Secondary Standard
Definition and meaning of Secondary Standard in chemistry.
A secondary standard is a chemical solution with a concentration found by testing it against a primary standard. It serves as a daily working reference liquid for general chemistry experiments. You must figure out its exact strength right before you plan to use it.
In more detail
Secondary standards are usually less pure and less stable than high-quality primary standards. Their exact concentration can drift over time because they react with air or water. Because of this drift, you must standardize them through a process called titration.
During titration, you carefully mix the secondary standard with a known primary standard. This tells you the exact molarity of your secondary liquid on that specific day. You might wonder why anyone would use a secondary standard if it changes so much.
The main reason is that primary standards are often very expensive and hard to handle. Secondary standards are much cheaper and very easy to prepare in large liquid batches. A common misconception is that a secondary standard is just a poorly made chemical.
In reality, it is a perfectly good tool once you measure it correctly. For example, sodium hydroxide changes strength because it grabs water and carbon dioxide from the air. You just have to test it against a stable acid before starting your main experiment.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Main purpose | Used as a daily working titrant in lab experiments |
| Common flaw | Concentration changes over time due to air or water exposure |
| Preparation step | Must be standardized against a primary standard before use |
| Cost benefit | Usually much cheaper and easier to make than primary standards |
| Classic example | Sodium hydroxide solution |
You want to test how much acid is in a bottle of store-bought vinegar. You prepare a large jug of cheap sodium hydroxide to use as your testing liquid. First, you titrate the sodium hydroxide against a trusted primary standard like potassium hydrogen phthalate. Once you know the exact strength of the sodium hydroxide, it becomes your secondary standard. You then use it to accurately test the vinegar.
Frequently asked questions
Why do secondary standards change their concentration over time?
Many secondary standards easily absorb water vapor or carbon dioxide directly from the room air. This extra mass changes the ratio of the chemical in the solution.
Why not just use primary standards for everything?
Primary standards are very expensive and often difficult to dissolve in water. It is much cheaper to buy a large amount of a secondary standard for routine daily testing.
How often do I need to test my secondary standard?
You usually need to test it against a primary standard on the exact day of your experiment. If it sits on a shelf for a week, its strength has likely changed.