What is a standard solution?

A standard solution is any solution whose concentration is known with an acceptable degree of precision and accuracy . These are solutions in everyday use in the field of analytical chemistry and are used in many chemical analysis applications, ranging from titrations or titrations to the preparation of calibration curves for instrumental analysis.

In principle, any solution can be considered a standard solution, as long as its concentration is known and it is reasonably certain that it will remain approximately constant during the time it is used. This implies that the solutes in standard solutions must have a minimum degree of stability that prevents them from decomposing or transforming into another compound before the solution is used.

Properties of standard solutions

In addition to having a known concentration, standard solutions must have some particular properties that largely depend on the type of chemical analysis for which they will be used. For example, in the case of standard solutions used in titration techniques (acid/base titrations, redox, etc.), the standard solutions must:

• Remain stable for sufficiently long periods of time, thus ensuring that their concentration remains constant during analysis.
• The concentration should be comparable to the suspected concentration of the analyte (the substance being tested). Otherwise, the titration will have a greater margin of error.
• They must react quantitatively with the analyte or substance whose concentration is being determined. This implies that the reaction must be complete.
• Only one chemical reaction representable by a balanced chemical equation should occur. That is, there must be no undesired side reactions either with the analyte or with the other components of the sample matrix.
• The reaction with the analyte must be rapid.

For other applications of standard solutions such as calibration curves for instrumental analysis (in techniques such as atomic emission or absorption spectroscopy, UV-visible absorption, etc.), these solutions are not necessarily required to have the same properties.

For example, in the case of calibration curves, the standard solutions do not react with the analyte, but contain the analyte in known concentrations in order to establish the instrumental response for said concentrations (known as calibration curves) and thus being able to later determine the concentration of the analyte in the sample by extrapolation. In these cases, a series of standard solutions with concentrations both higher and lower than the expected concentration of the analyte is required.

In other analytical methods known collectively as back-up techniques, standard solutions are added in known amounts to the analyte to allow the substances to react with each other, and the excess of the added standard is then titrated or otherwise determined. In these cases, the reaction with the analyte need not be rapid as it only needs to occur once before excess analysis, and not after each titrant addition during the titration itself.

Types of standard solutions

Depending on the characteristics of the solute and its chemical stability over time, we can distinguish two classes of standard solutions, primary and secondary standard solutions.

Primary standard solution

A primary standard solution is a solution prepared from a primary standard. This consists of a high purity chemical substance that remains stable over time, so the concentration of its solutions is constant. Primary standards have the following general characteristics:

• They are high purity reagents that are not spontaneously contaminated with substances present in the atmosphere.
• They have a precisely known composition, that is, we know their chemical formula, their purity, and the identity and concentration of the main contaminants.
• They are chemically stable substances both in their pure state and in solution. This ensures that stoichiometric calculations carried out from the mass or volume of the pure reagent will be exact, and that the concentration thus calculated from the solutions we prepare (the primary standard solutions) will be constant.
• They must not absorb water vapor or other gases from the atmosphere and must be able to be dried in an oven to constant weight without decomposing to remove any traces of moisture.
• Ideally, they have a high equivalent weight. This minimizes weighing errors as a larger mass of reagent is required to be weighed for the same final normal concentration.
• They must react rapidly and stoichiometrically with the analyte.

Primary standard solutions are the ideal solutions for chemical analysis as they can be prepared directly by weighing and dissolution (and, if necessary, dilution) and their concentration can be determined directly from the mass of the pure reagent and the final volume. of dissolution. This allows these solutions to be used for chemical analysis directly, without the need to prepare additional solutions or carry out other standardization steps. However, many primary standards are expensive due to the high degree of purity required.

Secondary standard solution

Primary standards are the ideal reagents for chemical analysis, but it is not always possible to find a suitable primary standard for certain analytical methods. Additionally, in many cases, especially in the routine analysis of samples, the cost of the primary standard is prohibitively high, especially considering that there are other substances that, although they do not meet all the conditions to be primary standards, have the appropriate chemical characteristics. but at a much lower cost. These are the secondary standards, and solutions prepared from them are called secondary standard solutions.

Secondary standards are substances that react quickly and quantitatively with the analyte, but do not meet the other conditions to be a primary standard. In some cases, secondary standards cannot be obtained with an adequate or even known level of purity since they are not completely chemically stable in the atmosphere.

A typical example is sodium hydroxide, which is a low-cost reagent that absorbs water and carbon dioxide from the atmosphere, reacting with both to become sodium carbonate. This means that sodium hydroxide is always contaminated with sodium carbonate, and the same happens with its aqueous solutions.

In the case of hydrochloric acid, this is not a primary pattern either, as both commercial solutions and standard solutions used in chemical analysis slowly lose solute in the form of hydrogen chloride gas.

Preparation of standard solutions

By dissolution of pure reagents

The simplest and most direct way to prepare a standard solution is to weigh the pure reagent on an analytical balance that is well calibrated (to ensure good accuracy) and has a good appreciation of the order of 0.1 mg (10 -4 g) ^and then dissolving it in a known final amount of solution (through the use of a balloon or volumetric flask). In the case of liquid reagents, these are usually measured using volumetric pipettes, although they can also be weighed, as long as they are not too volatile.

Then the relevant calculations are carried out to determine the real concentration of the solution from the actually weighed mass of the reagent, and not from the previously calculated mass. In other words, if we calculate that we need to weigh 0.1382 g of sodium carbonate to prepare 1 L of 10 ^-3 molar standard solution but we weigh 0.1389 g, we should use the last mass (the one we actually weigh) and not the first. in calculating the concentration of the standard solution.

As mentioned above, only primary standard solutions can be prepared directly by weighing and dissolving, since only with primary standards can we be sure that the mass we weigh actually corresponds to the reagent.

By dilution of concentrated solutions

A second very common way to prepare standard solutions is by means of the dilution procedure. In this case, a measured volume is taken with a volumetric pipet and transferred to a balloon or volumetric flask of suitable capacity and diluted to the graduation mark with water or with the solvent being worked on.

Concentrated solutions in some cases are commercially available or can be prepared by weighing and direct dissolution as explained in the previous section.

By standardization against a primary pattern

In the case of secondary standard solutions, these cannot be prepared directly by weighing and dissolving the pure reagent for the reasons explained above. This is because, due to the presence of different impurities and due to the instability of the reagent or its solutions, the concentration calculated from the measured amounts of reagents can deviate considerably from the actual concentration of the solution. In other words, even though we weigh the reagents to prepare the solution very precisely and as accurately as possible, we don’t actually know the true concentration of the solution. This implies that these are not yet standard solutions.

For this reason, in these cases, after preparing the secondary standard solution, a chemical analysis must be carried out to determine the real concentration of this solution using another standard solution (of already known concentration). This process is known as standardization since, once the real concentration of the solution is determined, it becomes a standard solution. But, since this solution was standardized from another solution consisting of a primary standard solution, the standardized solutions are called secondary standard solutions.

Examples of standard solutions

Examples of Common Primary Standards

There are a large number of primary standards used to prepare primary standard solutions for use in different types of chemical analysis. Some examples of these reagents are given below along with the kind of analytical method for which they are used:

• Sodium carbonate (Na ₂ CO ₃ ): It is a very stable salt that serves as the primary standard for the titration of acids by volumetry.
• Potassium acid phthalate (KC ₈ H ₅ O ₄ ): This substance has a high molecular weight of 204.22 g/mol and is a stable, non-hygroscopic substance that serves as a primary standard for base titration.
• Potassium dichromate (K ₂ Cr ₂ O ₇ ): Potassium dichromate is a very stable salt both at room temperature and at high temperatures. Its solutions are stable for years if the necessary precautions are taken to prevent the solvent from evaporating. This substance is a strong oxidant, so it can be used as a primary standard in the redox titration of reducing agents.
• Sodium oxalate (Na ₂ C ₂ O ₄ ): Again, this is a primary standard for redox titrations. Oxalate is a reducing agent that oxidizes rapidly to carbon dioxide in the presence of an oxidant, making it a suitable primary standard for determining the concentration of oxidizing agents.
• Silver nitrate (AgNO ₃ ): Silver nitrate is an example of a primary standard commonly used in the determination of silver by inductively coupled plasma atomic emission spectroscopy (ICP-AES). It is used mainly for its purity, its stability and its high solubility in water.

Examples of Common Secondary Standards

Each of the primary patterns mentioned above can be used to standardize on one of the following examples of secondary patterns:

• Sodium hydroxide (NaOH): As explained above, sodium hydroxide reacts slowly with carbon dioxide in the air to form sodium carbonate, so solutions are not perfectly stable. This substance serves as a secondary standard in the titration of both strong and weak acids.
• Hydrochloric acid (HCl): Hydrochloric acid solutions are used as standards for the determination of different strong and weak bases. However, like NaOH, HCl solutions are not stable in the long term, so they are not primary standards.
• Potassium permanganate (KMnO ₄ ): Permanganate is a very strong oxidizing agent and is one of the most common titrants used for the determination of reducing species in redox titrations. However, although permanganate itself is not unstable in the atmosphere and can be obtained in good purity, it is such a strong oxidizer that it can oxidize water to molecular oxygen, reducing permanganate to manganese dioxide. solid. This reaction is very slow, but it makes permanganate solutions not totally stable, so they are not suitable as primary standards.
• Sodium thiosulfate (Na ₂ S ₂ O ₃ ): Sodium thiosulfate is a reducing agent used in redox titrations for the determination of various oxidizing analytes. Despite the fact that aqueous solutions are very stable, the commercial salt, which always corresponds to the pentahydrated salt with the formula Na 2 _S 2 _O 3 _· 5H ₂ O, has a tendency to lose the waters of hydration, which is why it is not serves as the primary pattern. Sodium thiosulfate solutions are usually standardized with a potassium dichromate solution, or, failing that, with a standardized potassium permanganate solution.

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