# Rationale for titant standardization in alkalinity measurements [ISO 9963-1:1994]

I work in a environmental chemistry laboratory and I will start to do some alkalinity measurements in my project. My research group is pretty big on following any and all standardization possible, which most often mean compliance according to ISO-standards.

In the ISO-document Water quality - Determination of alkalinity - Part1: Determination of total and composite alkalinity (ISO 9963-1:1994), you need to prepare your titrant, in this case $0.1 \text{M HCl}$ and a standard solution $0.025 \text{M Na}_{2}\text{CO}_{3}$. Then you need to standardize your titrant using either a potentiometric detection (titrate a dilute standard solution to pH 4.5 and note the volume used) or visual endpoint detection (titrate a dilute standard solution with bromocresol green-methyl red indicator until color change). And lastly, a blank using water as the titrand instead of the standard solution.

Using these two acid volume consumption you then recalculate your $0.1 \text{M HCl}$ to it's actual concentration. And if you keep your stock acid solution for a longer period you should perform this standardization every week. Then you can perform the actual titration and determination of the alkalinity in the sample.

My question is, why is this necessary? Why does the ISO stipulate this standardization?

I understand that if I prepare an acid it won't be super precise since there are effects that influence the preparation such as my handling, age of chemical, pipetting, graduated cylinder error intervals etc. But the same would apply to my preparation of the sodium carbonate. In principle I am shifting the error effects from the acid preparation to the sodium carbonate preparation which I am standardizing against.

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According to this listing, $$\ce{Na2CO3}$$ is on of the primary standards used in volumetric analysis, i.e. is easily available in a reproducible form and concentration, has a high molecular mass, is -- once the container is opened -- not (so much) hygroscopic; and is stable.