What are the types, Definitions, Properties, and Uses of Primary and secondary reference standards?

 

What are Reference Standards?

Solutions of exactly known strength are called standard solutions. A standard solution contains a known weight of chemicals in a definite volume of solution. A standard solution is prepared by dissolving an accurately weighed quantity of highly pure substance in a definite volume of solvent.

Types of Standard Solutions

There are two types of standard solutions first is primary standard and secondary standard.

Primary standard

Any reagent or chemical which has all these properties is suitable for use as a primary standard:

• It is extremely pure. 
• It is highly stable. 
• It is anhydrous. 
• It is less hygroscopic. 
• It has a very high molecular weight. 
• It can be weighed easily. 
• It should be ready to use and available. 
• It should be preferably nontoxic

Following is the detail of a few properties

Purity: To qualify as being the primary reference standard, a chemical needs to be extremely pure. Preferably, it should have 99.98% of purity. Therefore, any chemical having less than 99.98% of purity cannot be categorized as a primary standard.

Stability: it should be highly stable, which means, it usually does not react easily when kept in its pure form, or in other words, it should have very low reactivity. Why stability is important? Because, if a chemical reacts easily with oxygen or water in the atmosphere or changes its property over time, then it is an unreliable chemical to use as a primary standard.

Anhydrous: it means, it does not contain any water molecule in its molecular structure. For example, magnesium sulfate is found with formula magnesium sulfate with seven molecules of water. Therefore, to prepare a primary standard solution of magnesium sulfate its anhydrous form is required. Which should be of analytical reagent grade with purity greater than 99.98% means you cannot prepare a primary standard solution of magnesium sulfate with its hydrated form. We need an anhydrous form of magnesium sulfate or any other chemical which we want to use as a primary standard.

Less hygroscopic: the chemical should be less hygroscopic that is on opening the container it should not absorb water molecules from the atmosphere.

High molecular weight: it should have a very high molecular weight to minimize weighing errors. Here are some examples of primary standards. These kinds of materials that are drying under certain specified conditions are recommended for use as a primary standard in the standardization of volumetric solutions. For instance, the primary standards for acid-base titration include potassium hydrogen phthalate, anhydrous sodium carbonate. The primary standards for redox titrations include arsenic trioxide sodium oxalate, potassium bromate. The primary standard for precipitation titration is Sodium chloride.

Uses of Primary Standards

• Used to standardize volumetric solutions. 
• Used for standardization of hydration solutions. 
• Used for calibration of secondary standards. 
• Used to calibrate analytical instruments or determine unknown concentrations.

Secondary Standards

A secondary standard is a solution that contains an exactly known amount of the substance in a unit volume of the solution which is determined by titrating against a primary standard. It means the concentration of the secondary standard is determined by a tight ratio against a primary standard. Or we can say that a secondary standard solution is a solution in which the concentration of dissolved solute has been determined by a reaction (means titration) with a primary standard solution. In simple words, it means a secondary standard can be only used when it is standardized against a primary standard.

Any chemical or reagent which has the following properties will be known as a secondary standard.

• It is less pure than the primary standard. 
• It is less stable and more reactive than the primary standard. 
• But its solution remains stable for a longer duration of time. 
• It is titrated against a primary standard

Uses of secondary standards

Secondary standards are commonly used to calibrate analytical equipment and analytical techniques as well it is used in titration.