Hydroxynaphthol blue

[BradB]

A DIY calcium test has been on my TODO list for a long time. I have 2 Salifert tests, one measures 350ppm and the other 450ppm!

I have Disodium-EDTA and NaoH, but never found a place to buy Hydroxynaphthol blue at a reasonable cost. I may run my own titration later with indicator from my test kits, but I don't save any money if I have to keep buying test kits for indicator.

Is there a different indicator I can use? Is there a good place to buy this? I know you are going to tell me to search, and this has been asked several times on this forum, but I don't think it has been answered yet.

 

[JimWelsh]

https://www.hach.com/calver-sup-sup-2-calcium-indicator-113-g/product?id=7640191003

 

[BradB]

Thanks! I had a lot of questions as well.

EDTA reacts with both Ca and Mg, and the NaOH precipitates Mg(OH)2 - right? But why doesn't Ca(OH)2 precipitate?

What about Potasium, Bromide, Boron, Strontium, Iron, etc? Do we just assume everything in our tank either won't bind EDTA, will precipitate with NaOH or is in too small a quantity to matter?

 

[Randy Holmes-Farley]

Thanks! I had a lot of questions as well.

EDTA reacts with both Ca and Mg, and the NaOH precipitates Mg(OH)2 - right? But why doesn't Ca(OH)2 precipitate?

What about Potasium, Bromide, Boron, Strontium, Iron, etc? Do we just assume everything in our tank either won't bind EDTA, will precipitate with NaOH or is in too small a quantity to matter?

Here are directions froma college lab course that shows details and some reasons:

http://www1.udel.edu/chem/beebe/Chem120/Chem120%20LAB2EDTATitration.pdf

"Calcium is determined at pH 12 where magnesium is quantitatively precipitated as the hydroxide and will not react with EDTA. "

At pH 12, magnesium is far less soluble than is calcium.

Of the other ions you mention:

potassium and all other ions that are not divalent cations (bromide, fluoride, boron,etc.) will not bind to hydroxynaphthol to cause a color change

strontium binds weakly and has a much smaller effect on the color (and is present at far lower concentrations than Ca)

iron is present at far too low a concentration to ever matter, and won't be soluble when the pH is raised

 

[JimWelsh]

But why doesn't Ca(OH)2 precipitate?

Excellent question. Answer is: It does! This is why if you stop at the first appearance of blue when doing a hobby Ca test, you will notice that it reverts to purple after just a minute or two.

Here's what is happening: When you add the hydroxide, some calcium does also precipitate out as calcium hydroxide. However, once the EDTA has complexed with the remaining calcium in solution, the equilibrium changes, and much of the precipitated calcium hydroxide re-dissolves, increasing the amount of calcium in solution a bit.

Regardless what the test kit instructions say, you should titrate until the pure blue color persists for at least 5 minutes, if you want an accurate result.

Now, if you have a good idea of what your calcium level is already, you can increase the accuracy and precision of the titration, and make the endpoint sharper by performing the test in the following manner: First, add about 95% of the amount of EDTA titrant necessary to complete the reaction, given the assumed approximate calcium level. Only then do you add the hydroxide and the dye. Then, complete the titration to the sharper blue endpoint.

What this does is it allows the EDTA to complex with the calcium first, leaving less free calcium to be precipitated as the hydroxide.

Reference: Pate, J.B. and Robinson, R.J., 1958. The (ethylenedinitrilo) tetraacetate titration of calcium and magnesium in ocean waters. I. Determination of calcium. J. mar. Res, 17, pp.390-402

EDIT: The alternate method I described is essentially the same as Paragraph #9 in the PDF that Randy linked.