Nickel forms single-phase solid solutions with some metals such as copper, iron, and chromium. Nicke — Organic Chemistry Chemistry Question

Nickel forms single-phase solid solutions with some metals such as copper, iron, and chromium. Nickel and copper feature unrestricted mutual solubility. Copper-nickel alloys, also referred to as cupronickels, possess different properties depending on their composition. The most used cupronickels contain 10 to 45 % of nickel.

The main properties of copper-rich alloys (70 – 90 % of copper) include high resistance to corrosion, electrical conductivity, ductileness, strength at elevated temperatures. These features make the alloys highly sought in various industrial applications: construction of sea water corrosion-resistant facilities including oil rig platforms (which is of particular importance for Azerbaijan!), condenser systems in desalination plants, cooling circuits, and ammunition. Small amounts of other elements are usually added for specific purposes; the two most popular alloys for marine applications contain up to 2 % iron and manganese. The alloy composed of copper with 45 % nickel content provide for almost exactly constant resistance regardless of temperature, and is thus used in the production of thermocouples and resistance wires in high precision resistors. The copper-nickel alloy containing 25 % nickel with an additive of 0.05 - 0.4 % manganese is commonly used in manufacturing coins and medals.

Precipitation of nickel as dimethylglyoximate followed by its weighing is widely used for accurate determination of this metal in steels and alloys. Nickel can also be assayed by treatment of a Ni²⁺ containing solution with KI and KCN followed by titration with silver nitrate.

Only metal ions (Zn(II), Cu(II), Ca(II) or Mg(II)) rapidly forming EDTA complexes can be determined by direct titration with EDTA. Since the EDTA complex of nickel(II) is formed slower, the back titration is used in the latter case: EDTA is added in an excess and the unreacted EDTA is back titrated with Ca(II) or Mg(II), the metals that also form colored complexes with a suitable indicator (such as Eriochrome black T), still less stable than the nickel one.

In this task you will precipitate nickel from its ammonia solution with dimethylglyoxime in the presence of citric or tartaric acid as a masking agent* and then determine the metal content by complexometric titration with Eriochrome black T as the indicator.

Notes.
*At this stage, gravimetric determination of nickel as dimethylglyoximate may be carried out. However, it requires drying to constant weight, which may be time consuming.
The method gives best results with the alloys containing less than 0.5 mass % of Cu.

Chemicals and reagents
* Alloy sample, ~0.5 g, or a test solution (a solution containing about 1 g dm–3 of Ni²⁺, 0.5 – 0.7 g dm–3, Fe³⁺, 5 – 6 g dm–3 Cu²⁺),
* Diluted nitric acid (1 : 1, v/v),
* Diluted hydrochloric acid (1: 1, v/v),
* Diluted sulfuric acid (1 : 1, v/v),
* Citric acid or tartaric acid,
* Ammonia solution, concentrated,
* Dimethylglyoxime (10 g dm–3 in ethanol),
* Ammonium chloride, 10%,
* Sodium hydroxide (200 g dm–3),
* Hydrogen peroxide solution, 3%,
* Eriochrome black T (as a mixture with NaCl, 1 : 100 w/w),
* Standard 0.05 mol dm–3 EDTA solution: Dissolve 18.61 g of solid ethylenediamine tetraacetate disodium dihydrate in 500 cm₃ of distilled water, place the mixture into a measuring flask and dilute to 1 dm₃.
* Ammonia – ammonium chloride buffer solution, pH 10: Dissolve 70 g of solid NH₄Cl in 600 cm₃ of concentrated (~15 mol dm–3) ammonia and dilute to 1 dm₃ with distilled water.
* Magnesium sulfate solution (0.05 mol dm–3): Dissolve 12.33 g of solid magnesium sulfate heptahydrate in 500 cm₃ of distilled water and dilute to 1 dm₃.

Equipment and glassware
* Analytical balance (± 0.0001 g),
* Glass beaker, 250 and 400 cm₃,
* Watch glass,
* Hotplate stirrer,
* Volumetric flask, 500 and 100 cm₃,
* Paper filters (3 ea.),
* Funnel,
* pH Indicator paper,
* Burette, 25 or 50 cm₃ (2 ea.),
* Funnels (to fill the burettes).
* Volumetric pipette, 10 cm₃,
* Erlenmeyer flask, 100 cm₃ (3 ea.),
* Graduated cylinders, 10 and 25 cm₃,
* Wash bottle with distilled water.

Procedure

A. Standardization of MgSO₄ solution
1. Fill a burette with the standard Na₂H₂EDTA solution. Transfer 5.00 cm₃ of the solution into the 100 cm₃ Erlenmeyer flask. Adjust pH to 10 with the ammonium buffer solution (3 – 4 cm₃). Add 20 – 30 mg of the Eriochrome black T indicator.
2. Fill the other burette with MgSO₄ solution. Titrate the prepared EDTA solution with MgSO₄ under continuous stirring until the blue color sharply turns into purple (the change must be irreversible). Record the volume of MgSO₄ solution consumed in titration. Repeat the titration until your get consistent results.
The concentration of MgSO₄ solution (M) is found from the following equation:
c₁ = V₀ · c₀ / V₁,
where
V₀ is the volume of Na₂H₂EDTA taken for titration, cm₃,
V₁ is the volume of magnesium sulfate consumed in titration, cm₃,
c₀ – the concentration of Na₂H₂EDTA solution.

B. Dissolution of the alloy sample (the experiment should be carried out under a fume hood)
(Skip this stage and proceed to part C if you are analyzing a test solution rather than an actual alloy sample.)
1. Accurately weigh out the alloy sample, place it into a 250 cm₃ beaker and carefully dissolve in 15 cm₃ of nitric acid (diluted with water, 1 : 1 v/v). Cover the beaker with the watch glass.
2. Gently heat the solution and boil it on a hot plate until dissolution is complete (the residual volume should be of about 5 cm₃). Transfer the solution into the 500 cm₃ volumetric flask, rinse the watch glass and the beaker with distilled water, add the wash water to the volumetric flask, and dilute to the mark using the wash bottle.
3. If the alloy sample cannot be totally dissolved (as it may contain W and/or Si), heat the mixture to dryness, add 10 cm₃ of HCl (1 : 1 v/v) and heat to dryness again. Dissolve the dry residue in 10 cm₃ of concentrated HCl and dilute it with 100 cm₃ of distilled water. Filter off the precipitated tungstic acid using two filters and a long-stem funnel. Wash the precipitate with hot diluted HCl solution (1 : 10 v/v) until no Ni²⁺ ions are detected in wash waters (test with dimethylglyoxime).
4. If the sample contains more than 0.1 mass % of Si, add 10 cm₃ of H₂SO₄ (1 : 1 v/v) and evaporate till abundant evolving of sulfuric acid fumes. Cool the slurry and carefully add about 10 cm₃ of cold water. Then add 100 cm₃ of hot water and dissolve the residue under heating. Filter off the precipitated silicic acid with filter paper and wash the precipitate with hot water. Transfer the filtrate obtained after W and/or Si separation to a 500 cm₃ volumetric flask and dilute to the mark.

C. Precipitation of nickel dimethylglyoximate
1. Transfer 50.00 cm₃ of the test solution into the 400 cm₃ beaker, adjust the volume with water to 200 cm₃ and add 6 – 8 g of solid tartaric or citric acid. Heat the solution on the hot plate until dissolution of the acid is complete. Neutralize the mixture to pH 4 – 5 with 5 – 10 cm₃ of ammonia solution (check against the pH indicator paper).
2. Add 25 cm₃ of the dimethylglyoxime solution in ethanol dropwise under intensive stirring, add 2 – 3 cm₃ of the concentrated ammonia solution to obtain pH 10 and then the other 2 – 3 cm₃ portion of the same solution to provide for NH₃ excess. You will observe the precipitation of nickel dimethylglyoximate.
3. If iron hydroxide precipitates as the solution turns alkaline, add more tartaric or citric acid.
4. Heat the solution with the precipitate on the hot plate until boiling (do not allow boiling!) and store it in a warm place for 40 – 50 min.
5. Filter the precipitate using the filter paper and wash it with 4 – 5 portions of hot distilled water. Then wash the precipitate off to the 400 cm₃ beaker, first with 30 – 50 cm₃ of HCl (1 : 1 v/v) and then hot water. If you failed dissolving the precipitate, heat the solution and slightly boil it with stirring.
6. Cool the solution down to room temperature, transfer to a 100 cm₃ volumetric flask and dilute to the mark with distilled water.

D. Determination of Ni 2+
1. Transfer 10.00 cm₃ of the prepared Ni²⁺ solution into the 100 cm₃ Erlenmeyer flask. Adjust pH to 10 with the ammonium buffer solution (4 – 6 cm₃) and add 10.00 cm₃ of the standard Na₂H₂EDTA solution from the burette. Add 20 – 30 mg of the Eriochrome black T indicator to obtain a blue colored solution.
2. Fill the burette with the standard MgSO₄ solution and take the initial reading of the burette. Titrate the blue nickel(II) solution with the standard MgSO₄ solution until the color becomes permanently purple. Take the final reading of the burette. Repeat the titration until your get consistent results.
3. Calculate the amount of Na₂H₂EDTA spent for Ni²⁺ titration based on the volumes of the Na₂H₂EDTA solution added and the MgSO₄ solution consumed in the titration.