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.
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.
I speculate, a dilute solution of HCl (0.1 mol/L), if stored over weeks in a bottle recurrently opened, maybe stored on a window bench (seen so frequently in the undergrad's labs) subject to ventilation and underneath heating by the radiators does not have this resillience.
Using a cartrige to prepare a standard solution of HCl that you rinse into a volumetric flask and fill up with deionised water may be seen valid as alternative for a secondary standard by convention. (ISO standards are settled agreements; similar to ASTM standards, if you see things differently, you may contact them.)  Obviously, alteration of a single step within a whole chain of procedures may invalidate the protocol's fit into ISO; a less severe issue in a teaching lab than for a GMP-accredited lab.
Addendum
&quot;How is the diluted solution of HCl obtained?&quot; brought an other element into play that I did not consider earlier: the use of concentrated hydrochloric acid. If purchased and used freshly, it may be a batch of concentrated (37%) or even &quot;fuming HCl&quot; (about 40%), and both are hygroscopic.  Small errors about the concentration of the stem solution of this corrosive reagent may indeed yield larger systematic errors, down the road of repeated dilutions.  (Obviously not so important if the sole intend is to fill up again the bottle of 1 M or 2 M HCl for the extractive workups ...)