Now, for the Salifert Calcium test kit:
This is also a titration, but instead of being a weak base / strong acid titration, now we have what is called a "complexometric titration". In this titration, the titrant "complexes" (read: combines) with the metal we are measuring (calcium), which means that it makes up a combination of the metal and the titrant that is kinda like being a molecule, but the components are not as strongly bound as when molecules are made. The "endpoint" of this titration is when the molar amount of free Ca ions is equal to the molar amount of free titrant (EDTA in this case).
This titration also has a color change for endpoint detection. The dye used for this purpose also has the ability to "complex" with Ca ions, just like the EDTA titrant. The dye has two forms: A red/pink colored form when it is complexed with Ca (or another metal), and a blue colored form when it is "free", or not complexed. The degree to which the dye and the EDTA titrant want to make these complexes with calcium are different; EDTA wants VERY STRONGLY to complex with Ca, while the dye just kinda sorta wants to complex with it. At the start of the titration, the dye is all complexed with the metals in the seawater (since it's the only thing around to do the complexing), making it red/pink. Once the hydroxide solution is added, then the dye is more or less just complexing with the free Ca, but it is still all red/pink. As the titration progresses, the EDTA titrant, which combines more strongly with Ca than the dye does, complexes with the Ca, eventually freeing up the dye near the end of the titration, as it starts to compete with the already complexed dye. Eventually, the titration gets to the "equivalence point", and the EDTA knocks enough of the dye off of the Ca ions to cause the vast majority of the dye to be in the blue-colored "free" form. That's when the solution turns blue, and the endpoint is achieved.
Now, EDTA and the dye can also complex with Mg ions, and since seawater has plenty of Mg in it, the Ca test has to somehow make the Mg to "go away", to make it invisible to the test, leaving only the Ca to be measured. Since magnesium hydroxide is quite insoluble, the Mg can be made to "disappear" by adding a reagent that is a strong hydroxide, such as potassium hydroxide or sodium hydroxide. This makes the pH of the solution go way up, and the magnesium precipitates out and becomes unavailable to the reaction in the form of the magnesium hydroxide molecule.
So, what the Salifert calcium test kit is doing is this: Step 1 is to take the sample. Step 2 is just adding the dye. Step 3 is adding the hydroxide solution. After swirling for 10 seconds, the Mg has been precipitated as the hydroxide (that's why there is a white cloudiness -- that's the precipitated Mg(OH)2), leaving only the Ca available to be titrated. Steps 4/5 is just the titration. Once again C1*V1 = C2*V2, and the concentration of the EDTA titrant is adjusted to make the chart accurate when the instructed 2 mL of sample is used.
