Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant or titrator, of a known concentration (a standard solution) and volume is used to react with a solution of the analyte or titrant, whose concentration is not known. Using a calibrated burette orchemistry pipetting syringe to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached.
An acid-base titration is the ...view middle of the document...
In order to achieve this, the indicator should change colour sharply at the equivalence point.
At the end point of titration,
(a) All the acid has been neutralized by the alkali,
(b) The solution in the conical flask contains salt and water only,
(c) The colour of the indicator will be midway between its colour in acidic solution and its colour in alkaline solution.
Calculation Involving Acid-Base Titration
* acid-base reactions involve a proton transfer
* the acid donates a proton to the base
* acid-base reactions are also known as neutralisation reactions
acid + base -----> salt + water
acid A + base B -----> conjugate acid of base B + conjugate base of acid A
* H+ + OH- ------> H2O is the most general neutralisation reaction
* Equivalence point is the point at which the moles of H+ is equal to the moles of OH-
An indicator is used to show the equivalence point during a titration
* A titration involves the progressive addition of one reactant from a burette
(usually the acid), to a known volume of the other reactant in a conical flask
(usually the base)
An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the iogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range. Weak acids are titrated in the presence of indicators which change under slightly alkaline conditions. Weak bases should be titrated in the presence of indicators which change under slightly acidic conditions.
Indicator | pH Range | Quantity per 10 ml | Acid | Base |
Thymol Blue | 1.2-2.8 | 1-2 drops 0.1% soln. in aq. | red | yellow |
Pentamethoxy red | 1.2-2.3 | 1 drop 0.1% soln. in 70% alc. | red-violet | colorless |
Tropeolin OO | 1.3-3.2 | 1 drop 1% aq. soln. | red | yellow |
2,4-Dinitrophenol | 2.4-4.0 | 1-2 drops 0.1% soln. in 50% alc. | colorless | yellow |
Methyl yellow | 2.9-4.0 | 1 drop 0.1% soln. in 90% alc. | red | yellow |
Methyl orange | 3.1-4.4 | 1 drop 0.1% aq. soln. | red | orange |
Bromphenol blue | 3.0-4.6 | 1 drop 0.1% aq. soln. | yellow | blue-violet |
Tetrabromphenol blue | 3.0-4.6 | 1 drop 0.1% aq. soln. | yellow | blue |
Alizarin sodium sulfonate | 3.7-5.2 | 1 drop 0.1% aq. soln. | yellow | violet |
a-Naphthyl red | 3.7-5.0 | 1 drop 0.1% soln. in 70% alc. | red | yellow |
p-Ethoxychrysoidine | 3.5-5.5 | 1 drop 0.1% aq. soln. | red | yellow |
Bromcresol green | 4.0-5.6 | 1 drop 0.1% aq. soln. | yellow | blue |
Methyl red | 4.4-6.2 | 1 drop 0.1% aq. soln. | red | yellow |
Bromcresol purple | 5.2-6.8 | 1 drop 0.1% aq. soln. | yellow | purple |
Chlorphenol red | 5.4-6.8 | 1 drop 0.1% aq. soln. | yellow | red |