Solutions are an indispensable element in the everyday work of a chemist. They play a particularly important role in volumetric chemical analysis, which is still successfully used for routine determinations. An important role is played by standard solutions, i.e. with an exactly known concentration.
The importance of standard solutions in chemistry
Standard solutions are solutions whose concentration is precisely known. They are of great importance in volumetric (titration) analysis, which is one of the sections of quantitative analysis. Compared to gravimetric determinations, where a precise amount of the reagent must be used to precipitate, in titration determinations equivalent amounts of a titration reagent solution are added. This involves the use of standard solutions to calculate the analyte content in the sample, based on the reaction equation. Titration methods are still used in everyday laboratory practice, e.g. in determining the content of chloride ions using the Winkler method (in this case, the standard solution reagent, i.e. the titration reagent, is a solution of silver (I) nitrate (V), AgNO3).
The concentration of the standard solutions must be known exactly. Most often, its accuracy is given to the digit of ten-thousandths (fourth decimal place). The concentration is usually expressed in moles of a given substance dissolved in one litre of solution (or the number of milli-moles in one millilitre). In volumetric analysis, a practical approach is to report the titre of the solution, which is the number of grams of solute per millilitre. It is especially useful in routine, serial analyses, where the result is usually provided in grams. In such cases, it is enough to multiply the number of millilitres of thereagent used during the titration by the titre.
Preparation of standard solutions
Standard solutions can be prepared in several different ways. We can use ready-made, commercially available so-called fixanals (you can read more about this method later in the article), prepare a solution from an exact sample weight, perform a titration against another solution of known concentration, or titrate against an exact sample weight of a reference substance. Accurate preparation of standard solutions and precise adjustment of their titre (i.e. determination of the exact concentration) is essential, especially in volumetric analysis.
Preparation of a standard solution from an exact sample
The basic and easiest way to prepare a standard solution is to make it from an exact weight of the reference substance. In most of the cases, aqueous salt solutions are prepared. Not every compound will be suitable. The reference substance must meet a number of conditions:
- its purity should be at least 99.99%;
- it must be a stable substance, which means that it cannot decompose in an aqueous solution within a certain period of time;
- when stored in atmospheric conditions, it cannot absorb water (no hygroscopy) or carbon dioxide;
- the reference substance must react strictly according to one reaction.
Examples of standard substances are: sodium carbonate (Na2CO3), potassium bromate (KBrO3), sodium chloride (NaCl).
A carefully prepared sample of the standard substance (the mass is known to the accuracy of ten-thousandths) is dissolved in a specific volume of distilled water.For this purpose, measuring flasks with a marked line on the neck are used, which are topped up taking the meniscus into account.We should remember to thoroughly rinse out the vessel, in which the reference substance has been weighed, several times with distilled water (quantitative transfer of the sample weight to the volumetric flask).
Preparation of a solution with an approximate concentration and titre adjustment to a sample of a standard substance
In this case, a solution of approximate concentration is prepared, e.g. by dissolving a less precise weight in distilled water. Then, several titrations are made with it (usually, to obtain reliable results three repetitions are made, and they should not differ significantly) of the appropriate size of the standard substance. Knowing the volume of the adjusted solution, used for the titration of the sample, the exact concentration of the standard solution is calculated.
Titration to another standard solution
The exact concentration of a solution can also be determined against another standard solution. Accurate volumes of the solution to be titrated are measured and titrated against an indicator. Then, the molar concentration is determined from the formula.
Fixanals
A convenient method of preparing standard solutions is to prepare them using so-called fixanals. A fiksanal is a precise analytical weight (solid or in the form of a solution), which is enclosed in an ampoule made of glass or plastic. The shape of the ampoule facilitates precise transfer of the contents to the volumetric flask. They can be easily purchased commercially.
Solutions prepared using fixanals do not need to be additionally titrated and can be successfully used in many volumetric analyses. To ensure the exact concentration of the solution resulting from the content of the ampoule, special attention should be paid to the proper transfer of the contents to the flask. It’s not just about accurately pouring over the contents. It is necessary to ensure the glass is clean, rinse the ampoule several times and transfer the washings quantitatively, as well as topping up the volumetric flask to the mark, taking the meniscus into account.
The fixanal content is specified by the manufacturer and indicated on the label. Usually these are quantities intended for 1 litre. Thus, using such a finished product, 1 dm3 of a standard solution is obtained.