Written by: J.A.D.I.Thathsarani, BSc.(Hons) special in Chemistry, University of Kelaniya (UOK)
The process of addition of standard solution (titrant) from the burette into the solution of unknown concentration (analyte) taken in a conical flask is known as titration. Following requirements should be completed for a titrimetric analysis.
Titrimetric methods are generally accomplished of high precision and wherever applicable retain noticeable advantages over gravimetric methods. It necessities simpler apparatus, are generally, quickly performed.
The following apparatus and solutions are required for titration.
To carry out the titration, these steps must be followed.
In general, each titration is to be performed three times, first to get a rough reading, the other two for accurate determination of the equivalence point. For the calculation, the average (arithmetic mean) of second and third readings is used.
Nevertheless, of the type of reaction used in the titration experiment, the subsequent calculation is established on the known stoichiometry with which the titrated substance (unknown) reacts with the titrant. While it can be accomplished in several ways, usage of the following simple formula is recommended,
Note: The values of the C and V on both sides of the equation must be in the same units. In the case where the stoichiometry of the titration reaction differs from a simple 1:1 ratio, it must be taken into account in the calculation as well.
An unknown sample of sulfuric acid (H2SO4) was titrated with the known KOH solution. It was found that 12 mL of the KOH (C = 0.1 molL-1) was necessary for just complete neutralization of 10 mL of the unknown sample of H2SO4. What is the concentration of the sulfuric acid in the sample?
Calculation : nH2SO4 KOH
The concentration of H2SO4 would be 0.12 mol L-1. However, it goes from the reaction above that 2 moles of KOH are required for neutralization of 1 mol of H2SO4. Therefore, the result, 0.12, must be divided by 2, giving the concentration of sulfuric acid in the unknown sample 0.06
In a titration, certain chemicals are used frequently in defined concentrations as reference solutions. Such substances are referred to as primary standards or secondary standards.
A compound that can be weighed and diluted to get an extract concentration solution is known as the primary standard.
In practice, an ideal primary standard is difficult to obtain and a compromise between the above ideal requirements is usually necessary.
Bases - Following Bases are used for standardizing acids
Acids - Following Acids are used for standardizing bases
Other common primary standards
NaOH, KOH, HCl, HNO3, H2SO4, H3PO4, KMnO4, Na2S23 are not primary standards
Hydrated salts, as a rule, do not make good standards because of the difficulty of efficient drying. However, those salts which do not effloresce, such as sodium tetraborate Na2B4O7.10H2O and copper sulphate CuSO4.5H2O are found by experiment to be satisfactory secondary standards.
A solution with an approximate concentration is prepared and the exact concentration is established using a primary standard. (standardized)
The point where enough titrant (stoichiometric amount) is added to completely react with the titrant (analyte).
The product of the equivalence point volume (Veq) and the titrant's concentration (CT), gives the moles of titrant react with the analyte.
Knowing the stoichiometry of titration reaction , we can calculate the moles of the analyte.
Unfortunately, in most titrations, we ordinarily have no obvious indication that an equivalence point has been reached. Instead, we stop adding titrant when we reach an endpoint of our choosing. Often this endpoint is indicated by a change in the colour of the substance added to the solution containing the analyte. Such substances are known as indicators.
The difference between the endpoint volume and equivalence point volume is known as titration error.
The indicator and the experimental conditions should be so selected that the difference between the visible endpoint and the equivalence point is as small as possible.
Titrate the solvent without analyte
A titration in which a reagent is added to a solution containing the analyte, and the excess reagent remaining after its reaction with the analyte is determined by titration.
Add an excess of the standard to the analyte (known amount, but it's too much). Use a secondary standard for titrating excess of the first standard.
Questions asked by students