Quantitative Analysis Chemistry | Definition, Methods & Examples
Quantitative analysis is an important aspect of 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 standard set of conditions.
Example: If you got a antacid powder sachet, you can notice the list of chemicals and also 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 pharmacopoeia For instance if it is United states, it will be done as per United states pharmacopoeia (USP), in UK by British pharmacopoeia (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 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 production of drugs, pharmaceuticals, nutraceuticals, chemicals, bio-molecules etc.. both qualitative and quantitative analysis are used to comply 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 ability to conduct electricity or get oxidized or reduced are used as function of quantity analysis.
c) Measurement of certain optical properties Some compounds have properties like ability absorb or emit a specific wave length of light. This function of the compound is utilized for quantitative analysis of substance.
More on Quantitative analysis relying on chemical reaction.
Gravimetric analysis: Here the said substance is precipitated into insoluble from which is filtered, dried and weight is measured as a function of quantity.
Ex: Barium sulphate + Sodium carbonate →Barium carbonate + Sodium sulphate.
Here Barium sulphate is soluble and is converted to Barium carbonate an insoluble form by 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 where in the volume of reagent required to complete the reaction with 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, end point is indicated by a change in colour of an indicator added in the reaction mixture.
Quantitative analysis by electrical measurements include
Amperometry here the sample which have ability to conduct current are tested. Wherein the current and time needed to complete an electro-chemical 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 are 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 solution. The end point or equivalence point is indicated by a sudden change in potential of the graphical plot of electro-motive force (emf) plotted as change in potential against the volume of the titrating solution.
Conductimetry which considers the electrical conductivity as function of quantity of substance of interest. Here the substance under test has ability to conduct and on addition of reacting substance, the conductivity goes down gradually till the end point. 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 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 specific wave length. This wave length of light is called as 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.. where in the characteristic light emitted by substance is recorded as a function of quantitative analysis.
Fluorimetry is based on the ability of sample to absorb and re-emit light of certain wave length. When light is passed on to the sample at specific wave length, the electrons in the atoms get into exited state. They come back to ground state by emitting light of certain wave length as fluorescence. This fluorescence emitted is measured to estimate quantity of the sample.