Procedure
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 25oC, 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.
B. 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 crystallizing dish, cover the dish with a light dust cover (a filter paper is suitable) and let it stand.
- At a room temperature of about 25oC, crystallization 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, crystallization 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.
Aim
To make a sample of crystals of ammonium cobalt (II) sulfate, and/or of ammonium nickel sulfate.
Materials and Equipment Required
- Saturated solution of nickel chloride (approx 6 g of NiCl2.6H2O crystals, dissolved in 5 mL of water at 25oC) and/or saturated solution of cobalt nitrate (approx 8 g of Co(NO3)2.6H2O crystals, dissolved in 5 mL of water at 25oC)
- Purified water
- Graduated pipette, 2 mL or 5 mL, with filler bulb
- Two small flasks or beakers (10 mL is ideal)
- Filter (optional)
- Crystallizing dish
- Dry filter papers
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 solubility in water than any of the single salts that might be used in the preparation, or of ammonium chloride, NH4Cl, or ammonium nitrate, NH4 NO3, 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 crystallization of the required double salt.
Suitable amounts are suggested here in the form of a recipe. Solubility 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.
Data That Could Be Used in Analyzing the Recipe
A. Solute concentration in saturated solution at 25oC:
|
Compound (as anhydrous substance)
|
Formula mass
|
grams solute per
100 mL water
|
grams solute per
100 g solution
|
|
NH4NO3
|
80
|
208
|
|
|
NH4Cl
|
53.5
|
39
|
approx 28
|
|
(NH4)2SO4
|
132.1
|
76
|
approx 42
|
|
Co(NO3)2
|
182.9
|
102
|
approx 51
|
|
CoSO4
|
155
|
38
|
approx 28
|
|
NiCl2
|
129.6
|
66
|
approx 60
|
|
NiSO4
|
154.8
|
41
|
approx 28
|
|
(NH4)2Co(SO4)2.6H2O
|
395
|
|
|
|
(NH4)2Ni(SO4)2.6H2O
|
395
|
|
|
|
(NH4)2Ni(SO4)2
|
287
|
|
approx 30
|
B. Approximate densities of saturated solutions at 25oC:
- 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 slow
Description
A "double salt" has two different cations in its 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.
Author
Mike Clark
