Here is a question that sounds silly until you think about it: how do you know the strength of the liquid you are measuring with? In a titration, everything depends on it. If the bottle says the liquid is a certain strength and it is actually two percent weaker, you use more of it to reach the end, every result you report comes out about two percent high, and nothing in the titration will warn you.
The label tells you what the liquid was meant to be when it was made. It does not tell you what it is today. Bottles get opened. Liquids pick up water, lose a little to evaporation, or react with the air. Sodium hydroxide, one of the most common of these liquids, quietly absorbs carbon dioxide from the air and gets weaker while it sits on the shelf.
Checking it against something you can trust
So chemists check. The check is called standardisation, and it works by titrating the liquid against something whose amount you can know exactly just by weighing it. That something is called a primary standard: a substance so pure, so stable and so easy to weigh that its weight alone tells you how much of it you have.
For sodium hydroxide, the usual choice is a white powder called potassium hydrogen phthalate. For work that has to stand up to scrutiny, the best supplies come with a certificate from a national standards laboratory, such as NIST in the United States, stating exactly how pure it is and exactly how to prepare it. That last part matters. The certificate for one of these powders says to grind a portion by hand for a minute or so and dry it at 120 °C for two hours before use, because the crystals hold traces of water that would throw the weight off. Skip the steps and you lose the accuracy you paid for.
The number that comes out
The result of all this is a single number called the titre factor: the liquid's real strength divided by the strength on its label. A factor of 0.998, for example, means the liquid is 0.2 percent weaker than the label says, and every calculation uses the real value from then on.
A few habits keep that number honest. Do the check several times and use the average. Do it at the same temperature you will work at, because a liquid expands as it warms and each millilitre carries slightly less of it. And do it again regularly, more often for liquids that change quickly, like sodium hydroxide, and less often for stable ones.
The idea worth keeping
The best habit of all is to write the factor down every time and plot it. Over weeks, that chart tells you exactly how stable your liquid is, and the day it jumps, you will know something changed before it reaches a single result.
It is a small step that most people outside a lab never hear about. But every number a lab reports from a titration is only as good as this one check. A label is a promise. Standardisation is how you find out whether the promise was kept.
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References
- NIST, Certificate of Analysis, Standard Reference Material 84L, Potassium Hydrogen Phthalate, 2024.
- Metrohm, "What to consider when standardizing titrant", blog, 16 April 2020.
- Mettler Toledo, "The effects of carbon dioxide on alkaline titrants", note TS_00016.