HA + H2O <=> H3O+ + A- (For Acid)
BOH <=> B+ + OH- (For Base)
If the above reactions have a large equilibrium constant, the corresponding acid or base is strong and if the equilibrium constant is small then the corresponding acid or base is weak.
Acid Base Titration is carried out to determine the molar concentration of the acid or base. In the titration process, the OH- from the base added acts as the absorber of the proton generated from the acid, converting the acid into its conjugate base which reacts with the conjugate acid from the base to form a salt.
The solution whose concentration is known is called the standard solution and is always kept in the buret. Whereas the unknown solution is kept in the flask and it is titrated by drop wise addition of the standard solution. At the equivalence point of the reaction the equivalents of the acid are equal to the equivalents of the base. (Kenkel 1994) The calculations are given as:
Normality x Volume (for ACID) = Normality x Volume (for BASE)
For the monoprotic acid and base like HCl and NaOH, the above equation would be:
Molarity x Volume (for ACID) = Molarity x Volume (for BASE)
So from these calculations the concentration of the unknown acid or base can be calculated directly.
Equivalence point is the point at which this reaction is just complete and the acid and base are completely neutralized. It is distinguished from the end point of the titration as the point at which the observation is made or the point where the indicator tends to show the corresponding color change. The end point is generally higher than the equivalence point.
In the acid base titration, the amount of base added to reach the equivalence point is independent of the concentration in the solution and the strength of the acid (Lower n.d.).
From the curve two things can be distinguished (Lower n.d.):
· Stronger the acid or base, higher is the slope of the curve at the equivalence point.
· Weaker the acid or base, higher would be the deviation of the pH from the neutrality at the equivalence point.
The equivalence point in the curve is detected by the presence of distinct break in the titration curve. In case of strong acid there is a sudden increase in the pH value of the curve. The pH value of the solution (or the equivalence point of the curve) can be measured by two ways:
· Using the electronic pH meter.
· Using the indicator dye.
In the pH meter, the measurement of voltage change due to replacement of Na+ (present in the thin glass membrane dipped in the solution) with H+ ions takes place. This method is automated and accurate. It measures the value of the pH for which the second differential of pH with respect to the volume of the titrant is zero, which is the equivalence point.
In the indicator dye test, small amount of indicator is added to the solution, which shows different color on being protonated and de-protonated i.e. the color of the acid or base is different from the color of its corresponding conjugate base or conjugate acid. The changes its color should takes place at the approximate equivalence point of the titration.
But this is generally not possible. The indicator change their color at certain level of pH. So depending up on the acid and base that are being titrated the indicated is varied. That is, for the titration of a strong acid and a weak base, the color change pH of the indicator is in the acidic zone (pH<7) and the opposite is true for the strong base. The indicator dye titrations are generally not as accurate as the pH meters. But they can calculate the approximate value easily (Lower n.d.).
For the polyprotic acid, the titration curve has multiple points of infection or distinct breaks. That is the titration curve will have multiple equivalence points.
This curve shows that the polyprotic acids lose their proton in steps resulting in multiple equivalence point.
So the titration or the volumetric analysis is the general and standard method for the determination of the concentration of the unknown acid or base for all practical purposes. The results are accurate if the readings of the experiment are taken accurately.
Kenkel, John. “Acid Base titration and Calculation.” In Analytical chemistry for technicians. 1994.
Lower, Stephen K. “A Chem1 Reference Text.” In Acid-base Equilibria and Calculations, by Stephen K. Lower. Simon Fraser University.
Spark_Notes. Spark Note Titration Terms. http://www.sparknotes.com/chemistry/acidsbases/titrations/terms.html.