Acid base titrations
On youtube you can find lots of examples of titrations, like the automatic one here.
Note that many of these demonstration films on youtube are bad ones. And don't forget to consult Wikipedia.
In another module we come back to the titrations.
I prefer to use an electronic pH-meter, but doing it with an indicator also is a very good idea.
With a pH-meter you write the pH-value every time after adding, for example, 0.5 ml of the titrant, until reaching and passing the endpoint.
After that, I would demonstrate the same titration without pH-meter, but in the presence of an indicator, knowing already the whereabouts of the endpoint. I am prepared to stop at the very right moment.
A titration is a rather quick method to determine the concentration of certain, in water dissolved substances, where apparatus are applied that can read very precisely the volumes of solutions, using pipettes, burettes and graduated cylinders etc.
Another word for titration could be: volumetry
The fundament of an acid base titration is of course the acid base reactionl. An acid an a base react in a fixed mol proportion.
As soon as the two have neutralised each other, you joined 'equivalent' amounts (and you reached the equivalence point).
Normally, two dissolved substances react quietly in a titration until the reaction stops: at the moment that equivalent amounts are joined.
In practice - unfortunately - it is quite impossible to stop exactly at the moment that the equivalence point is reached.
Mostly we add just a bit too much, we pass that point. Just the last drop will contain a bit too much.
In other words: the endpoint of the titration very often will pass the equivalence point. Always there will be added just a bit too much of the titrant (from the burette), never more than one drop (if you work well).
Officially, the error may not exceed 0.5%.
The indicator of course is our big helper to know when to stop. Att.: chosing the right indicator is not always that easy.
At simple titrations, the concentration of the titrant is well known.
In that way we can calculate the concentration of the unknown solution.
More about titrations and its applications can be found in another module (14, Quantitative analysis).
In an acid base titratin, the pH of the equivalence point not always will be 7. The pH dependson the properties of the product (mostly a salt).
The product can be built up of particles having an acid or basic character.
For example: if the product is Sodium acetate (a salt), the Sodium ions do not influence the pH, but the acetate ions do.
Acetate is a weak base and so the final solution - after the titration of HAc met NaOH (with NaAc as product) - will have a basic environment.
The titration will stop at a pH>7.
We can state the next general rules:
- titrations of a strong acid with a strong base will have a final pH = 7
- titrations of a strong acid with a weak base will have a final pH < 7
- titrations of a weak acid with a strong base will have a final pH > 7
Now we hope that it will be clear to you that the choice of indicator for any titration is very important and depends on the kind of titration:
You must always check if the product causes a neutral, an acid of a basic solution, and chose an indicator that will change color at the right moment.
We also know special titrations:
For example, if direct determination of a concenctration is not possible. The substance to investigate could be a gas, or an insoluble solid, or it might be unstable. What to do then?
It is possible to execute an indirect titration.
The substance to determine must first completely react with a "substitute".
- You know exactly how much substitute you first had.
- In a titration you determines exactly how much substitute was left over after the first reaction.
- Then you can calculate how much of the original substance there was.
Marble contains a high amount of Calcium carbonate. that is unsoluble in water and cannot as such be titrated directly.
We can do now the following: you weigh an amount of marble very precisely.
Later, after calculating how much calcium carbonate was in the marbel, you know the percentage. But how to do this in practice?
You bring the weighed amount of marble in an exactly known amount (excess, more then enough) strong acid solution of which you know well the concentration.
All calcium carbonate in the marble will react with the acid until finishing. A certain amount of the acid will be left over.
With a titration you determine now how much acid was left over. That you subtract from the original amount of acid, and you know the amount of acid that reacted with the calcium carbonate.
You know the reaction equation of the acid with calcium carbonate, and thus the mol proportion.
So now you can execute the calculation, find the amount of Calcium carbonate and the percentage of it in the marble.
Acid base indicators
The indicators in table VIII are all acid base indicators.
We also know redox-indicators to be used in redox titrations (other module).
The acid base indicator mostly is a weak organic acid with a rather complex structure. Abbreviated formula: HIn.
HIn + H2O
H3O+ + In- (evenwicht IND)
HIn has color 1 In- has color 2
Imagine this indicator in acid environment, for example in a solution of hydrochloric acid.
In this environment dominate the (H3O+)-ions and the equilibrium IND dislocates to the left.
You can also say that in acid environment the indicator takes the HIn performance; color 1 dominates.
Every indicator has a zone of change that can be found in the table.
An example: Methyl orange has a change zone of 3.1 - 4.4 (red - orange)
Imagine a strong acid solution with pH=1.
Add some drops of Methyl orange solution, that gets the color red.
Now we add strong base like (KOH(aq), the acid will bit by bit be neutralised, the pH-value will gradually increase.
It started with pH=1, slowly this value increases.
As soon as the value passes 3.1, then you start to see a color change from red to orange.
Only after reaching 4.4 the color has definitly become orange.
Within the change zone, the color is a mix of the two colors.
Again: the color of the indicator depends on the environment of the solution.
In acid environment the equilibrium IND dominates at left, as also does the color of the HIn molecules.
Note that the change zone certainly not always is close to 7!