Ca EDTA Titration

[brad]

I have some problems (best kept as a separate thread) calling into question the accuracy of my test kits. I want to think about doing DIY test kits or a full chemistry lab (would help with other crazy but non-reef projects). Even if I don't go this route, this understanding will help me better determine the accuracy of my tests using kits. So I wanted to start with a Calcium titration based on my Salifert kit. As I understand, the basic process is:

1) Start with a measured amount of tank water.
2) Add Hydroxynaphthol blue.
3) Add NaOH
4) Measure the amount of EDTA needed to change color from pink to blue.
5) Calculate Ca in ppm as 40 * moles of EDTA used per ml of tank water.

Questions:
For step 2) I don't think it matters how much Hydroxynaphthol blue I use, how old it is, or how long it has been stored, as it shows the correct color. Is this correct?

For step 3), I don't understand how much NaOH I need to add, but would think there is no harm adding too much so long as I am not adding ridiculous amounts. If I add way too little, I don't think it will turn pink (I haven't tried this). I believe I could add enough for it to turn pink without adding enough to precipitate magnesium, this would cause my test to read high, as I'd be binding that with EDTA along with calcium. Could this be a source of error in the test? Or is a pH high enough to turn pink high enough to guarantee I've precipitated out magnesium?

Can step 2) and 3) be done way ahead of time by premixing NaOh and Hydroxynaphthol blue? If so, I don't understand why Salifert wouldn't do this in their kits. If not, then if these chemicals are not stable together, can the time and amount of mixing before step 4) affect the results?

Does EDTA have a practical shelf life? If so, how does this affect the results? I would assume this degrades linearly (or reverse exponentially) if it does. The idea that a test kit will measure perfectly 47 months after purchase, but be useless 48 months after purchase, is unrealistic.


I got some of this information from a popular forum that reefcentral blocked posted a DIY Calcium test kit, similar to the Salifert kit:

http://www.*********.com/forums/reef-chemistry-forum/106147-do-yourself-calcium-test-kit.html

[blanden.adam]

question for step 2) Not within reason, the dye turns pink in the presence of calcium ion, the EDTA binds the calcium ions and thus turns the solution blue.

question for step 3) The point of adding the hydroxide is to make the pH range between 12-13 ish so the dye will function correctly. You shouln't add a crazy excess, and you shouldn't add too little, but it's not all that sensitive. Mg has little to do with this particular situation as EDTA binds calcium with an affinity constant 2 orders of magnitude higher than magnesium (on the order of 10^10 and 10^8 M-1 respectively), so it essentially won't bind magnesium until it has bound all the calcium it can.

answer to Step 2 and step 3) Looking at the structure of the dye, I suspect it would photobleach or otherwise decompose over time if stored in solution. Chemicals in general, particularly fragile absorbant molecules like dyes, tend to be more stable long-term if kept as a solid powder rather than a solution, particularly when kept at room temperature in a lighted room. Also, storing that molecule in solution in the presence of hydroxide is just asking for oxidation problems :)

Answer for step 4) Only if you wait an unreasonable amount of time. For best results it's probably best to just mix it right before you use it.

I have never heard of EDTA going bad. I've used 3 year old solutions in the lab before without issues. The only real thing I'd worry about is if something started growing in it.

[brad]

Thanks! BTW - my formula (step 5) is wrong, as this seems to require an unreasonable quantity of ETDA. I am also confused on whether Salifert used disodium EDTA, tetrasodium ETDA, or just ETDA, and whether there is an advantage of using one over another. Several write ups online suggest combining disodium EDTA and magnesium chloride - I am not sure what the reasoning is for this or how relevant it is to our tanks.

Can someone tell me what form of EDTA Salifert uses, which form(s) are best for reef alkalinity tests, and correct my formula for the correct form to use?

[blanden.adam]

for step 5 you have the right idea, the math is just a little different.
Ca in ppm is approximately 40 * [EDTA] (M) (in your sample) * 1000

There will be no difference in the form of EDTA -- they interconvert depending on the pH. Personally I'd go with disodium or tetrasodium just to avoid acidifying the solution. I'm wondering if you've done a cost analysis to determine if this is going to be a fruitful endeavor in the end, or if you are just doing it for fun :).

[brad]

for step 5 you have the right idea, the math is just a little different.
Ca in ppm is approximately 40 * [EDTA] (M) (in your sample) * 1000

There will be no difference in the form of EDTA -- they interconvert depending on the pH. Personally I'd go with disodium or tetrasodium just to avoid acidifying the solution.


To clarify, do you mean Ca in ppm is approximately 40 * moles of EDTA * 1000 for a 1 ml sample of tank water? Am I also correct this means there is no point including Magnesium Chloride?

I'm wondering if you've done a cost analysis to determine if this is going to be a fruitful endeavor in the end, or if you are just doing it for fun :).

Given what I spent on this hobby over the past 12 years and the cost over the rest of my life, the cost of calcium testing is de minimis either way - so mostly doing it for fun.

When you consider that:
1) A higher degree of confidence in my tests = fewer retests and fewer times buying a 2nd test kit just to check the first.
2) Once I have a proper set up I am familiar with, this can save time.
3) Glassware and reagents can be shared between tests.
4) I might find other hobbyists interested in sharing supplies and costs.
5) I have (and will develop) other hobbies (beer and wine come to mind) that can benefit from the same knowledge, glassware and reagents.
6) I plan to have children and teach them everything I know about everything before they get to be my age.

- then having a lab might actually save money.

[dkeller_nc]

As someone that has a (private) chemistry lab, I can absolutely assure you that it will not save you any money - quite the opposite in fact. ;-)

Regarding the degredation of kits over time - the expiration date of a chemistry or clinical test kit is typically the last time point that the manufacturer tested where the kit performed within certain pre-determined limits. That's not the same as saying that the manufacturer tested it to failure (though they might have), and not the same as saying that the kit is no longer accurate after the expiration date. It just means that there's no assurance that the kit will perform after the expiration date.

Also, EDTA will not degrade under the normal storage conditions in a household for decades, if not longer. The main concern is that you keep it dry. It's hygroscopic, and will turn into a puddle at the bottom of the container if exposed to high humidities.

[blanden.adam]

To clarify, do you mean Ca in ppm is approximately 40 * moles of EDTA * 1000 for a 1 ml sample of tank water? Am I also correct this means there is no point including Magnesium Chloride?


Almost, [EDTA] (M) means concentration of EDTA (indicated by brackets), in Molar (unit in parentheses). Molar is units of moles/liter, calcium has a molar mass of 40 grams/mol, EDTA chelates calcium 1:1, meaning that [EDTA] = [Ca2+] at the end of the titration. Thus, 40 g/mol * [EDTA] mol/L * (1000 mg / 1 g) gives you some number of mg/L, which is approximately equal to ppm. You are titrating in a known concentration of EDTA, so you know how much volume you are adding per concentration you are adding. At the end of the titration, you calculate the total concentration of EDTA in your new volume. You then take THAT number, and back calculate what the concentration was at the start of your titration (lest you fall victim to the error plaguing students in analytical chemistry lab -- failing to account for titrant volume)

This approach has the advantage of the absolute volume of your test not mattering because we are working in units of concentration.

Alternatively, we can do the calculation in terms of mass and then back-calculate a concentration:

[Ca2+] (ppm) = 40 g/mol * EDTA added (mols) * (1000 mg/ 1 g)/ Original Volume (L)

Where EDTA added = [EDTA] (M) * Volume Titrant (L)

Either way works.

[disc1]

Whether you use the free acid, the disodium or tetrasodium EDTA isn't going to make any difference in the chelation reaction. The only difference it makes to the actual test is what molecular weight you use to determine how many moles of EDTA you've used.

Where it will make a big difference is making the solution. The tetrasodium will be the easiest to dissolve. The disodium EDTA should dissolve OK, especially if the solution is a little bit basic. The free acid doesn't dissolve well in water and usually requires a couple of molar equivalents of base to go into solution. You can sometimes dissolve the free acid with a lot of heat.

If it were me I'd go with the tetrasodium salt.