Titration Curves:
Indicators:
Indicators are used to determine the pH of a substance. In a titration, an indicator is used to show when neutralisation occurs between an acid and a base. When neutralisation occurs, it is described as the “end point” of the titration. A pH indicator is a halochromic chemical compound. It detects the number of protons in substance. The more protons, the more acidic the substance is.
pH indicators themselves are either slightly acidic or slightly alkaline. This means that the pH readings cannot be exact. For more accurate readings, pH meters are used.
This is due to them binding with the hydrogen ion or the hydroxide ions. The different electron configurations of the bound indicator cause the indicator’s colour to change. For more accurate readings pH meters are used
Types of Indicators:
Litmus
Litmus has a very complicated structure. However, when look at it properties that makes it a good pH detector, we view it as “HLit”.
Litmus is a weak acid and when dissolved in water, an equilibrium will be established.
The un-ionised litmus is red, whereas the ion is blue.
Adding hydroxide ions:
Adding hydrogen ions:
When the concentrations of the two colours is equal, to represent that the pH is neautal, a mixture of the two colours will be produced:
Methyl Orange
Because of its simple structure, we can clearly see how the compound loses or gains hydrogen ions.
This shows the loss and gain of hydrogen ions.
Phenolphthalein
Phenolphthalein is a weak acid. It is colourless compound and its ions are pink. When hydrogen ions are added to the solution, the position of the equilibrium shifts and the colour of the solution and indicator change from pink to a colourless compound. The pink solution is made colourless when hydroxide ions are added. These ions remove the hydrogen ions, changing the equilibrium.
Choosing the appropriate indicator for my titration experiment.
In my experiment I will be adding a strong acid to a weak base. The two indicators that I could use for my experiment are methyl orange and phenolphthalein. Their ranges are plotted on the graph below with the titration curve for my reaction. The equivalence point of a titration is where the two substances have mixed in exact proportions. The best indicator for a reaction is the indicator that changes colour as close as possible to that equivalence point.
From the graph it is clear that I cannot decide which indicator is best by comparing it to the equivalence point. This is because it is virtually the same volume of acid added for both, as the graph is very steep in that region. As Methyl Orange is commonly used in titration experiments, this is the indicator I am going to use. Because methyl orange is slightly acidic, I am going to add only 3 drops to my solution, to prevent the indicator affecting my result too much. This also means I must stop my titration as soon as the indicator colour changes.
Apparatus:
- Anhydrous sodium carbonate
- Weighing bottle
- Glass rod
- Sulphuric acid
- Distilled Water
-
250 cm3 conical flask
-
250 cm3 volumetric flask + cork
- Burette
-
250 cm3 beaker
-
25 cm3 pipette
- Pipette filler
- 2 Small funnels
- Electronic scales
- Clamp
- White tile
- Methyl orange indicator
- Safety goggles
- Small pipette
- Spatula
- Digital Scale
Method:
- Wash Beaker, Volumetric Flask and Conical Flask with distilled water.
- Record the weight of a weighing bottle using a digital scale accurate to 2d.p.
- Accurately add 2.65g of anhydrous sodium carbonate to the weighing bottle. Record new weight of bottle.
-
Transfer the solid to a 250cm3 beaker and reweigh the bottle. Record this new weight. Check that no solid has been left in the bottle. If this is the case, record the correct amount of solid used.
-
Add distilled water to the beaker up to the 200cm3 mark. Stir with a glass rod. When the solid is completely dissolved, transfer to a 250cm volumetric flask using a funnel. Add washings from funnel, glass stirring rod and beaker.
- Add more distilled water to the volumetric flask until the meniscus is above the 250cm line. Do this using a small pipette. If solution goes above meniscus line. Throw away solution and redo steps 1-6.
- Clean out burette with sulphuric(VI) acid. Let some run through
- Fill burette with acid solution. Ensure there are no air pockets. Record initial reading at eye level, reading the value at the bottom of the meniscus.
- Shake volumetric flask.
- Clean pipette with some of the sodium carbonate solution.
- Shake volumetric flask again.
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Pipette, with a pipette filler, 25cm3 of the solution into a conical flask.
- Add 3 drops of methyl orange solution to the conical flask. Place conical flask on top of a white tile.
- Add the acid solution from the burette into the conical flask and swirl flask, monitoring the colour against the white tile. Keep adding sulphuric acid slowly until there is a slight colour change. Once there has been a colour change, add the sulphuric acid drop by drop and continue to swirl the conical flask. If the colour changes with a single drop, stop titrating and close the tap
(By doing a “dry run” you can find out approximately how much acid to add, so that when the real attempts are done they can be done quicker as you can add a certain amount of acid with no colour change. It also gives you a practice to get used to the equipment)
- Read off final value on burette at eye level.
- Wash out conical flask using distilled water and repeat titration.
Once there are concordant titres, which are the same or are within 0.1cm3 of each other, no more repeats will need to be done.
Risk Assessment:
- Sulphuric is a strong toxic and corrosive acid. Protective eyewear must be worn throughout experiment. If the acid comes in contact with skin wash skin with plenty of water.
- Sodium Carbonate is a weak base, which is slightly corrosive. Care should be taken if it comes in contact with skin and should be washed off with water.
- Ensure all equipment is placed away form the edges of the tables to avoid accidents. If any glass is broken, clean it immediately to prevent injuries.
- Long hair should be tied up so it does not restrict vision or come in contact with harmful substances.
- Carry all the chemicals carefully to avoid spillages. Methyl orange indicator will stain clothes and sulphuric acid and sodium carbonate are irritants.
- Any coats and bags should be placed away from the work station so people don’t trip over them.
