Question on DIY Alk test

[dartier]

I noticed a user had posted a DIY version of an ALK test here (URL="http://www.reefcentral.com/forums/showthread.php?p=25233374#post25233374"] (http://www.reefcentral.com/forums/showpost.php?p=23718143&postcount=1)). What I can't figure out is the purpose of including the NaOH in Reagent A.

Anyone have any idea as to the reason for including the Sodium Hydroxide?

Dennis

[bertoni]

I think this has been answered over on the other thread. As has been stated, the dye release H⁺ ions, which might be the main reason. It's also organic, which could lead to decay without a preservative. I'm not sure about the chemistry, so I can't add much more.

[Dan_P]

The 1.6 g of sodium hydroxide is slightly more base than needed to neutralize the carboxylic acid group of methyl red and the sulfonic acid of the ionized form of bromocresol green and to ionize the phenol group of bromocresol green which turns the molecule blue.

[dartier]

Thank you to both of you for your answers.

Dennis

[orcafood]

http://www.sigmaaldrich.com/catalog/product/sial/114367?lang=en&region=US

I think BCG might be usually sold as a sodium salt or as the stabilized cyclic sulfonate ester? I agree that the phenol would be a weak acid, the phenolate is stabilized significantly by the benzoquinone resonance. Perhaps the dye(s) are more stable in basic conditions?

[dkeller_nc]

Dartier - If you have an interest in very high accuracy alkalinity tests, the standard way to do this in a lab is with a volumetric titration with a standardized acid solution and a pH meter. It used to be that the equipment to do this would be quite expensive, but that's no longer true (though admittedly, "expensive" is a relative term).

The essential equipment/reagents is a burette and a stand to hold it, a stir plate, stir bar and beaker to hold the sample, a small volumetric flask to accurately measure the volume of the sample, a pH meter with a calibrated probe, and a standardized acid solution.

Randy Holmes Farley wrote an article on this standard method, but I can't link to it b/c of the TOS. But you can find it easily with a web search.

[dartier]

Dartier - If you have an interest in very high accuracy alkalinity tests, the standard way to do this in a lab is with a volumetric titration with a standardized acid solution and a pH meter. It used to be that the equipment to do this would be quite expensive, but that's no longer true (though admittedly, "expensive" is a relative term).

The essential equipment/reagents is a burette and a stand to hold it, a stir plate, stir bar and beaker to hold the sample, a small volumetric flask to accurately measure the volume of the sample, a pH meter with a calibrated probe, and a standardized acid solution.

Randy Holmes Farley wrote an article on this standard method, but I can't link to it b/c of the TOS. But you can find it easily with a web search.

Thanks dkeller_nc. No need to post randy's article. I have read it before. I agree that would be the best way to get a high accuracy test. I have just been looking for alternate methods to get as good or better than manual testing through a DIY alkalinity tester. This particular DIY alk test recipe caught my eye as it appears to be as good as the Salifert one, and with the removal of human fallibility may be able to exceed the commonly obtained results. Because I would also like to incorporate Nitrate, Phosphate and turbidity testing into the device, sticking to a colorimeter for the measurement stage would eliminate the need for a PH probe and a colorimeter.

Dennis

[orcafood]

I don't know if I trust a pH meter more or a dye changing colors. If using a pH meter you should calibrate it every time you do the titration which is a bit annoying. Burrette is very necessary, stirring can be done by hand. Any glass will do as the titration vessel. Could even use the pipette as the volumetric device.

Not sure if I am way off track, but I usually preferred to use sulfuric acid as the titrant, it is much less volitile, I hate being around fuming HCl. Making a standard is not complicated.

The real trick is the dye mixture chosen, the rest is nothing special. If you mix multiple dyes with varying pH indicating ranges you can get a color change far closer to the true endpoint of your titration.

If you want to increase the resolution of the test, what you do is titrate up to almost the very end, then switch over to a lower concentration of standard. Thus you could get resolution into the hundreths if you wanted. Also use a large sample size to get more accurate results and better resolution. Salifert tests use a freaking syringe, ~10 of mL of water, and they can tell you down to the .1 dkH. If someone can get that accurate with a syringe imagine them with a burrette...

[dartier]

Because I am working towards an automated device, strictly DIY with no commercial aspirations, I am trying to get away from being dependent on volumetric measurements. The reason being is that they are hard to do accurately. My plans is to reply instead on an integral weight measurement capability so that the amounts are no longer crucial and can be calculated even when using imprecise hardware. Well as long as the load cell that I will be incorporating is reasonably precise. This would work for both PH probe based titrations and for colormetric ones.

I also plan to experiment with using the Hanna alkalinity reagent to do colormetric tests with no titration required. After all the reagent is relatively inexpensive, and it should be possible to plot calibration curves using it. By removing some of the uncertainty through automation, the sample size and reagent use may also be able to be lowered.

Somewhere on here there is a user in Italy that noticed that you can use the Hanna colorimeter to preform tests with 3rd party alkalinity reagents. My plan is similar, just the other way around. For that matter I could even try the 3rd part reagent with my own colorimeter if they are an easy to use liquid.

Dennis

[orcafood]

I'm excited to see the end result! I don't see a need to test n/p levels in a functioning reef tank. Calcium is linked to alkalinity thus I see measuring alkalinity as the optically testable parameter.

All reagents are dirt cheap if you know what your doing (why doesn't anyone sell there own testing kits). Any kit costs a few dollars to make tops.

I agree that colorimeters are the way to go. Titration would be necessary for any non-volatile molecule.