Quantitative analysis is an important aspect of the analysis of any product, substance, a chemical or a drug formulation.
Analysis (Scientific) is a qualitative and quantitative estimation of any compound or substance by a defined and accepted procedures under a standard set of conditions.
Example: If you got an antacid powder sachet, you can notice the list of chemicals and also the quantity of each chemical present in the sachet. This quantity is approved by the government bodies and also the analysis of each constituent is carried as per the norms described in the respective nation pharmacopeia For instance if it is the United States, it will be done as per the United States Pharmacopeia (USP), in the UK by British Pharmacopeia (BP), etc.
“Qualitative analysis is meant to check/specify the actual chemical or substance and also chances of impurities if any.”
It relies on nature or chemistry of the compound. So the quality of the compound can be known.
In the above formulation, qualitative analysis tells you the purity of the sample and its nature
Quantitative analysis indicates how much of the given substance or chemical is present in said sample. It relies on the concentration of the said compound in a given sample. Here the actual quantity of the compound in the sample is known.
In firms involved in the production of drugs, pharmaceuticals, nutraceuticals, chemicals, bio-molecules, etc.. both qualitative and quantitative analysis are used to comply with the end product with desired or recommended standards.
Quantitative analysis methods:
This is done by 3 basic methods by either.
a) Measuring the amount of reagent needed to complete the chemical reaction, or ascertaining the amount of reaction product obtained through suitable chemical reactions. Ex: Acid-base titrations.
b) Appropriate electrical measurements: Here the property of sample like its ability to conduct electricity or get oxidized or reduced are used as a function of quantity analysis.
c) Measurement of certain optical properties Some compounds have properties like the ability to absorb or emit a specific wavelength of light. This function of the compound is utilized for quantitative analysis of the substance.
More on Quantitative analysis relying on a chemical reaction.
Gravimetric analysis: Here the said substance is precipitated into an insoluble form which is filtered, 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 ion in the given sample.
Titrimetric analysis: 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 color of an indicator added in the reaction mixture.
Quantitative analysis by electrical measurements include
Amperometry here the sample which has the ability to conduct current is tested. Wherein the current and time needed to complete an electrochemical reaction is noted.
Potentiometry: Those substances which have an ability to oxidize or reduce are measured by this technique. In potentiometry, the changes in potential or EMF due to oxidation or reductive reaction are measured. During potentiometric titrations, a standard electrode and reference electrode is used. During the course of reaction progress, the standard electrode potential doesn’t change but that of reference electrode changes. The addition of reacting reagent either oxidative (preferably) or reducing agent, changes the potential of the solution. The endpoint or equivalence point is indicated by a sudden change in potential of the graphical plot of electro-motive force (emf) plotted as a change in potential against the volume of the titrating solution.
Conductimetry which considers the electrical conductivity as a function of the quantity of substance of interest. Here the substance under test has the ability to conduct and on the addition of reacting substance, the conductivity goes down gradually till the endpoint. From there the conductance of externally added titrating agent rises.
Optical methods of quantitative analysis
Absorption methods like visible spectroscopy i.e colorimetry, ultraviolet spectrophotometry, infrared spectrophotometry, etc. Here the substance in the sample absorbs a certain wavelength of light and this absorption is determined as a function of quantity.
Visible spectroscopy and UV spectroscopy rely on the light-absorbing property of the substance. Every substance absorbs light to a maximum extent at a specific wavelength. This wavelength of light is called lambda max.
Infrared spectroscopy is a bit different than the above. It is used to check the bonds in between the atoms and molecules.
Emission methods involve techniques like emission spectroscopy, fluorimetry, etc.. wherein the characteristic light emitted by a substance is recorded as a function of quantitative analysis.
Fluorimetry 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 exited 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.