Acid Base Titration An acid–base titration is the determination of the concentration of an acid or base by exactly neutralizing the acid or base with an acid or base of known concentration. Acid–base titrations can also be used to find percent purity of chemicals. The key equipment used in a titration are: Burette White tile – used to see a colour change in the solution Pipette pH indicator (the one used varies depending on the reactants) Erlenmeyer flask/ Conical flask Titrant or titrator (a standard solution of known concentration, a common one is aqueous sodium carbonate) Analyte or titrand (solution of unknown concentration) Methodology Before starting the titration a suitable pH indicator must be chosen. The equivalence point of the reaction, the point at which equivalent amounts of the reactants have reacted, will have a pH dependent on the relative strengths of the acid and base used. The pH of the equivalence point can be estimated using the following rules: A strong acid will react with a strong base to form a neutral (pH = 7) solution. A strong acid will react with a weak base to form an acidic (pH < 7) solution. A weak acid will react with a strong base to form a basic (pH > 7) solution. When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour. The point at which the indicator changes colour is called the end point. A suitable indicator should be chosen, preferably one that will experience a change in colour (an end point) close to the equivalence 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 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 neutralize the unknown solution. The solution should be let out of the burette until the indicator changes colour and the value on the burette should be recorded. This is the first (or rough) titre and should be excluded from any calculations. At least three more titrations should be performed, this time more accurately, taking into account roughly where the end point will occur. The initial and final readings on the burette (prior to starting the titration and at the end point, respectively) should be recorded. Subtracting the initial volume from the final volume will yield the amount of titrant used to reach the end point. The end point is reached when the indicator just changes colour permanently. This is best achieved by washing a hanging drop from the tip of the burette into the flask right at the end of the titration to achieve a drop that is smaller in volume than what can usually be achieved by just dripping solution off the burette. Acid–base titration is performed with a bromthymol blue indicator, when it is a strong acid – strong base titration, a phenolphthalein indicator in weak acid – strong base reactions, and a methyl orange indicator for strong acid – weak base reactions. If the base is off the scale, i.e. a pH of >13.5, and the acid has a pH >5.5, then an Alizarine yellow indicator may be used. On the other hand, if the acid is off the scale, i.e. a pH of
Posted on: Thu, 01 Jan 2015 12:33:31 +0000
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