Primary Standards

Primary standards are substances that have high purity, stability toward air, high molecular mass, are weighed easily, are low-toxic, inexpensive, and non-hygroscopic reagents that are used in chemical reagents. These are used in chemical reactions with another substance. As these are pure and stable, we can obtain pure solutions using these compounds. Primary standards having special chemical and physical properties. As these are pure and accurate, they are used in the calibration of analytical instruments like pH meters, spectrophotometers (IR, UV, NMR, and mass), and titration equipment.

Primary standards are used to standardize the volumetric solution and are mainly used in the titration method for standardization of titration solvents. It can be used both in acid and base titrations. These are used to standardize the secondary standards. The main advantages of primary standards are reliability, traceability, and accuracy. In general, primary standards go through four types of reactions. Below are the examples for primary standard reactions.

1. Acid-base reactions: sodium carbonate Na2Co3, sodium tetraborate Na2B4O7, potassium hydrogen phthalate KH(C2H4O4), constant boiling point hydrochloric acid, potassium hydrogeiodate KH(IO3)2, benzoic acid (C6H5COOH).
2. Complex formation: silver, silver nitrate, sodium chloride, various metals (ex: spectroscopically pure zinc, magnesium, copper, and manganese), and salts, depending upon the reaction used.
3. Precipitation reactions: silver, silver nitrate, sodium chloride, potassium chloride, and potassium bromide (prepared from potassium bromate).

4. Oxidation-reduction reactions: potassium dichromate K2Cr2O7, potassium bromate KBrO3, potassium iodate KIO3, potassium hydrogeniodate KH(IO3)2, sodium oxalate Na2C2O4, arsenic(II1) oxide As2O3, and pure ions.

   To use as a reagent in chemical reactions, primary standards should have the following qualities:

• They are made from highly pure materials. These should have 99.98 percent purity. 
• Because of their well-defined chemical standards, they enable precise computations. 
• Pure chemicals are highly stable in nature. When the chemical is exposed to water or     air, it exhibits little reactivity. Chemicals that are unstable in nature are not considered standards. 
• To minimize weighing errors, these standards should have a high relative molecular mass.  
• The chemical is less hygroscopic, meaning it retains its weight even when exposed to    air or water. 
• High purity allows for easy weighing. 
• Simple solvents, such as water or dilute acids or bases, are used to create them. So this is still less hazardous and less expensive. 

 Sodium carbonate (Na2CO3), potassium bromate (KBrO3), potassium iodate (KIO3), and sodium tetraborate (Na2B4O7) are some of the most common standards used in chemical reactions. 

Sodium chloride (Nacl) serves as the principal standard for silver nitrate reactions.

 Standardization of potassium permanganate (KMnO4) solutions involves using oxalic acid (H2C2O4). 
Hydrated salts are not employed as major standards due to their instability in nature. When exposed to air conditions, their volume fluctuates.