Need help with Nutrafin KH test

[Elbereth]

I'm confused with the Nutrafin KH test. The directions say to stop / read the value when the colour turns lime or yellow but because it takes 2 or 3 drops to go from green to completely yellow, I'm not sure at what point to stop. I used to use the API KH test and that turns yellow much faster so it wasn't an issue.

Do I read the value once the colour change is complete (i.e. it's completely yellow) or when the colour change begins (i.e. it's green/lime)?

Thanks in advance!

 

[Dkeller_nc]

For most of these sorts of test kits (titrimetric determination of carbonate hardness with a dye indicator), the correct carbonate hardness is calculated from the number of drops required to change the sample from its original color (in this case, blue) just to the point where the sample+dye mixture starts to color shift. In this case, based on the instructions, that would be "lime green". A comment about this sort of test, though - the quantity of sample specified, the number of drops of indicator and the concentration of the titrant (and therefore the number of drops to titrate an average sample) are all compromises based on trading off usability versus precision.

For example, if the manufacturer wanted to introduce more precision to the assay, they would make the titrant less concentrated - that would increase the number of drops required and reduce the amount of error associated with +/- one drop. Similarly, increasing the volume of sample and therefore increasing the number of drops required would increase the precision of the assay.

In a reef tank, stability of alkalinity is more important than the absolute value of the alkalinity, at least within reason. So for someone using a titration test, it's typically more important that the titrate to the same color change every time versus which particular color you select as the end point.

 

[4tanks]

Well said above. I just go with as soon as it changes from blue to green I use red sea the green does get stronger the more you add until it goes to pee coloured. But then there's a bit of a margin so easier and more stable I think to go with as soon as colour changes .

 

[Randy Holmes-Farley]

If you have a pH meter, you can ignore the color and just go by the pH, aiming for about pH 4.3.

 

[Dkeller_nc]

Yep. If those viewing this thread want to get a nerdily-accurate alkalinity value for their tank water, Randy has published a DIY alkalinity assay based on pH measurement and a standard acid titrant.

I've mentioned this before, but I'll add a bit to Randy's excellent article. What he's explaining in the article is the standard method for doing acid/base titration - in this case, the base is the aquarium water. Or more specifically, the carbonate/bicarbonate in the aquarium water and a few other anions that are present in low amounts and don't make a difference for a reefer's purpose. The acid is the titrant - you could use any acid (well, most of them anyway) as long as it has a known normality/molarity, but most often hydrochloric acid is used in a lab, and is readily available to reefers from on-line sources.

What I've mentioned is that there are some standard laboratory equipment items (in addition to the pH meter that Randy notes in the article) that makes performing the procedure that Randy's outlined in the article a bit more convenient. Specifically, you'd like to have a piece of glassware called a burette, a laboratory stand and holder for the burette, a magnetic stir plate, and a magnetic stir bar. The burette provides an accurate and convenient way of measuring the total amount of standard acid that you've added drop-by-drop to the sample to get the pH down to about 4.5, and the magnetic stir plate and bar keeps your sample thoroughly mixed as you add the acid.

The burette, ring stand, burette holder, magnetic stir plate and stir bar are much, much cheaper than they were just 15-20 years ago, and you can easily buy them off of Amazon. If you decide to go this route, note that burettes come in different liquid capacities. In Randy's article, he notes that if you use 0.1 N HCl, your aquarium water has an alkalinity of 7 dKH, and you use a tank water sample size of 250 mL, you'd need 6.25 mL of 0.1 N HCl to complete the titration. For this particular example, you'd like to have a 10 mL burette, since you'd like to use at least 1/2 of the total capacity or more of the burette during your titration for the best accuracy of reading off how much acid titrant you used. Generally, glass burettes come in standard capacities - e.g., 1 mL, 5 mL, 10 mL, 25 mL, etc... Presuming that you use Randy's recommended 0.1 N HCL titrant, and use a reasonable water sample from your aquarium (i.e., 250 mL), the 10 mL burette size will be the most useful. However, if you use a larger volume of sample, the 25 mL burette size may be more appropriate, while if you use a smaller size sample such as 50 mL, the 5 mL burette size may be more convenient.

 

[Elbereth]

Thank you all for such informative replies!

Hopefully the Nutrafin test is a little more accurate because the solution is less concentrated - it takes about twice as many drops to get a colour change as with the API. But I'm planning to get a Hanna KH checker in the future. I'm assuming the colorimeters would be more accurate?