When carrying out volumetric analysis, the concentration of one of the chemical reactants is already known whilst the other's concentration is unknown and it is this value that is to be found through the volumetric method - titration.
Quantitative Reactions
However, to make the reaction quantitative, the chemical equation between the two reactants must be known, and made balanced. Such an equation is known as a stoichiometric equation, and it is these that are involved in titrations, as they show how many moles of the reactants there are, and how many moles of the products will be formed. For the reaction to be described as quantitative, the reaction must react completely in line with the mole ratios shown by the stoichiometric equation.
Titration
A chemical titration is a technique used to determine the concentration of an unknown reactant. It is carried out in the laboratory using a pipette, burette and a conical flask. The solution of known concentration is measuring accurately and quantitatively into the conical flask using the pipette and an indicator may be added at this point(also of known volume), whilst the solution of unknown concentration is measured into a burette. This burette is normally secured into place using a clamp stand, and the starting point noted. The solution is then dispensed into the conical flask, which is stirred to ensure equal distribution of the reactant.
The chemical reaction is complete when a permanent colour change occurs in the conical flask, normally due to the indicator added, and this is known as the end-point of the reaction. However, some titrations need no indicator, as one of the reactants themselves act as an indicator e.g. Potassium permanganate.
There is a point where the reaction is just complete, which can quite often be hard to pinpoint, and this is known as the equivalence point. The best case scenario is of course where the equivalence and end points are almost, if not, identical, and occur at the same moment, but this can be hard to achieve. However, the success of titrations relies on the level of accuracy they are carried out at, and on the correct indicator being chosen. Both of these factors increase the likelihood of the equivalence and end-points being as close together as possible.
Calculations Involving Titrations
These calculations are easiest to carry out when all of the quantities are converted into moles. The formula for substances in solution is "n=V x c," where "n" is the number of moles, "V" is the volume of the the solution(measured in liters) and "c" is the concentration measured in moles per liter.
To find out the concentration of an unknown solution then, the equation can be swapped around to "c = n / V."
The formula for substances that are solids, the mass is measured in the formula, and is "n = mass / GFM," where "n" again is the number of moles, the mass is the mass of the substance measured in grams, and the "GFM" is the gram formula mass of the substance, again measured in grams.
This equation can be used to find the number of moles in a substance and if the volume is known, this can then be substituted into the above equation, to find the unknown concentration.
The Solution of Known Concentration
The solution of known concentration is otherwise known as a standard solution. These solutions are made from a primary standard solution which have certain, strict characteristics, these being:
- Purity -- a primary stand must be readily available in high levels of purity;
- Stability -- it must be stable both when solid and when in solution;
- Solubility -- it must be readily soluble in water; and
- Mass -- to decrease errors when weighing, primary standards must have a high formula mass.
Standard solutions are made from primary solutions by first weighing out the primary standard, then dissolving it in a beaker of deionised water. A standard flask is washed out with deionised water, before being allowed to dry completely. The primary standard solution, is then poured into this standard flask, along with all of the rinsings (to ensure that none of the primary standard solution is left behind). Deionised water is then used to fill the standard flask with the appropriate volume.
Concordant Results
To improve the accuracy and reliability of the results, it is the mean (or average) of the concordant results that are used in the calculations. Concordant results are obtained through first carrying out a 'rough' titration -- the first titration, one where the accuracy is impaired by not knowing the approximate volume of the dispensing reactant to use -- followed by several more accurate titrations, until concordant results are reached. These are results that are within a very close range of one another, depending on the accuracy needed for the particular experiment.
Sources
- The Interactive Lab Primer, managed by the RSC.
- Archie Gibb, David Hawley, BrightRed Advanced Higher Chemistry, pg 30, 31.
Hayleigh Hunter - Hayleigh describes herself as the eternal student - she loves to learn. Her articles aim to provide the best quality information ...
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