MAKING DOUBLE SALTS

A "double salt"has two different cations in their crystal lattice, in a definite proportion. Double salts commonly form large crystals more readily than do the single salts that make them up.

Ammonium cobalt(II) sulfate, (NH₄)₂Co(SO₄)₂.6H₂O, and ammonium nickel sulfate, (NH₄)₂Ni(SO₄)₂.6H₂O, are examples of double salts.

AIM To make a sample of crystals of ammonium cobalt(II) sulfate, and/or of ammonium nickel sulfate.

PLANNING AND PREDICTIONS

Cobalt(II) ions are obtained easily from cobalt chloride, CoCl₂, or cobalt nitrate, Co(NO₃)₂. Nickel ions can be obtained as nickel chloride, NiCl₂. Ammonium sulfate, (NH₄)₂SO₄, is a readily available compound. Both double salts have lower solubilities in water than any of the single salts that might be used in the preparation, or of ammonium chloride, NH₄Cl, or ammonium nitrate, NH₄ NO_3, that may be formed in the solution. Careful selection of the quantities and concentrations of solutions of the single salts that are mixed should lead to ready crystallisation of the required double salt.

Suitable amounts are suggested here in the form of a recipe. Solubilities and other data are listed at the end of this set of these instructions, sufficient to allow an interested student to understand how the recipe was created.

MATERIALS AND EQUIPMENT REQUIRED

• Saturated solution of nickel chloride (approx 6 g of NiCl₂.6H₂O crystals, dissolved in 5 mL of water at 25^oC) and/or saturated solution of cobalt nitrate (approx 8 g of Co(NO₃)₂.6H₂O crystals, dissolved in 5 mL of water at 25^oC).

Purified water.

Graduated pipette, 2 mL or 5 mL, with filler bulb.

Two small flasks or beakers (10 mL is ideal).

Filter (optional).

Crystallising dish.

Dry filter papers.

INSTRUCTIONS.

A. Ammonium nickel sulfate, (NH₄)₂ SO₄. NiSO₄.6H₂O

• Measure 2.0 mL of saturated nickel chloride solution into a small flask or beaker, and dilute it with an equal volume (2.0 mL) of purified water.

Measure 5.9 mL of saturated ammonium sulfate solution into a small flask or beaker, and dilute it with an equal volume (5.9 mL) of purified water.

If either solution contains suspended insoluble impurities, filter it.

Pour both solutions together into a crystallising dish, cover the dish with a light dust cover (a filter paper is suitable) and let it stand.

At a room temperature of about 25^oC, crystallisation should commence within about ten minutes, and be advanced within about one to one and a half hours.

Drain off the (pale green) supernatant solution, scrape the crystals onto a small wad of dry filter paper and sponge them dry.

The colour of the residual supernatant solution should be very pale green, indicating that the amount of nickel being discarded with the supernatant solution is very small. The yield from this preparation should be close to 90% of the theoretical maximum.

 

2. Ammonium cobalt(II) sulfate, (NH₄)₂ SO₄. CoSO₄.6H₂O

• Measure 2.0 mL of saturated cobalt nitrate solution into a small flask or beaker, and dilute it with an equal volume (2.0 mL) of purified water.

Measure 4.4 mL of saturated ammonium sulfate solution into a small flask or beaker, and dilute it with about 3.0 mL) of purified water.

If either solution contains suspended insoluble impurities, filter it.

Pour both solutions together into a crystallising dish, cover the dish with a light dust cover (a filter paper is suitable) and let it stand.

At a room temperature of about 25^oC, crystallisation should commence within less than ten minutes, and be advanced within one hour. (If up to 1.5 mL of additional water is used to dilute the initial solutions, crystallisation will be slowed down, taking over half an hour to start, and over two hours to be more or less complete.

After several hours, or on the next day, drain off the (red) supernatant solution, scrape the crystals onto a small wad of dry filter paper and sponge them dry.

The yield should be between 70% and 80% of the theoretical maximum. Greater yield can be obtained by allowing more of the supernatant solution to evaporate over two or three days.

 

 

DATA THAT COULD BE USED IN ANALYSING THE RECIPE

A. Solute concentration in saturated solution at 25^oC:

B. Approximate densities of saturated solutions at 25^oC:

• Ammonium sulfate ------1.26 g mL^-1

Cobalt nitrate -------------1.6 g mL^-1

Nickel chloride -----------1.4 g mL^-1

Consider the recipe for making ammonium cobalt(II) sulfate :

A first decision must involve the total amount that will be made. This may be a quite arbitrary decision. In this case, it is based on a need to keep total quantities small: for economy in use of raw materials, for minimising the amount of waste for disposal, and for ease, safety and efficiency of laboratory practice. The amounts made are quite large enough for easy handling and inspection.

2.0 mL of saturated cobalt nitrate solution has a mass of 3.2 g and contains a little over 1.6 g of anhydrous cobalt(II) nitrate, or 8.7 X 10^-3 mol.

For conversion to ammonium cobalt(II) sulfate, two moles of sulfate are required for every mole of cobalt, so 0.017 mol of ammonium sulfate are required, or 2.3 g.

If 100 g of ammonium sulfate solution hold 42 g, then 5.5 g of saturated solution contain 2.3 g. From the density of saturated ammonium sulfate solution, the volume required is about 4.4 mL.

If 2.0 mL of saturated cobalt nitrate solution is mixed directly with 4.4 mL of saturated ammonium sulfate solution, the total volume is 6.4 mL, containing the equivalent of 1.4 g of cobalt(II) sulfate, or 22 g/100 mL solution. While this is well below saturation concentration for pure cobalt(II) sulfate, it is well above saturation for the double salt, causing excessively rapid crystallisation. Dilution of the initial solutions slows the rate of crystallisation. The rate of dilution suggested above achieves a balance between a moderate rate of crystallisation and a fair yield of crystals at the first crystallisation, without additional evaporation of the supernatant solution.

Ammonium nitrate is extremely soluble in water, so is unlikely to be a significant contaminant of the product if crystallisation is allowed to proceed at a moderate rate.

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