Reactions of positive ions with sodium hydroxide (microscale version)
This is a microscale version of the common test-tube practical reacting various positive ions with sodium hydroxide.
The main advantages of the microscale version are the tiny quantities of chemicals consumed, and there are no test-tubes to wash up. Instead of test-tubes, students have a results sheet which looks like a large results table. This is laminated or put inside a plastic document wallet and can be re-used many times.
Read our standard health & safety guidance
Lesson organisation
This version is far quicker than the more traditional test-tube version and only takes a few minutes to do.
The main management issue is likely to be students wandering around looking for the various reagents. This can be avoided if you supply enough bottles – ideally one bottle of each chemical for each bench of students. Make sure that there are plenty of bottles of sodium hydroxide, as this is used the most.
Students need to make or be given a second copy of the results table to record their observations.
Apparatus and chemicals
Eye protection
Each working group requires:
Results table either laminated or in a plastic document wallet (see note 1). Download it
here (107 KB).
Access to:
Red litmus paper
Bottles of (see note 2):
Sodium hydroxide, <0.5 mol dm-3 (Irritant at this concentration)
Iron(II) sulfate, 0.2 mol dm-3, in 0.1 mol dm-3 sulfuric acid (Low hazard at this concentration)
Iron(III) nitrate, 0.2 mol dm-3 (Low hazard at this concentration)
Copper(II) sulfate, 0.2 mol dm-3 (Low hazard at this concentration)
Aluminium nitrate, 0.2 mol dm-3 (Low hazard at this concentration)
Calcium chloride, 0.2 mol dm-3 (Low hazard at this concentration)
Magnesium chloride, 0.2 mol dm-3 (Low hazard)
Ammonium chloride, 0.2 mol dm-3 (Low hazard at this concentration)
Technical notes
Sodium hydroxide (Irritant at this concentration) Refer to CLEAPSS Hazcard 91 and Recipe card 65
Iron(II) sulfate (Low hazard at this concentration) Refer to CLEAPSS Hazcard 55B and Recipe card 40
Iron(III) nitrate (Low hazard at this concentration) Refer to CLEAPSS Hazcard 55C
Copper(II) sulfate (Low hazard at this concentration) Refer to CLEAPSS Hazcard 27C and Recipe card 19
Aluminium nitrate (Low hazard at concentration used) Refer to CLEAPSS Hazcard 2B
Calcium chloride (Low hazard at concentration used) Refer to CLEAPSS Hazcard 19A
Magnesium chloride (Low hazard) Refer to CLEAPSS Hazcard 59B
Ammonium chloride (Low hazard at concentration used) Refer to CLEAPSS Hazcard 9A
1 Laminated copies of the results table last longer. Results tables in document wallets are very likely to get wet and dirty. If this practical is done regularly, it is worth laminating the tables.
2 Bottles of chemicals are required for each bench of students. If enough bottles are supplied, students do not need to wander round looking for the reagents. Dropper bottles are best. The concentrations are not crucial, but the sodium hydroxide should be below 0.5 mol dm-3 to minimise the hazard. Exactly which salt is used is also not critical, ie sulfates, chlorides or nitrates could be used, as available.
Procedure
HEALTH & SAFETY: Wear eye protection
a Take a copy of the results sheet. If it is not laminated, put it into a plastic pocket. Put two drops of sodium hydroxide onto each of the empty boxes and then two drops of the positive ion solution, and observe what happens.
b Hold a piece of damp red litmus paper over the ammonium chloride and sodium hydroxide box.
c Add more sodium hydroxide, dropwise, to the aluminium nitrate and sodium hydroxide box. Observe what happens.
Teaching notes
It is important that students do not add more than a couple of drops of each solution to the boxes. If this happens, the drops spread and mix, obscuring the results.
If the red litmus is not held near the ammonium chloride and sodium hydroxide box soon after the solutions have mixed, it may be difficult to see litmus changing colour.
The solids formed are: iron(II) hydroxide; iron(III) hydroxide; copper(II) hydroxide; aluminium hydroxide; calcium hydroxide; magnesium hydroxide. The gas made is ammonia.
Equations
Fe2+(aq) + 2OH-(aq) → Fe(OH)2 (s)
Fe3+(aq)+ 3OH-(aq) → Fe(OH)3 (s)
Cu2+(aq)+ 2OH-(aq) → Cu(OH)2 (s)
Al3+(aq) + 3OH-(aq) → Al(OH)3 (s)
followed by the aluminium hydroxide dissolving in the excess hydroxide to give a solution of sodium aluminate
Al(OH)3(s) + 3OH-(aq) → (Al(OH)6)3-(aq)
Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)
Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
NH4+ + OH- → NH3 + H2O
Expected results
Here are the expected results, if students use the worksheet you can download from this page. Some of the boxes on the worksheet are shaded to increase the visibility of the white precipitates.
Positive ion solution | Positive ion solution and sodium hydroxide solution |
Iron(II), Fe2+ | Grey green solid is formed |
Iron(III), Fe3+ | Orange solid is formed |
Copper(II), Cu2+ | Blue solid is formed |
Aluminium, Al3+ | White solid is formed which dissolves in excess sodium hydroxide |
Calcium, Ca2+ | White solid is formed |
Magnesium, Mg2+ | White solid is formed |
Ammonium, NH4+ | A gas is evolved which turns damp red litmus paper blue |
Health and Safety checked, February 2008
Web links
Further information and ideas for microscale experiments can be found on Learnnet
www.chemsoc.org/networks/learnnet/microscale.htm
and Inspirational chemistry
www.chemsoc.org/networks/learnnet/inspirational/home.htm
(Websites accessed August 2007)
Updated 29 Oct 2008
4 out of 5
I think that this experiment was well done, but more information was needed.
Submitted by: MONIQUE MILLER on 31 March 2010