Acid-base titration
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An acid-base titration is a method in chemistry that allows quantitative analysis of the concentration of an unknown acid or base solution. It makes use of the neutralization reaction that occurs between acids and bases, and that we know how acids and bases will react if we know their formula.
Contents |
Equipment
The key equipment used in a titration are:
- Burette
- Pipette
- Acid/Base Indicator (the one used varies depending on the reactants)
- Conical Flask
- Standard Solution (a solution of known concentration, a common one is aqueous Na2CO3)
- Solution of unknown concentration
Method
Before starting the titration a suitable indicator must be chosen. The endpoint of the reaction, when all the products have reacted, will have a pH dependent on the relative strengths of the acids and bases. The pH of the endpoint can be roughly determined using the following rules:
- A strong acid reacts with a strong base to form a neutral (pH=7) solution.
- A strong acid reacts with a weak base to form an acidic (pH<7) solution.
- A weak acid reacts with a strong base to form a basic (pH>7) solution.
When a weak acid reacts with a weak base, the endpoint solution will be basic if the base is stronger (Kb>Ka) and acidic if the acid is stronger (Ka>Kb). If both are of equal strength (Kb=Ka), then the endpoint pH will be neutral.
A suitable indicator should be chosen, that will experience a change in colour close to the end point of the reaction.
First, the burette should be rinsed with the standard solution, the pipette with the unknown solution, and the conical flask with distilled water.
Secondly, a known volume of the unknown concentration solution should be taken with the pipette and placed into the conical flask, along with a small amount of the indicator chosen. The burette should be filled to the top of its scale with the known solution.
The known solution should then be allowed out of the burette, into the conical flask. At this stage we want a rough estimate of the amount of this solution it took to neutralise the unknown solution. Let the solution out of the burette until the indicator changes colour and then record the value on the burette. This is the first titre and should be discluded from any calculations.
Perform three more titrations, this time more accurately, taking into account we know roughly where the end point will occur. Take note of each of the readings on the burette at the end point, and average these at the end. Endpoint is reached when the indicator changes colour permanently.
Calculations
Knowing the average of the three results recorded the number of moles can be determined, and hence the concentration of the unknown solution.
First, work out and balance the reaction to determine how many moles of reactants are needed to neutralize a mole of the solution.
The number of moles of the standard solution used is known(concentration multiplied by volume) and can then be used to determine the moles in the unknown solution. By dividing this by the volume of the unknown solution, the concentration can be found
When making a graph of the titration, two key points that are helpful in calculations are the half-way point and the equivalence point. At the half-way point, the pH (-log(H+)) is equal to the pKa/pKb, the -log(Ka/Kb). Using this information, it is possible to determine the Ka or Kb of the acid/base from the graph of the solution. At the equivalence point, the acid and base have neutralized each other to produce water and a salt (in most acid-base reactions). Therefore, since all of the acid/base added has been neutralized, the moles of acid equals the moles of base. This is helpful in standardizations, when only the molarity of the acid or only the molarity of the base is known. In such cases, knowing the molarity of one solution (moles/liter), as well as the volume (liters), moles can be determined; this number of moles is the same for the other solution of unknown molarity. Then, by dividing the moles by the volume of this solution present, the molarity can be calculated.
<math>N1*V1=N2*V2</math>, where N1 is the normality of the base, V1 the volume of the base, N2 the normality of the acid and V2 the volume of the same.