Main Difference – Primary vs Secondary Standard Solution

Standardization refers to finding the exact concentration of a prepared solution using a standard solution as the reference. Standard solutions are solutions of accurately known concentrations, prepared using standard substances. There are two types of standard solutions known as primary solution and secondary solution. A primary standard solution is a solution with a high purity and less reactivity. A secondary standard is not that pure and is chemically reactive than primary standards.

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This is the main difference between primary and secondary standard solution.

Key Areas Covered

1. What is a Primary Standard Solution – Definition, Specific Properties, Examples 2. What is a Secondary Standard Solution – Definition, Properties, Examples 3. What is the Difference Between Primary and Secondary Standard Solution – Comparison of Key Differences

Key Terms: Hygroscopic, Primary Standard, Reference Material, Secondary Standard, Solvent, Standardization

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What is a Primary Standard Solution

Primary standard solutions are solutions made out of primary standard substances. A primary standard is a substance of known high purity (99.9% pure) which may be dissolved in a known volume of solvent to give a primary standard solution. Primary standards are reagents that can involve in chemical reactions. These compounds are often used to determine the unknown concentration of a solution that can undergo a chemical reaction with the primary standard.

Primary standards comprise special chemical and physical properties. These compounds are extremely pure and highly stable. Therefore, we can obtain pure solutions using these compounds. For example, if we want to prepare a standard solution of 0.1 molL-1 concentration, we can calculate the weight of the primary standard required for this and then dissolve that amount in a suitable solvent. This gives exactly the 0.1 molL-1 solution with a high purity.

Standardization of solutions is a concept of analytical chemistry that is required for the accuracy of a titration. Before we use any solution in a titration process, all the solutions should be standardized with a primary standard solution. This is because, even though we weigh the exact amount of a compound that is required to prepare a 0.1 molL-1 solution, it will not give the exact concentration (due to the presence of impurities). However, since the concentration of the primary standard solution is 99.9% accurate, we can titrate the prepared solution with a suitable primary standard solution to find the exact concentration of the prepared solution.


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Figure 1: Standardization is done as a Titration


Primary standards are highly pure due to their low reactivity. If these compounds were highly reactive, they could get contaminated with many other chemicals, forming impurities. Primary standards are less hygroscopic. Therefore they do not absorb moisture from the air in considerable amounts. This also makes the primary standards highly pure compounds.

Examples of Primary Standard Solutions and their Applications

Solution

Application

Potassium bromate (KBrO3)

Used for the standardization of sodium thiosulfate solutions

Sodium chloride (NaCl)

Used as a primary standard for silver nitrate

Zinc powder

Used to standardize EDTA solutions


What is a Secondary Standard Solution

A secondary standard solution is a solution that is made specifically for a certain analysis. A secondary standard is a substance whose active agent contents have been found by comparison against a primary standard. This means it is usually standardized against a primary standard.

Secondary standard solutions are used to calibrate analytical equipment and analytical techniques. These solutions do not fulfill the requirements of a primary standard. A secondary standard has a less purity than a primary standard. These are less stable and chemically reactive than primary standards. Therefore these compounds can get contaminated.


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Figure 2: Potassium permanganate solution is a secondary standard solution.


Anhydrous sodium hydroxide is a secondary standard. It is highly hygroscopic. Potassium permanganate is another compound that is often used as a secondary standard. It is less stable and is reactive. Therefore when a Potassium permanganate solution is being prepared, it has to be standardized with a primary standard.

Difference Between Primary and Secondary Standard Solution

Definition

Primary Standard Solution: Primary standard solutions are solutions made out of primary standard substances.

Secondary Standard Solution: Secondary standard solutions are solutions made specifically for a certain analysis.

Purity

Primary Standard Solution: Primary standard solutions are extremely pure (about 99.9%).

Secondary Standard Solution: Secondary standard solutions are not very pure.

Reactivity

Primary Standard Solution: Primary standards are less or not reactive.

Secondary Standard Solution: Secondary standards are reactive than primary standards.

Water Absorption

Primary Standard Solution: Primary standards are not hygroscopic.

Secondary Standard Solution: Secondary standards are somewhat hygroscopic.

Applications

Primary Standard Solution: Primary standard solutions are used to standardize secondary standards and other reagents.

Secondary Standard Solution: Secondary standard solutions are used for specific analytical experiments.

Conclusion

Standard solutions can be divided into two groups as primary standard solutions and secondary standard solutions. Primary standard solutions are solutions made out of primary standard substances. Secondary standard solutions are not as pure as primary standard solutions. Purity is the main difference between primary and secondary standard solution.

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References:

1. Helmenstine, Ph.D. Anne Marie. “Learn About Primary and Secondary Standards in Chemistry.” ThoughtCo, Available here.2. “Standard solution.” IUPAC Gold Book, Available here.3. Libretexts. “5.1: Analytical Standards.” Chemistry LibreTexts, Libretexts, 23 Dec. 2016, Available here.

Image Courtesy:

1. “Titration Apparatus” By Ivan Akira – Own work (CC BY-SA 3.0) via Commons Wikimedia2. “Kmno4-oldva” By Naszy – Saját kép (CC BY-SA 2.5) via Commons Wikimedia