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Titration Curve

There are two varieties of postulating laws and mechanisms of the chemical behaviors. One is theoretical based on imaginations, intuitions and expectations. The other is by ascertaining the fact after observing practically after experimental studies and recordings of lots of data.

Analytical chemistry is a branch which is entirely dedicated to the practical aspects and study. Putting forward a statement after a thorough analysis is the requirement of analytical chemistry. Pure science will not accept any postulate until it is substantiated by analysis.
Titration is one of the techniques by which the characteristics of any substance can be analyzed. The strengths of acids, oxidizing and reducing properties, solubility, concentrations and a lot of other parameters can be studied by the process of titration.
A titration graph is a graph obtained by plotting a known quality on one axis and plotting the graph with the one of the components for which a certain property is to be established on the other axis. The graph obtained is termed as the titration graph. In this topic we will study and discuss various titration graphs and titration curves.

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Titration Curves

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The technique of finding the concentration of a particular component in a solution whose concentration is unknown is called titration. Titrant and the titrate are the two components in the process of titration. Titrate is generally the liquid form and of unknown concentration and titrant, which is also in the form of a solution, the concentration of which is known. By adding titrant in small quantities the process of titration takes place.

End point is a situation where the stage looking for is arrived. Acid base titration, oxidation reduction (redox) titrations, precipitation titrations, adsorption titrations and complex titrations etc., are some of the variety of titrations one will perform to find the concentrations.

A titration graph or curve is obtained in a graph. The graph is plotted with the readings of a titration. Titrations are measured in of volumes and one of the volumes is generally fixed. The fixed volume which is the unknown concentration is taken in one axis say 'x' axis. The other axis 'y' is for the volume that is added to achieve the end point. 

A titration curve is the line joining the volumes to the changing quality on the other axis. An accurate measure and value of the titration is obtained by the titration curve.

pH Titration Curve

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Some Ponits about the pH titration curve:
  1. Acids are defined as the substances that release protons or H+ ions when in solution.
  2. While alkalis or bases are those which react with this H+ ions by the presence of OH- ions and form a water molecule.
  3. The strength of an acid is measured in terms of pH. pH is derived from German word potenz which means strength and it is the strength of hydrogen ions in the given acid solution.
  4. When all the H+ ions are consumed by the OH- ions, the solution is termed to be neutral (pH=7).
  5. In pH titration curve one can establish this neutralization point.
  6. Like H+ ion concentration in the solution when it is in acidic medium (pH 1 to 6.9), OH- ions will also be there in the medium when it is basic (pH 7.1-14), which means there are no H+ ions in the solution but only OH- ions are present.
  7. The change over from acidic to basic passes through gradual change in the pH which can be plotted on the graph where volume of acid or base added is taken in x axis and the pH in y axis. Such a graph is called pH  titration curve. 
pH Titration Curve

pKa Titration Curve

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Dissociation constant of acid is given by Ka and according to the law of mass action is defined as 

Ka = $\frac{[H^+][A^-]}{[HA]}$

For a strong acid the dissociation is complete and therefore the dissociation constant of acid is the H+ ion concentration [H+] pH of a solution is the inverse of hydrogen ion concentration. 

Thus pKa = -log Ka.
Thus instead of pH, if the graph is drawn against pKa it is called pKa titration curve.

Acid Base Titration Curve

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Acid base titration curve is the most common curve to find the pH at different points of the addition of one in to the other. There are three varieties of acid base curves. 

They are:
  1. Strong acid -strong base curve
  2. Strong acid- weak base curve
  3. Weak acid- strong base curve
Weak acid strong base curve starts at a pH higher than 1 which is generally the case with strong acids. This is because of the incomplete dissociation of the acid. There is a partial dissociation of the acid in to H+ and A- ions. A- combines with the B+ of the base while H+ and OH- get neutralized. The AB salt formed acts as a conjugate acid and base which will assist further dissociation of weak acid. Once the equivalence point is reached the strong base affects the increase of pH rapidly. An example is given in a graph.

Equivalence Point Titration Curve

The acid base titration curve which shows the pH 7, is known as equivalence point titration curve. In the same way if the titration is for any quality other than the acid base, the end point is called the equivalence point and the curve which shows the end point is called equivalence point titration curve. 

Weak Acid Strong Base Curve

Histidine Titration Curve

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Histidine is an amino acid and is a polyprotic acid. This means the dissociation is not complete and there are different equivalence points in the histidine titration curve. The steps of dissociation can be shown as

Different Equivalence Points in Histidine Titration Curve

Hence the histidine titration curve will be

Histidine Titration Curve

Glycine Titration Curve

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Glycine molecular formula is H2N-CH2-COOH and the zwitter ion structure is  H3N+-CH2-COO-. The glycine titration curve is 

Glycine Titration Curve

Arginine Titration Curve

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Arginine is a natural α-Amino acid.

The arginine titration curve and its structure is as shown in the diagram.

Titration Curve of Arginine

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