An indicator of pH or other function is added to the calibrated solution that allows the determination of the concentration of a given substance. The solution calibrated in this way is called a secondary standard or secondary standard.
To be used as a primary standard or standard, a reagent must meet a series of requirements, which include, among others, purity greater than 99.9% and that it is not reactive with water, oxygen and carbon dioxide present in the air.
Characteristics of a primary pattern
A primary standard must meet a set of requirements that allow it to be used in this role. These requirements can be summarized as follows:
A reagent to be used as a primary standard must have a purity greater than 99.9%, indicating that the concentration of allowed impurities must range between 0.01% and 0.02%. The concentration of the impurities must be determined by analytical methods.
The primary standard must be chemically stable and non-reactive. Reactive substances during storage combine mainly with air components; such as water vapor, carbon dioxide, and oxygen, thus being able to alter the net weight of the reagent.
This causes the actual weight of the substance to be less than the calculated or theoretical weight; which disables its use as a primary pattern. For this reason, sodium hydroxide, a hygroscopic substance, cannot be used as a primary standard in the standardization of acid solutions.
High equivalent weight
It is desirable that the substance used as the primary standard has a high equivalent weight. This minimizes the error that can be made when weighing the substance.
Other desirable characteristics have been noted in a substance used as a primary standard, but of less importance, such as that these substances are not very expensive and have little toxicity.
Differences with the secondary standard
A solution of the primary standard is prepared with a substance that meets the above requirements. Meanwhile, a solution of a secondary standard is prepared at an approximate concentration and must be standardized by using a solution of the primary standard.
Standardization of the secondary standard solution is carried out by titration of the secondary standard solution, using the primary standard solution as a titrant. This enables the concentration of a secondary standard solution to be prepared with the desired accuracy.
The secondary standard is prepared by titration using a primary standard as a titrant; while the use of the secondary standard as a titrant produces a working solution.
In public health, the concept of primary and secondary standards has another connotation. The primary standards are aimed at protecting public health, while the secondary standards are aimed at aesthetics.
There are four titrations used for secondary standard solution preparations. They are acid-base, redox, precipitation, and complexation titrations.
In this titration, sodium carbonate, Na 2 CO 3, which has a molecular weight of 105.99 g / mol, and Tris (hydroxymethyl) aminomethane (TRIS or THAM) are used as primary standards, having a molecular weight of 121.14 g / mol.
Sodium hydroxide cannot be used because it is a hygroscopic substance, so it is difficult to obtain an exact weighing of this substance.
Bases or alkalis
In the titration of the bases, potassium hydrogen phthalate or potassium acid phthalate are used as primary standards, which is abbreviated as KHP, and whose chemical formula is C 8 H 5 KO 4, with a molecular weight of 204.23 g / mol. This is the quintessential primary standard for standardization.
Likewise, potassium hydrogen iodate is used, with the chemical formula KH (IO 3 ) 2 and a molecular weight of 389.92 g / mol; and benzoic acid, with the chemical formula C 7 H 6 O 2, with a molecular weight of 122.12 g / mol.
Another widely used primary standard, in the absence of KHP, is oxalic acid; specifically its hydrate, C 2 O 4 · 2H 2 O.
Hydrochloric acid and sulfuric acid are not used because there is no exact information about their concentrations. Nitric acid presents the same problem but also contains traces of nitrous acid (HNO2 ), which acts on some pH indicators, causing their alteration.
Phenolphthalein is almost always used as an acid-base indicator, since it offers a fairly noticeable endpoint, and its use is quite simple.
Several primary standards or standards are used for redox reagents. Arsenic trioxide, As 2 O 3, is used as the primary standard in the titration of potassium permanganate, KMnO 4, and molecular iodine, I 2.
Iron with oxidation state +2 is used as the primary standard in the titration of potassium permanganate, cerium sulfate, Ce (SO 4 ) 2, and potassium dichromate, K 2 Cr 2 O 7. K 2 Cr 2 O 7 is used as the primary standard in the titration of sodium thiosulfate, Na 2 S 2 O 3, an essential reagent for iodometric determinations.
On the other hand, copper is used as the primary standard in the titration of sodium thiosulfate.
Silver in solution precipitates in the presence of X – ions :
Ag + + X – → AgX (s) (precipitate)
X – represents the chloride, bromide, iodide, or thiocyanate ion, SCN –. Thus and for example, sodium chloride is used as a titrant or primary standard for the standardization of silver nitrate solutions, AgNO 3.
EDTA (ethylenediaminetetracetate) is standardized with a calcium carbonate solution, CaCO 3 , which acts as the primary standard.
Ca 2+ + EDTA 4- → CaEDTA 2-