the other two pH regions
b) Compare the relative stability of nickel-tetra cyanide and Ni-EDTA complexes

I already figured out the answer for b) However, can anyone helps me with a)?

a) To suggest an appropriate pH for step 1, we need to consider the nature of the reactants involved. In this case, a solution of cyanide ions is being mixed with a solution of nickel ions to form the Ni-cyanide complex.

To eliminate pH regions, we can consider the properties of cyanide ions and nickel ions. Cyanide ions are typically weak bases, which means they can accept protons (H+) and tend to prefer an acidic environment. Nickel ions, on the other hand, can form complexes with strong ligands such as EDTA, which are more stable in alkaline conditions.

Considering this information, we can eliminate the neutral pH region because neither the cyanide ions nor the nickel ions favor a neutral environment.

Therefore, an appropriate pH for step 1 would be acidic. This is because cyanide ions are more stable in an acidic environment, allowing for the formation of the Ni-cyanide complex.

b) To compare the relative stability of nickel-tetra cyanide (Ni(CN)4^-2) and Ni-EDTA complexes, we can look at the stability constants of these complexes.

The stability constant is a measure of how strong a particular complex is. Higher stability constants indicate stronger complexes. By comparing the stability constants, we can determine the relative stability.

However, since the stability constants are not provided in the question, we cannot calculate the absolute values of stability constants for comparison.

In general, it is known that EDTA forms highly stable complexes with many metal ions, including nickel. EDTA has a stronger affinity for metal ions compared to cyanide ions.

Based on this information, we can infer that the Ni-EDTA complex is relatively more stable than the Ni-tetra cyanide complex. This is because EDTA forms stronger complexes with metal ions like nickel compared to cyanide ions.

Please note that the actual comparison can only be made accurately when the stability constants are known.