titration

a quantitative analysis in which the concentration of a substance is determined by measuring the volume of one solution required to react with a given known volume of another solution.

titration

standard solution

dilution dissolving a known mass of a substance into a known volume of deionized water.

● Standard solution with a desired concentration can be prepared from a more concentrated stock solution. ● Certain volume of the stock solution is mixed with deionized water in a volumetric flask. ● Adding water to a concentrated solution: ○ changes the concentration of the solution ○ does not change the number of moles of solute present No. of moles of solute before dilution = No. of moles of solute after dilution C1 x V1= C2 x V2 ○ C1 and C2 - concentrations (in mol dm-3) ○ V1 and V2 - volumes of solution (in dm3

use a pipette to obtain 25.0cm^3 of 2.0 mol dm^-3 H2SO4 transfer the acid to a 100cm3 volumetric flask containing a little distilled water. Shake to mix the solution. add more distilled water to the flask, with constant swirling water is added dropwise to bring the solution to the calibration mark. stopper the flask and shake well to ensure thorough mixing

● Use a pipette to measure out exactly 25.0 cm3 of stock solution. ● Use a burette to measure out any other volumes that are less than 50.0 cm3 .

• The mass of solute needed is calculated and weighed. • The solute is dissolved in some distilled water in a beaker. • The solution is transferred into a volumetric flask. • More distilled water is added to top up to the required volume. • The flask is stoppered and inverted a few times.

Titration is used to determine the concentration of an analyte (substance to be analysed).

A reagent, termed the titrant, is prepared as a standard solution of known concentration and volume. ● The titrant is commonly delivered by burette into a known volume of the solution with the analyte (called aliquot) in a conical flask. ○ Note that the analyte can sometimes be delivered by burette.

● The titrant is added to the solution of analyte until it has reacted in the exact quantity stoichiometrically with the analyte - the equivalence point. ● However, it is difficult to detect the equivalence point practically.

● Instead, the end-point of the titration will be detected. ● It is achieved when an additional drop of titrant is added to the solution in the flask. ● This point is marked by an indicator, which is added to the solution in the conical flask before the start of titration. ● If the point is reached, a colour change of the indicator will be observed.

● Burette - transfers varied known volume of titrant to the conical flask ● Conical flask - holds the aliquot (transferred by a volumetric pipette) with several drops of indicator added. ● White tile - provides a white background for better observation of colour change at the end-point.

HCl + NaOH methyl orange or phenolphthalein

HCl + NH3 methyl orange

CH3COOH + NaOH Phenolphthalein

CH3COOH + NH3 no suitable indicator

acidic : red to orange basic : yellow to orange

acidic: colourless to pale pink basic: pink to colourless

● The first trial of titration is a rough titration - the titrant volume recorded is unreliable as the volume added to reach the end-point is usually an overshot value. ● The titration must be repeated, until concordant results (values are ±0.10 cm3 of one another) are recorded. ● The concordant results are then used to calculate an average value of titrant volume - so the unknown concentration of analyte could be determined afterwards.