“Analytical Chemistry is the art of separating and recognising different substances and determining the constituents of a sample”
(Whilelm Ostwald, 1894)
Since then, Analytical Chemistry has evolved from an art to a science. It is not an end in itself but is applied to basic chemistry research, and has practical application in industry, medicine and other branches of science based on the general study of materials.
These few examples help us realise what a central role Analytical Chemistry plays.
“Analytical chemistry is the branch of chemistry concerned with the identification, chemical-physical composition and the qualitative and quantitative analysis of the components in a given sample”.
Some of the most frequently-encountered terms in analytical chemistry are:
sample: the object of the analytical procedure (for example: a blood sample);
analyte: the substance that is of interest in the analysis (for example: amount of hemoglobin in blood);
matrix: the constituents, apart from the analyte, of the given sample (for example: all the constituents of blood except hemoglobin);
method: the procedure used for the analysis, which may be standard or non-official;
qualitative analysis: reveals the presence and chemical identity of the analyte in a sample;
quantitative analysis: establishes, in numerical terms, the quantity of one or more analytes in a sample;
revealability limit: the minimum quantity of analyte that can be determined through analysis;
sensibility: the appreciable variation in quantity of analyte depending on the technique used for analysis.
“The process of analysis often requires preliminary operations (treatment of sample) to transform or eliminate other constituents in the sample that are not of interest (the matrix) so there is no interference.”
“Both quantitative and qualitative chemical analysis can be based on chemical reactions between reagents to form a product and /or on determining the chemical -physical barriers in an analyte. Most chemical analysis these days is of the quantitative type. Qualitative analysis is a procedure that is only used for macroscopic or mesoscopic investigation (or in the classroom). However, it is completely inadequate for revealing how much analyte is present if the amounts are microscopic. If this is the case, instrumental methods of analysis have to be used.”
Based on the information given above, we can define the current objectives of Analytical Chemistry:
The aim of this course is to help students develop the mindset of the chemiscal analyst and to provide them with the basic knowledge for solving probblems relating to Analytical Chemisty, mainly in the food industry. Students should also be aware that the knowledge and techniques they learn can be successfully applied to other fields.
Since this is a course about practical application of knowledge, students are encouraged to attend the laboratory sessions which include quite a lot of practical activities.
One of the main aims of food legislation is to make sure human life and health are protected. This can be achieved through proper application of the HACCP system (hazard analysis critical control point). The main points of this system are:
In many cases, it is current legislation that dictates critical control limits but they can also be set arbitrarily:
This is how analytical chemistry fits in to the food industry.
1. What analytes are present in the sample?
2. What is the concentration of analyte in the sample?
There are two different types of analysis:
Qualitative analysis → The process of identifying what types are present in the sample.
Quantitative analysis → The process that serves to establish in numerical terms the quantity of one or more of the components in a sample.
Classical Analytical Chemistry (uses traditional laboratory materials like glassware, scales etc.)
Traditional analytical methods → Gravimetric and titrimetric methods are particularly useful for determining the main components, that is, those up to 10-3 10-4 M.
Instrumental analytical techniques → Useful for determining substances present in trace amounts (less than 10-5 10-6 M) and they are carried out by measuring a physical signal either directly from the analyte or from one of its derivatives produced by chemical transformation.
Instrumental Analytical Chemistry (using electric-powered tools).
Analytical chemistry uses knowledge from General and Inorganic chemistry and applies it to its own ends.
Theory of solutions: solute, solvent, solution.
Homogeneous and heterogeneous solutions.
End point of titration indicators.
PH of water solutions.
Concept of mass and weight.
An analyte reacts with a reactive in solution, called the titrant, using any chemical reaction possible that gives us the required answer, namely, what is the volume of reactive necessary to make the analyte react completely?
Different types of reaction can be used:
H+ + OH- → H2O
Ce4+ + Fe2+ → Ca(EDTA)2+
Ca2++EDTA → Ca(EDTA)2+
Ag+ + Cl- → AgCl
The end point has to be demonstrated clearly, either with visual indicators or with instrument methods.
Gravimetric analysis involves measuring how much analyte is present in a sample before and /or after drying. The instrument needed therefore, is an analytical balance. A classic gravimetric analysis measures the moisture present in a sample, the amount of water being calculated as the difference in weight between the original sample and once it has been dried.
Inorganic qualitative analysis refers to a systematic procedure used to confirm the presence of ions or other elements by using different chemical reactions such as:
selective: applied to only a few analytes;
specific: reactions which are only applicable when looking for a single analyte.
Based on the measurement of physical (e.g. absorption of light) and chemical (e.g. oxidability) properties of an analyte and its derivatives, obtained through chemical-physical transformation.
The instrument measures these properties and sends a signal that varies according to analyte concentration. Electronics and computing have favoured the development of instrumental methods to the extent that:
Quantification is through the dose-response curve, or standardisation curve.
This is the most important stage in qualitative and quantitative analysis, and requires experience and, above all, intuition. Of vital importance are:
Analytical chemistry falls into the category of scientific methods and so is considered a proper science. This is why is has to satisfy the requirements of reproducibility and universality, making it possible for anyone who wants to check the results to do so. It is a science within everyone’s reach.
We need to consider the precision and accuracy of the method.
Precision: the result is repeatable if the experiment is carried out repeatedly. It depends on random errors and is represented by standard deviation.
Accuracy: concordance between the result obtained and the real value. It depends on random error as well as system errors.
A high level of accuracy and precision requires large amounts of time and money. This is why the chosen method is often a compromise between accuracy and cost.
13. Mohr method
14. Vohlard method