Make my own alk reagent for salifert kit

[SDguy]

I've got more than half the bottle still of the Salifert KH indicator (only use one drop per test). But I'm running low on the titration reagent. Can I make some in my lab at work? It's just, what, a weak acid?

[Young Frankenstein]

Do you know how to make it ?

[Billybeau1]

<a href=showthread.php?s=&postid=12022843#post12022843 target=_blank>Originally posted</a> by 0 Agios
Do you know how to make it ?

barley, malt, hops..........

Oh, you mean indicator solution ? :lol:

[SDguy]

<a href=showthread.php?s=&postid=12022843#post12022843 target=_blank>Originally posted</a> by 0 Agios
Do you know how to make it ?

Well, I wasn't asking if I am allowed to make it..... ;)

[KevChem]

I've seen and used sulfuric acid titrant. Hydrochloric acid also would work. Both are strong acids.

The key is standardizing it with a sodium carbonate solution - you can't just dilute it down since there is a lot of uncertainty in the concentration of the concentrated acids.

Kevin

[SDguy]

I do have a standard to test with.

So, if I have water at 4 meq/l, or 200(?) ppm, I should be able to figure out approximately what concentration of HCl I should use to titrate 2mL using about 0.3mL (what the current kit uses), correct?

[SDguy]

OK, here's what I did. Chemist gurus, be gentle :o

200ppm alk:

200mols CO3/1,000,000 mols water =

2molsCO3/10,000 mols water or 2molsCO3/180,000 g (mL)

For my 2mL water samople, that gives 0.000022mols CO3

That would require 2 fold molar ratio of H, or 0.000044 mols H

in 0.4mL of volume (the amount I use from the original kit) =

0.000044mols/0.0004L = x/1 or 0.11M HCl.

which we conveniently already have here at work used for titration :D

What do you think?

[Randy Holmes-Farley]

If you can make or buy a standard acid, you might as well not use the kit at all and just use a pH meter. I show how to do that here using 0.1 N HCl:

What is Alkalinity
http://www.advancedaquarist.com/2002/2/chemistry

[SDguy]

Because it's easier when I still have sleep in my eyes at 5am and I want to quickly test my tank before work, for me to use the kit :D

OK, here's what I got:

First, I'm using a standard I made with Na2CO3 anhydrous. 0.312g into 1L. According to the reef chem calculator, that comes to about 16.6dKH. I use it diluted 1:1 with DI H2O, so it should be 8.3dKH.

My current Salifert kit, using all its reagents, and a 2mL sample, gives:
(1.0 - 0.73) = 0.27 x 16 x 2 = 8.64dKH close enough for me.

OK, so 0.1N HCl was too strong :o

Using 0.02N HCl I get:
(1.0 - 0.7) = 0.3 x 16 x 2 = 9.6dKH

to correct for the error, I do:

0.3 x 13.83 x 2 = 8.3 dKH

Any problem with me using this reagent and the new multiplication factor to test my tank water?

[Randy Holmes-Farley]

I don't really follow what you did with those corrections, but assuming you made your standards correctly and get the right answer for them (8.3 dKH), it should be correct. :)

[SDguy]

Just one correction...instead of multiplying by 16 like in the salifert instructions to get dKH, I multiplied by 13.83.

OK, just to confirm, I tested my standard full strength and got

(1.0-0.41) = 0.59 x 13.83 x 2 = 16.32dKH (close enough to the 16.6dKH it should be).

[Randy Holmes-Farley]

Sounds good. The 8.3 and 16.6 dKH are correct for the standard you made (I get 16.5 dKH, but that's close enough). :)

[SDguy]

Now let's see how it does with tank water... :D

[Randy Holmes-Farley]

:thumbsup:

Good luck. :)

[SDguy]

So it works great! Incidentally Randy, any idea what the indicator dye is made of? Can I make my own, and just forgo buying alk kits altogether? :D

[Randy Holmes-Farley]

A suitable indicator dye for an alkalinity kit is one that has a color transition around pH 4.2.

What is Alkalinity
http://www.advancedaquarist.com/2002/2/chemistry

from it:

Alkalinity using Test Kits

Of course, most reefkeepers measure alkalinity with a test kit, not with a pH titration. How does that work?

Well, in effect test kits do a pH endpoint titration. They all include pH indicating dyes (providing a color change) and an acid (frequently dilute sulfuric acid) to lower the pH. You typically add acid until the dyes turn color. Since these dyes are selected to have a color change in the pH = 4 to 5 range, what you get is a measurement of how much acid it takes to lower the pH to that range. This color change is used to approximate the endpoint of the titration.

Interestingly, many test kits use more than one pH indicating dye. Using more than one dye at the same time permits the endpoint to be sharper. For example, bromcresol green has a broad color transition between pH 3.8 (yellow) and 5.4 (blue-green) and methyl red has a broad transition between pH 4.4 (red) and 6.2 (yellow). A mixture of the two (used in the Hach alkalinity kit) has a sharp transition (orange to blue-green) around pH 5.1 in fresh water (which may be slightly different in salt water).

Five point 1 you say? Based on the discussion above, is that low enough? Well, the Hach kit was designed for use in fresh water where the pKa of the bicarbonate is much higher than in seawater, and in that situation, it is appropriate. In seawater, however, it is marginal. My tank water took 3.4 meq/L to get down to pH = 5.03, and then an additional 0.4 meq/L to get down to pH 4.00. Consequently, this kit (and others with a similar dye mix) may be missing out on 10% of the alkalinity simply because it isn't titrating low enough. This difference obviously isn't significant to most reef keepers, but is something to keep in mind when doing such things as comparing test kits to standards (in seawater) or to each other.

Some test kits also provide a different dye for a different measure of alkalinity. Frequently, this other dye is phenolphthalein. This dye has a color change between pH 8.2 and pH 9.8. In fresh water, carbonate is almost completely converted into bicarbonate at pH 8.3, and that is the purpose of phenolphthalein titrations: to determine alkalinity in freshwater due to carbonate only (discussed in detail below). This test serves no purpose in a reef tank or seawater for two reasons: 1) the water is probably already more acidic than the endpoint of this dye, and 2) the carbonate in seawater is not completely converted into bicarbonate at this pH anyway. That is, even if the pH were higher than 8.3 (say, 8.6), titrating down to the phenolphthalein endpoint will not effectively "count" all of the carbonate because in saltwater there will still be substantial carbonate present at the phenolphthalein endpoint.

[Conesus_Kid]

Put me first in line to buy a reliable Alk test, Peter!

[Young Frankenstein]

All that stuff to replase a kit ???? why you dont just go out and buy the salifert kit ?

[SDguy]

DP

[SDguy]

Thanks Randy...I knew I read that somewhere about a mixture of dyes...couldn't find it though.

[Randy Holmes-Farley]

:thumbsup:

Happy Reefing. :)

[SDguy]

So I found this handy little table....any of these readily available and non toxic?

http://chemistry.about.com/library/weekly/aa112201a.htm

[Randy Holmes-Farley]

Not sure what you mean by readily available. All can probably be obtained from a place like Sigma Aldrich. :)

[SDguy]

Sorry, I meant for anyone off the street, in case others are following this thread and want to do some DIY action.

[Randy Holmes-Farley]

I don't know of any suitable ones that are generally available. :)

[toaster77]

nice thread, i'm running low on ALK titration reagent as well.

time to mix up some HCl !!!

now, onto calcium kits? javascript:smilie(':)')