Pharmaceutical Chemistry

By: Sk Abd Razak

Topic 11 – Part 1Acid – Base Titrations

(Aqueous)

Because volume measurements play a key role in titration, it is also known as volumetric analysis

TitrationTitration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant.

Reactants are elements and compounds that appear on the left side of a chemical equation.

HCl + NaOH = NaCl + H2O reactant reactant product

Reagent ( titrant or titrator)

titrant 

Analyte (titrand)

Titrand (analyte)

An analyte is a substance or chemical constituent that is determined in an analytical procedure, such as a titration.

They are the species that react or undergo a chemical change into other elements and compounds called products.

by-product

Analytes :

For instance, blood glucose testing, the analyte is glucose.

Any substance undergoing analysis.

Blood uric acid testing, the analyte is uric acid

A burette containing the titrant

A reagent, called the titrant or titrator, of a known concentration (a standard solution) and volume is used to react with a solution of the analyte or titrand, whose concentration is not known. An Erlenmeyer flask

in which the solution to be analyzed (titrand)

An analyte (titrand) is a substance or chemical constituent that is determined in an analytical procedure, such as a titration.

Phenolphthalein is used as an acid or base indicator where in contact or presence of acid it will turn colorless and with base, it will turn into a pinkish violet color.

Using a calibrated burette would be able to determine the exact amount (volume) of titrant to reach the endpoint, and so the equivalence point.

Titrant

The endpoint (similar, but not the same as the equivalence point) is the point at which the titration is complete, as determined by an indicator.

Indicators are substances whose solutions change color due to changes in pH. These are called acid-base indicators.

Color on acidic side

pH Range of color change

Color on basic (alkaline) side

Phenolphthalein Colorless 8.3 – 10.0 Pink

Indicator

pH = potential of hydrogen ion= concentration of hydrogen ion

Equivalence point (Stoichiometric point) The point in a titration where the same

chemical quantity of substances (titrant) has been added. For example, in an acid-base titration it is the point where mol H+ = mol OH- . An acid-base titration is a neutralization reaction that is performed in the lab in order to determine an unknown concentration of acid or base. The moles of acid will equal the moles of base at the equivalence point.

For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3 :N2 + 3H2 =

2NH3

When an indicator dye is used to detect the changes, its endpoint should be close to the equivalence point.

Titration - apparatus, technique, calculationApparatus

Conical flask

Burette

Pipette

Volumetric flask

Before : wash with distilled water and then with the solution going into it.

After : wash with distilled water only.

The Technique of Titration

The technique of titration is used to find out accurately how much of a chemical substance is dissolved in a given volume of a solution, that is, the concentration of the solution.The technique uses a particular set of apparatus with which volumes of solutions can be measured to an accuracy of greater than 0.1 cm3. Three important pieces of apparatus are:

Burette to measure accurately the volume of a solution added. The scale can be read to an accuracy of half a division, that is to 0.05 cm3.

Pipette  to deliver an accurate volume of a solution. Often this is 25 cm3.

Volumetric flask

to make up an accurate volume of a solution, for example, 250 cm3. This could be a standard solution.

A standard solution is one of exactly known concentration and of a known solute.

For each piece of apparatus volume readings are taken at the bottom of the meniscus.

This is called the end point of the titration (or equivalence point of the reaction). There needs to be a way of knowing when the end point is reached. An indicator of some kind may be needed. For example, in the titration of a strong acid and a strong base a few drops of methyl orange or phenolphthalein could be used.

In a titration the pipette is used to transfer 25 cm3 (usually to ±0.05 cm3) of a solution into a conical flask. Another solution that reacts with the pipetted solution in the conical flask is carefully added from a burette until it has all exactly reacted.

Often a titration is repeated until successive titres are within 0.1 cm3.