Volumetric analysis, (titrations), involves reacting chemical opposites with each other, in order to determine unknown information about one of the substances. Titrations in Leaving Certificate chemistry are of two main types: (i) acid/alkali, and (ii) oxidising/reducing agent. In the water analysis section of the course, there is also the titration of metal ions with edta.
The meaning of the following must be understood:
Titration calculations are easy, when done logically. The following are the steps:
- Write out the titration conclusion - the information that resulted from the experiment.
- Write out the balanced equation for the titration reaction - this is invariable given.
- Substitute the data from (i) and (ii) into the volumetric analysis formula (V1M1 etc.).
- Calculate the value of the unknown quantity which will generally be one of the molarities.
Remember that this works out in moles per litre - which may not be the final required answer.
The above four steps should become automatic in doing any volumetric analysis calculation.
Procedures to be known include:
- How to prepare a standard solution accurately
- How to fill a burette and a pipette
- Precautions to be taken for accurate titrating
- Choosing the right indicator
The most common oxidising agent used for volumetric analysis is potassium permanganate. It reacts with, and is used for the determination of reducing agents, the Leaving Certificate example being iron(II) ions, Fe2+, which are oxidised to iron(III). Such titrations need no indicator, since the permanganate is reduced to colourless Mn2+ ions during the reaction and so acts as its own indicator.
The other redox titration involves iodine oxidising sodium thiosulphate. Starch is the indicator for titrations involving iodine, since iodine and thiosulphate produce a dark blue-black compound. The techniques mentioned under acid-alkali titrations apply to redox titrations.