Quantitative Analysis
Quantitative analysis involves measuring the proportions of known components in a mixture. Whereas qualitative analysis involves identifying what materials are present in a sample (quality), quantitative analysis involves determining how much of a material is present in a sample (quantity).
Chemists did not begin to use quantitative methods of analysis in earnest until the mid-eighteenth century. The development of chemistry into a science unto its own with robust theories and laws mirrored the introduction of quantitative analytical methods. Chemists could argue more strongly their views about the composition of substances when they could provide numbers and exact amounts. The two chief classes of quantitative analysis, gravimetric and volumetric, were developed in the eighteenth and nineteenth centuries.
The first quantitative analyses in the mid-eighteenth century were gravimetric. The experiments conducted with gravimetric techniques helped develop the theory of combustion, the law of chemical combination, and the concept of equivalent weight. Volumetric analysis was also developed in the eighteenth century, but its use in determining quantities of a substance in a mixture was limited until chemists reached a better understanding of solutions in the nineteenth century.
Gravimetric analysis involves weighing substances both before and after chemical reactions.Antoine-Laurent Lavoisier ushered in the new system of chemistry by dismantling the phlogiston theory of combustion using gravimetric analysis to determine that the weight gained by metals upon combustion was equal to the loss of weight of oxygen in the reaction vessel. Gravimetric analyses today frequently involve precipitation reactions. In these reactions, the amount of a species in a sample is determined by precipitating it from solution. The precipitate is then filtered from the solution, dried, and weighed. Gravimetric analyses tend to be very simple and accurate, but because they involve much time-consuming work, chemists today tend to use modern instrumental methods like gas chromatography- mass spectrometry.
The other main branch of quantitative methods is volumetric analysis. These analyses are based on titration. Titration is a procedure for determining the concentration of a sample by carefully adding a just enough of a solution with known concentration to react with the sample. If you know the volume and concentration of substance with known concentration that just reacts with sample, you can determine the concentration of the sample in solution. This method of analysis is called titration.
To find when a reaction is just complete, an indicator is added to the solution. An indicator is a substance that changes colour when a reaction approaches completion. An example of volumetric quantitative analysis is the following: a flask contains a solution with an unknown amount of HCl. This solution is titrated with 0.207M NaOH. It takes 4.47 mL of NaOH to complete the reaction. What is the concentration of the HCl?
Most quantitative techniques, such as gravimetric and volumetric techniques, were replaced in the 1960s with instrumental methods. These instrumental techniques usually include both qualitative and quantitative analytical methods. Gas chromatography-mass spectroscopy, for instance, can give both quantitative and qualitative information. This is a technique for separating mixtures of gases, liquids, or dissolved substances into their constituents and then identifying what substances make up the mixtures and their relative amounts. Other techniques that will give both qualitative and quantitative analyses are spectrophotometry.
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