Standard solutions are used in analytical chemistry to compare the concentration of solutions, determine the composition or rule out the presence of certain substances. They’re commonly used in laboratory experiments such as titration, as well as larger-scale processes like the calibration of chemical products for quality control purposes.
In this article, we look at some common examples of standard solutions and their uses.
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How Do You Identify a Standard Solution?
You can buy standard solutions from a chemical supplier. The easiest way to identify a standard solution is to look at the label on the container, where you’ll find details of the type of substance, solution, and concentration, along with instructions on how to use it.
Another way to identify a standard solution is to test its purity and concentration. While concentration and purity might seem similar, they’re actually different concepts. A solution can be considered pure if it contains zero or very negligible extraneous substances. There must be at least 99% of the intended substance present.
A concentrated solution, however, contains a high amount of the solute that’s been dissolved in the solvent or solution. For example, a solution with 2M of a particular solute is twice as concentrated as a solution that contains only 1M of the same substance.
Primary Standards in Chemistry
Not all substances can be used to prepare primary standard solutions. To be suitable a substance must:
- Be inexpensive and easy to obtain
- Not toxic or harmful, especially to humans
- Have a high molar weight for convenient weighing
- Be very stable and not degrade easily
- Have relatively low reactivity with other substances
- Have a very high level of purity
- Not easily absorb water or moisture from the air (non-hygroscopic).
So, what is a standard solution example? Here are some substances that qualify as primary standards based on the above criteria.
- Sodium carbonate
Otherwise known as soda ash or washing soda, sodium carbonate (Na2CO3) is a common cleaning agent. It’s also used to remove water hardness and can be dissolved in water and taken as an antacid. In industry, sodium carbonate is used to manufacture glass and paper. It should not be confused with sodium bicarbonate, which is better known as baking soda.
- Sodium oxalate
Sodium oxalate (NaC2O4) is toxic when ingested and generally harmful if it makes contact with the skin. It’s commonly used as a reducing agent and is a byproduct of the metabolism of glyoxylic acid or ascorbic acid.
- Sodium tetraborate
Otherwise known as borax, sodium tetraborate (Na2B407) has several practical applications. As well as being used as a cleaning agent, it’s an ingredient in lotions, moisturisers and acne care products. It’s also used in the manufacture of herbicide, paint and ceramic glaze.
- Potassium dichromate
Potassium dichromate (K2Cr2O7) is mainly used as a sanitiser and oxidising agent.
- Potassium bromate
Potassium bromate (KBrO3) is an oxidising agent under certain conditions. It’s mainly used to strengthen dough and increase the rise.
- Potassium iodate
Often used during radioactive leak emergencies, potassium iodate (KIO3) prevents the thyroid from absorbing radioactive iodine.
What is a Secondary Standard?
Secondary standard solutions can be prepared from primary standard solutions through the dilution process (more on this below). Some secondary solutions also act as conjugates of primary standard solutions. This is true for both acids and bases.
Secondary standard solutions can be calibrated using primary standard solutions through titration. They’re not as stable or as pure as primary standard solutions and therefore more reactive.
Some examples of secondary standard solutions are listed below.
- Hydrochloric acid
Aside from being a cleaning agent, hydrochloric acid (HCL) is used in the manufacture of batteries, fireworks and gelatin products.
- Sulphuric acid
Sulphuric acid (H2SO4) is virtually ubiquitous in manufacturing processes, either as a precursor or a reagent. Aside from car battery production, it’s mainly used to manufacture fertilisers and other chemicals, including medicines.
- Sodium hydroxide
Sodium hydroxide (NaOH) is commonly used in the production of soap, paper and dyes.
- Potassium hydroxide
Potassium hydroxide (KOH) has a wide range of applications but it’s mainly used as a precursor to other potassium compounds. It also has several applications in the food industry, where it’s used as a stabiliser and food thickening agent.
What’s the Difference Between Primary & Secondary Standards?
Primary standard solutions are prepared from substances that are pure, stable, water-soluble and have a high molar mass. Ideally, a standard substance shouldn’t be toxic or expensive.
You can make a primary standard by weighing exactly one molar mass of a standard substance and dissolving it in one litre of pure water to get a concentration of one mole. This process is known as the weighing method.
You can prepare a secondary standard solution by following what’s known as the dilution method. In this method, a primary standard solution is used to calibrate the secondary standard solution. A secondary standard can also be directly prepared from a primary standard solution. In the dilution method, water is added gradually until the desired concentration is achieved. You can calculate the total volume of water required by using the formula:
M1V1 = M2V2
- M1 – molar concentration of the original solution
- V1 – the volume of the original solution in litres or cubic decimetres
- M2 – the desired molar concentration of the diluted solution
- V2 – this is usually the unknown value, which is the final volume of the diluted solution that’s required to achieve the desired new concentration.
You can read more about the weighing and dilution methods in our article, How to Make a Standard Solution.
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