In chemistry, a primary standard is a reagent that is very pure, stable, not hygroscopic, and has a high molecular weight. Ideally, it’s also non-toxic, inexpensive, and readily available. A primary standard provides a reference to find unknown concentrations in titrations and is used to prepare secondary standards and working solutions.
Primary Standard Properties
The ideal primary standard meets all of the following criteria. However, the most important properties are high purity and high stability. Reagent grade chemicals should be used to prepare primary standard solutions.
- High purity
- High stability/low reactivity
- High equivalent weight (to reduce mass measurement error)
- Not hygroscopic (to reduce mass changes from water absorption)
- Non-toxic or low toxicity
- Readily available
Why Primary Standards Are Used
Chemicals react according to mole ratios. A titration determines the concentration of an unknown solution based on volume of a solution with known concentration needed to react with the solution of unknown concentration. But, the accuracy of the calculation relies on truly knowing the concentration of one solution.
So, for example, sodium hydroxide (NaOH) reacts in a 1:1 ratio with hydrochloric acid (HCl). But, sodium hydroxide is not a primary standard because it’s typically impure. Sodium hydroxide is highly hygroscopic and absorbs carbon dioxide from air, so if you weigh out a sample, some of that mass is actually water and carbon dioxide. That throws off any calculation involving the mole ratio because you really have less sodium hydroxide in the solution than you think. Meanwhile, sodium carbonate (Na2CO3) is a good primary standard for a reaction with hydrochloric acid because it’s available at high purity, has a higher molecular weight than sodium hydroxide, and isn’t as hygroscopic.
Using a primary standard offers a high degree of confidence in the concentration of the unknown solution. Because this solution has been standardized against the primary standard, it can be used as a secondary standard. The degree of confidence in the concentration is slightly lower because of error in the process (for example, overshooting the mark for a titration). But, for some chemicals, this type of standardization is the best way to get reliable concentration value.
Primary standards are used to make dilutions for working solutions for exactly the same reason. Because the concentration of the primary standard is known with a high degree of confidence, the concentration of solutions made from it is also known with a high degree of accuracy.
Examples of Primary Standards
There are many primary standards. They are not one-size-fits-all. For example, sodium carbonate is good for standardizing hydrochloric and sulfuric acid, but not acetic acid. So, which primary standard you use depends on the chemical you’re reacting it with. Here is a list of some common primary standards:
- Sodium chloride (NaCl): for silver nitrate (AgNO3) reactions
- Sodium carbonate (Na2CO3, mol wt. = 105.99 g/mol): for titrating acids
- Potassium hydrogen phthalate or KHP (C8H5KO4, mol wt. = 204.23 g/mol): for titrating bases or perchloric acid and an aqueous base in an acetic acid solution
- Potassium hydrogen iodate [KH(IO3)2, mol wt. = 389.92 g/mol]: for titrating bases
- Potassium dichromate (K2Cr2O7, mol wt. = 294.19 g/mol): for redox reactions
- Tris(hydroxymethyl)aminomethane (TRIS or THAM) [(HOCH2)3CNH2, mol wt. = 121.14 g/mol]: for titrating acids
- Zinc powder (after dissolving it in hydrochloric acid or sulfuric acid): to standardize EDTA (ethylenediaminetetraacetic acid) solutions
A secondary standard is a reagent that has been standardized against a primary standard. In other words, a secondary standard’s concentration is known by titrating it against a measured volume of a primary standard instead of by weighing it out and dissolving it in a solvent. A secondary standard may be less pure and more reactive than a primary standard, but it still upholds some of the properties of a standard. It’s stable enough that its concentration remains known for a long time. Sodium hydroxide (NaOH) is a common secondary standard.
- Hemenway, M. K. (2002) Holt Science & Technology: Physical Science (1st ed.). Holt, Rinehart and Winston. ISBN 78-0030519574
- Skoog, Douglas A., West, Donald M.;Holler, F. James (1995). Fundamentals of Analytical Chemistry (8th ed.). Harcourt Brace College Publishers. ISBN 0-03-035523-0