Supporting Practical Science, D&T and Art
- in schools and colleges
pH and indicators - using the plastic Comboplate®
Procedure
1. Wear eye protection - solution B is IRRITANT.
2. Use the pipette to fill the wells E1 – E6 and F1 – F5 as follows:
E1
20 drops of A
E2
18 drops of A + 2 drops of B
E3
16 drops of A + 4 drops of B
E4
14 drops of A + 6 drops of B
E5
12 drops of A + 8 drops of B
E6
10 drops of A + 10 drops of B
F1
8 drops of A + 12 drops of B
F2
6 drops of A + 14 drops of B
F3
4 drops of A + 16 drops of B
F4
2 drops of A + 18 drops of B
F5
20 drops of B
F6
empty
1
2
3
4
5
6
8
7
9
10
11
12
A
B
C
D
E
F
1
2
3
4
5
6
3. Add distilled water to each of the wells so the level is about 3mm from the top.
4. Rinse a pH meter* in clean water. Remove as much water as possible then dip it into the liquid in well E1.
Note the reading (to 1 decimal place).
5. Dip the pH meter into water and take a reading of well E2. Continue in this way up to F5.
Write the readings on the diagram above.
6. Fill each well A1, B1, C1 and D1 with 3 drops from E1.
Fill each well A2, B2, C2 and D2 with 3 drops from E2.
Continue until wells A11, B11, C11 and D11 are full.
If you have used well F6, you can fill A12, B12, C12 and D12.
*A calibrated Checker pH
meter (about £30) can be
used to find the pH values.
7. Add 1 drop of Universal indicator to wells A1 to A11.
Repeat for each row using a different pure indicator (eg, methyl orange for row B1 - B11).
Extracts from flowers and vegetables can also be used (e.g. red cabbage; petunia flowers).
8. If you have another Comboplate®, more wells can be filled.
9. Photograph the Comboplate® from above for a lasting record. Label the photograph.
Solutions
·
·
Solution A: 3.1 g of boric acid + 2.65 g of citric acid made up to 250 cm3 of solution.
Solution B: 9.0 g of disodium hydrogen phosphate-12-water + 1 g of sodium hydroxide made up to
250 cm3 of solution.
·
Universal indicator solution. Other chemical indicators (e.g. methyl orange). Plant extracts.
ASE 01/14_DRAFT
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©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
Some results
The following pictures were sent to CLEAPSS by a technician. The pupils did this in a science club.
ASE 01/14_DRAFT
Page 2 of 2
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk