Quantitative analysis determines the exact amount of ingredients in a given chemical or drug formulation sample.
For example, if you see an oral rehydration packet powder sachet, you can notice the list of chemicals and the quantity of each chemical present.
This gives an idea of how much each of the ingredients is present in the packet out of the whole material in it.
These ingredients’ quantity, in most cases, complies with the norms of the respective country.
If it is the United States, it will be done as per the United States Pharmacopeia (USP) or FDA guidelines or in the UK as per British Pharmacopeia (BP), etc.
Quantitative analysis methods
The concentration of the sample can be estimated by four basic methods, as mentioned below.
- Titration methods
- Spectroscopic method
- Electrochemical method and
- Chromatographic methods.
By measuring the amount of reagent needed to complete the chemical reaction with a specific ingredient in the sample, one can determine the concentration of the ingredient.
There are four types of titration methods to analyze samples: acid-base, redox, precipitation, and complexometric titrations.
Ex: Acid-base titrations can help us determine the extent of iron present in a sample of ferrous gluconate.
Fe-Sample + H2SO4 ————–>FeSO4 + H2O + -sample.
Titration is a method wherein the volume of reagent required to complete the reaction with the substance of interest is noted using reactions like acid-base titration, oxidation-reduction, complex-forming, or precipitation reactions.
Ex: HCl + NaOH → NaCl + H2O. Once the reaction reaches completion, the endpoint is indicated by a change in the color of an indicator added to the reaction mixture.
2. Spectroscopic method
Every substance has a tendency to absorb electromagnetic radiation of a specific wavelength. This absorption is directly proportional to the concentration.
Hence, by measuring the extent of the absorbed wavelength of light, the sample component is estimated.
The spectroscopic methods are
a) Visible spectroscopy.
b) UV spectroscopy.
c) Fluorimetry (here, the emission is measured instead)
Fluorimetry is an emission method that is based on the ability of a sample to absorb and re-emit light of a certain wavelength.
When light is passed on to the sample at a specific wavelength, the electrons in the atoms get into an excited state.
They come back to the ground state by emitting light of a certain wavelength as fluorescence.
This fluorescence emitted is measured to estimate the quantity of the sample.
3. By electrochemical measurements
Here, the electrical property of the sample molecules is used to determine the concentration.
This includes the ability to conduct electricity or get oxidized or reduced, which is used to determine the concentration.
The methods are conductometry, amperometry, and potentiometric methods.
Amperometry here, the sample that has the ability to conduct current, is tested. Wherein the current and time needed to complete an electrochemical reaction is noted.
Potentiometry: Those substances that have the ability to oxidize or reduce are measured by this technique.
The changes in potential or Electron Motive Force (EMF) due to oxidation or reduction reactions are measured.
The addition of a reacting reagent, either an oxidative (preferably) or reducing agent, changes the potential of the solution.
The endpoint point is indicated by a sudden change in potential of the graphical plot of EMF plotted as a change in potential against the volume of the titrating solution.
Conductimetry measures the electrical conductivity as a function of the quantity of substance of interest.
Here, the substance with the ability to conduct is tested, and with the addition of the reacting substance, the conductivity goes down gradually till the endpoint. From there, the conductance of externally added titrating agents rises.
4. Chromatography methods
Chromatography is a widely used technique for quantitative analysis. Here, the sample components are subjected to differential movement through a stationary phase.
The compounds get separated based on the affinity between the stationary phase and mobile phase liquid.
The separated compounds are collected individually, and their concentration is determined.
There are many techniques in chromatography, such as column chromatography, HPLC, Gas chromatography, ion exchange, gel chromatography, and more.
Other methods of Quantitative analysis
Gravimetric analysis: Here, the said substance is precipitated into an insoluble form, which is filtered and dried, and weight is measured as a function of quantity.
Ex: Barium sulfate + Sodium carbonate →Barium carbonate + Sodium sulfate.
Here, Barium sulfate is soluble and is converted to Barium carbonate, an insoluble form by the addition of sodium carbonate.
The insoluble precipitate is weighed for the quantity of Barium ions in the given sample.
Here, the ability of a compound to rotate the light in a clockwise or anticlockwise direction is measured using a polarimeter.
The ability to rotate light depends on the concentration of the optically active compounds.
Examples of compounds that can be measured include sucrose, morphine, etc.