Title: Making your own acid-base indicator

Transcription

1 Lo Fong Him(19) Chan Lik Sang(2) Title: Making your own acid-base indicator Date: 26 th April, 2010 Objective: To determine what substances can be used as acid-base indicator Theory: Acid-base indicators are organic dyes that change colours at different ph values. Most indicators are synthesised from chemical reactions, but some indicators can be easily extracted from plant materials e.g. red cabbage, beetroot, blackcurrant and black bean using water or an organic solvent. Materials and Apparatus: 0.5 M sodium hydroxide solution Ethanol 0.5 M ethanoic acid 0.5 M hydrochloric acid Buffer solutions of ph 1-12 Distilled water 10 times diluted oven cleaner solution Common titration apparatus and laboratory glassware Microscale chemical apparatus (plastic pipette and well-plate) ph meter (or data logger with ph sensor) Cutting board Hotplate Precautions: Avoid direct contact with chemicals. Wash immediately with plenty of tap water if chemicals spill on your skin. Do not touch any hot objects with your bare hands. Flammable solvent should be heated in a hot water bath on a hotplate instead of direct heating using naked flame. Never ingest any plant materials used in experiments. Never use plants which are known to contain irritants that induce allergy or dermatitis on skin contact. Plant pigments may stain your hands. Wear disposable plastic gloves when handling the plant materials. When dealing with plant specimens which bear spines or thorns, care must be taken. Wear suitable protective gloves when necessary. Use sharp tools carefully. Wash hands after handling plant materials. 1

2 Lo Fong Him(19) Procedures: Chan Lik Sang(2) Part A: Extraction of plant pigments 1. Cut coloured plant materials into small pieces. 2. Place a measured amount of materials in a beaker and add 30 cm 3 of distilled water. 3. Use a hotplate to heat the materials for 10 minutes. Stir the materials occasionally while they are being heated. 4. After cooling down, obtain the plant extracts by filtration. Part B: Preparation of buffer solutions and making choice of suitable acidbase Indicator 1.Prepare buffer solutions of ph 1-12 according to this table. ph Chemical composition 1 50 cm 3 0.1M HCl 2 5 cm 3 0.1M HCl cm M HCl g potassium hydrogen phthalate cm M HCl g potassium hydrogen phthalate cm M NaOH g potassium hydrogen phthalate cm M NaOH g potassium phosphate monobasic cm M NaOH g potassium phosphate monobasic cm M NaOH g potassium phosphate monobasic cm M HCl g sodium tetraborate cm M NaOH g sodium tetraborate cm M NaOH g sodium hydrogencarbonate 12 5 cm M NaOH 2. Measure and record the ph value of the buffer solutions using ph meter. If the ph is too high, adjust the ph to the target value by adding 1 M HCl; if the ph is too low, adjust the ph to the target value by adding 1 M NaOH. Dilute each of the mixtures with distilled water to 50 cm Label 12 clean test tubes with the ph values of the solution. 4. Fill each tube one-third full with the corresponding solution of known ph. Add approximately 1 cm 3 of the plant extract to each test tube and mix the solution. 5. Record the colours of the plant extract at different ph values. 6. Repeat steps 1-4 with other plant extracts. 7. Determine which plant extract is suitable as an indicator for titration of strong alkalis with strong and/or weak acids. 2

3 Lo Fong Him(19) Chan Lik Sang(2) Observations: Dragon fruit peel (10.017g) 1 Light pink 2 Light pink 3 Light pink 4 Light pink 5 Light pink 6 Light pink 7 Light pink 8 Light pink 9 Light pink 10 Very light pink 11 Nearly colourless 12 Nearly colourless From left to right : ph12-1 cabbage(11.90g) 1 2 dish purple 3 dish purple 4 Purple 5 Light pale purple 6 Pale purple 7 Violet 8 Blue 9 Greenish blue 10 Bluish Green 11 Green 12 Greenish yellow From left to right : ph1-12 (no 9&10) 3 Only for 9&10(because they were fresh but the other were not)

4 Lo Fong Him(19) Chan Lik Sang(2) Grape juice Nearly Colourless 6 Light pink 7 Greyish green 8 Purple 9 Violet 10 Light red 11 Green 12 Green From left to right : ph1-12 Yellow ginger powder 1 Dark brown 2 Brown 3 Orange 4 Orange 5 Very pale yellow 6 Very pale yellow 7 Pale yellow 8 Pale yellow 9 Light yellow 10 Light yellow 11 Ligh yellow 12 Yellow From left to right : ph1-12 4

5 Lo Fong Him(19) Discussion: Chan Lik Sang(2) Error: 1. Error on measuring the volume of solution using pipette. 2. The color of the extracts is difficult to distinguish by eyes. 3. Some chemical substances such as potassium hydrogen phthalate is used for many times so these chemical may be polluted. 4. The plant material may contain impurities which will affect the final color in the result. Improvement: 1. Using apparatus with higher precision. 2. Colorimeter can be used instead of our eyes for observation of the color of the extracts. 3. Polluted chemical substances can be replaced by new one and handle it carefully in order to prevent pollution of it. 4. The plant material can be cleaned by distilled water before the experiment. Questions and Answers: 1.What is an acid- base indicator? A: Acid- base indicators are substances which change colour with the ph of their environment. They are usually weak acids or weak bases. 2.What is the chemical principle involved in acid- base indicators? A: Acid-base indicators are substances which appear as characteristic colours in differentph. They are usually weak organic acids or bases which have different colours in their ionized form and non-ionized form. In aqueous solution, the two forms of the indicator are in equilibrium. HIn (aq) H+(aq) + In-(aq) In an acidic solution, the equilibrium shifts to the left and the colour of the indicator is mainly the colour of the non-ionized form. In alkaline solution, the equilibrium shifts to the right and the colour of the ionized form predominates. 5

6 Lo Fong Him(19) Chan Lik Sang(2) 3.What substance inside plant materials can be used as acid-base indicator? What are their basic chemical structures? A: Many plants including from leaves (red cabbage); flowers (geranium,poppy, or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb) contain chemicals from the naturally-colored anthocyanins family of compounds. They are red in acidic solutions and blue in basic. 4.How to determine whether the indicator is suitable for a titration between a strong alkali and a weak acid? A: Indicators should be chosen in such a way that the ph at equivalence point lies within the ph range of indicator. For a titration between a strong alkali and a weak acid, the ph at equivalence point is below 7,so the ph range of the indicator should below 7 to give a sharp colour change at end-point. 5. What are the pros and cons of using your indicator over acid-base indicators commonly found in the laboratory? A: The indicators we used are natural. They are all non-hazardous and they can be prepared easily. These advantages make both red cabbage and blueberry extracts decent teaching materials for junior form students. But both plant extracts cannot be stored for a long time and they are not suitable for precise ph measurement. Conclusion: Both red cabbages and Grape juice can be used as acid-base indicator, but red cabbage is the better one. 6

7 Lo Fong Him(19) Chan Lik Sang(2) 7