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.
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