help in titration method

[gendiaaa]

Hello guys

I am new to the reef central forum actually I am a chemist and
I would like to do my calcium and magnisum test using complexametry
Titration I will put my thought and I would like to get your opinion about it

For this I will measure the hardness
First the total hardness by
50ml sample + 2ml buffer "ammonium chloride / ammonium soln" + EBT and titrate
With 0.01M EDTA from pink to blue


Second Ca hardness

50ml sample +2ml sodium hydroxide 1N (PH about 12) + meroxide and titrate
With 0.01M EDTA COLOR from pink to violet

Is this could work guys?

Uhh forget to measure magnesium we will (total hardness-calcium hardness=magnesium hardness)

Your help is appreciated

Thank you

 

[BigJohnny]

Your the chemist, you tell us if that will work! I just use test kits

 

[gendiaaa]

I will prepare the chemicals tomorrow and I will let you knows but I wonder if
We can open good discussion here about it I know that this is one of the biggest
Aquarium forum and too expert chemist in marine will be here so I am asking them to join this thread

 

[disc1]

There are published standard methods for these things. They come with a lot more detail than you've given here.

How will you standardize the EDTA solution?

 

[gendiaaa]

For 0.02M you should dissolve 3.75 EDTA in RO/DI water .. If you have good methods could you please share it here so I could try it I have small lab at home.. Or just post links .... Also regarding EDITA IT REQUIRE AOME HEAT TO BE DISOLVED COMPLETELLY

 

[gendiaaa]

What detail you required I could share anything its just open discussion for finding good method for calsium and magnesium.

 

[disc1]

What form of EDTA do you have? Is it the free acid? That's hard to dissolve without some base. The tetrasodium salt might be better.

EDTA has a tendency to draw some moisture. So you can't rely on it's weight for measurement like that. You should be titrating it against a known primary standard to get its exact concentration and adjusting from there. Calcium Carbonate is a good primary standard that you can count on the weight of. I'd start there.

 

[disc1]

But again, the academic discussion here is pointless. There are well established standard methods that have been published for the compleximetric titration of magnesium and calcium from the same sample. I'm not really interested in trying to reinvent the wheel.

 

[disc1]

IIRC, with erichrome black you would also want to add a little pinch of a manganese salt to keep the dye bound to the end of the titration. Otherwise you'll get a premature endpoint.

If I were going to try to DIY my own alkalinity kit, I wouldn't use an indicator dye. I'd use a standard acid and a good pH probe.

 

[gendiaaa]

IIRC, with erichrome black you would also want to add a little pinch of a manganese salt to keep the dye bound to the end of the titration. Otherwise you'll get a premature endpoint.

If I were going to try to DIY my own alkalinity kit, I wouldn't use an indicator dye. I'd use a standard acid and a good pH probe.

Yeah you are right about EDTA and its not in acid form also this test is not alkalinity its hardness as ca and mg carbonate ..... You said I should put some manganis salt to get good end point this one I can not understand how this could happen ... Also if you can help with any link to the standard method you are talking about just past it here and its not really academic discussion its just discussion to get the best way any way how you test you calcium and magnesium

 

[disc1]

Those two comments weren't meant to be connected. The first was about your hardness test and the second was about the alk test.

The manganese, or I think zinc can be used too, is to make sure your dye stays bound until the end point. Look at the equilibrium constant for erichrome black with calcium or magnesium and you'll see that it is going to have a significant amount of unbound dye even while the concentration of calcium or magnesium is still significant. The Mn will bind very tightly to the indicator while you are titrating the Ca and Mg. The EDTA will strongly prefer the Ca and Mg. Once all the Ca and Mg have been complexed to EDTA the next drop will strip the Mn from the erichrome black and it will change color. Your end point will be one drop before the color change.

Without that, the EDTA will strip and Ca or Mg from the dye and it will change color long before the true end point.

 

[disc1]

actually I am a chemist

Then you shouldn't have any problem using Google or other internet sources to find published methods. I would post it for you if I had it handy, but I'd have to pull it from the net. There's no reason you can't do that for yourself.

 

[gendiaaa]

What I do understand is the magnesium complex with ebt is more stable than calcium complex with ebt but less stable with EDTA so when u add EDTA to the sample containing mg and ca it will react with free ca and mg then it will start to react with mg ebt complex and it will change the color .. Is that means that the titration will not give the exact amount of ca and mg ? So I should add manganese to the ebt to do bond the ebt strong to the mg ? But if I do so how I will get color change ?

 

[gendiaaa]

Also we do use with ammonia buffer an ammonium chloride to reduce some of the metal to lowering the their oxidation state which will make it not reffer to the titration using ebt as indicator

 

[gendiaaa]

 

[gendiaaa]

 

[gendiaaa]

 

[disc1]

Like I said, not trying to reinvent the wheel.

Look on that last curve where your endpoints are. If that's OK for you, then you're in good shape.

 

[karimwassef]

Why reinvent the wheel? Saving cost vs standard tests? More accuracy? Automation?

 

[gendiaaa]

I did it and it show success