High Resolution API KH Test Kit

[hilgert]

This weekend I was looking at my old Salifert KH kits lying around, and then at my AP KH kits. I really like the ease of use and dramatic color change in the API kits, but the one "nuclear drop" at a time always worried me in terms of repeatability (what if one drop is not the same as the next?). One drop more or less is a pretty large swing in a KH measurement.

So, I popped the cap and dropper off of an API KH solution bottle, stuck in an old 1ml Salifert syringe (with the small tip attached), pulled in 1ml of API KH solution, and did a test using 5ml of tank water in the Salifert test cup (I dislike the API vials...too small to swirl, so they require shaking...I just swirl the Salifert vial while adding solution). I found that each droplet from the syringe equaled about 1/5 of an API "nuclear bomb" drop, which would make testing with the Salifert syringe/tip about 5 times more precise than with drops straight from the API bottle.

I happen to have 3 API kits of varying batches, and 3 Salifert kits of varying batches (one of which is pretty old, but still accurate). Tests from all 6 are close enough (after adjusting 10% up for API...see below) that I would not know which ones would be outliers. I found that the color change on the API (with the Salifert syringe) was more dramatic (in my opinion) than the Salifert color change, and occurred within 1-2 of the smaller syringe drops.

The 3 Salifert test kits were all clustered in a tight group about 10% higher than the 3 API test kits (which were also clustered in a tight group). Assuming the Salifert KH were "the" standard to go by (as Salifert is adjusted for saltwater and indicates about 10% higher than other kits from what I understand) I found that if I multiplied the amount of solution USED (subtract the reading on syringe from 1ml) is multiplied by 27 it gives a reading very close to what Salifert is indicating.

So, basically follow the following using Salifert equipment (vial/syringe/small tip) and the API reagent:

• Use 5ml of water in the Salifert vial (I like this one better than the API vial...can swirl while adding solution versus add/cover/shake)
• Skip the addition of the Salifert dye
• Add 1ml of API solution into the 1ml syringe (with the small tip attached)
• Add one tiny drop form the syringe at a time while swirling until the color changes from blue to yellow (and stays there)
• Subtract the remaining solution from 1ml...this is the amount of solution used.
• Take the amount of solution used and multiply by 27 instead of using the Salifert chart
• The result is KH in dKH

So, if the syringe indicated 0.7ml remaining after the color change, then 0.3ml would have been used. Multiply 0.3 by 27 and you get 8.1dKH.

I have NOT yet run any more extensive tests...this is just me testing my water which happens (at this point) to be around 8dKH.

One thing I can say is that my API KH tests have been very repeatable over the years, and they are inexpensive enough that I could toss a kit without crying at any point in time if I felt the least bit concerned (not so with Salifert or any of the more-expensive kits). Heck, as easy as this is I could always test with two API kits from different batches in less time (and cost) than one Salifert test. Then just check every month or so against a Salifert test.

-Hilgert

 

[RRaider]

I have two api kits that both read different and an Elos kit which reads about 1.5 dkh lower than one of the API and 2.5 lower than the other. :spin2:

 

[hilgert]

Actually I think the conversion factor is looking close to 26 instead of 27 based on some more quick testing...but certainly it's close (at least from my perspective). Interested in testing from anyone else...

-Hilgert

 

[Randy Holmes-Farley]

If you are interested in DIY, I'd just buy a standard acid and do the titration yourself. It is not expensive, it can be as precise as you are able to measure liquids, and is guaranteed accurate.

I describe how to do that in this thread (read from post 94):

http://www.reefcentral.com/forums/showthread.php?t=1743173&page=4

 

[hilgert]

Randy, thanks for the link...I had seen that some time ago. It's just that I had all those old Salifert syringes sitting around, and got to thinkin'. I'm less interested in DIY (well, I might try it) than I am in getting more precision from the API kits. Personally I like the color change in the API, the ease-of-use (only one thing to add) and the cost.

Your thoughts on API (with the syringe method) versus Salifert in terms of accuracy (for our purposes in reefkeeping)?

-Hilgert

 

[Randy Holmes-Farley]

I think that using syringes to allow smaller or more measured drop size is a fine plan.

 

[bertoni]

Do you know of a source of acid for hobbyists? Cole-Parmer doesn't seem to sell their products to individuals.

 

[Randy Holmes-Farley]

No? I've bought from them as an individual, but maybe that policy applies only to chemicals (which I've not bought from them) or is new. I'll see what I can find.

Maybe BRS can be convinced to carry it if we put together a simple DIY alk test using it.

 

[hilgert]

...

 

[hilgert]

Problem with DIY would be a precise and accurate pH measurement for those without an electronic pH meter, if I understand the DIY method.

Is there a DIY way to make an API-like solution?

 

[Randy Holmes-Farley]

Problem with DIY would be a precise and accurate pH measurement for those without an electronic pH meter, if I understand the DIY method.

You can buy an indicator dye, or use one from a test kit, but I think a meter would be preferred.

Is there a DIY way to make an API-like solution?

Not sure what that means? Make a standard acid yourself? Or are you referring to the dye?

 

[hilgert]

Is there a DIY way to make an API-like solution?

Not sure what that means? Make a standard acid yourself? Or are you referring to the dye?

The API kit is a single-solution...first drop turns sample blue, then keep dropping until it turns yellow. I am not a chemist...I am assuming there is more to the API solution than meets the eye...something not quite as simple as an acid that we can get ourselves?

I am unlikely to do DIY since the API kits are so inexpensive compared to others (and are reliable/repeatable), but just curious if it was possible. If so then a DIY (or BRS) kit might be doable.

-Hilgert

 

[Yogre]

What would the indicator dye be, Randy? I haven't done this sort of lab work in a long time, my memory fails me.

I'm also really interested in this semi- DIY Alk testing. I like the API's sharp end point but don't like the lack of precision.

Unfortunately, I really don't trust my pH meter (Hanna pH pen) much further than I can throw it, I certainly wouldn't make any dosing decisions based on its input.

 

[hilgert]

Lack of precision is from the dropping mechanism. If you use a small tip like the Salifert syringe tip you should get much more precision.

 

[Randy Holmes-Farley]

The API kit is a single-solution...first drop turns sample blue, then keep dropping until it turns yellow. I am not a chemist...I am assuming there is more to the API solution than meets the eye..

They just combined the indicator dye and the acid into a single solution. One less step, but not a better method.

I discuss what the dyes are (or can be) here:

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.

 

[Randy Holmes-Farley]

FWIW, I just proposed it to BRS:

http://www.reefcentral.com/forums/showthread.php?p=16527803#post16527803

 

[Yogre]

Lack of precision is from the dropping mechanism. If you use a small tip like the Salifert syringe tip you should get much more precision.

Right, and I do have a spare Salifert syringe. The "iffy" factor of course is how does say, 0.5ml of API solution translate into alkalinity. You found that approx. 26 drops times ml used gives a comparable result, correct? Maybe I'll try a little testing this afternoon myself, see if i come up with the same result.

 

[hilgert]

I have been testing away with several different batches of API and Salifet kits. Thus far they are all consistent. I have started only drawing 0.5ml into the syringe...unless KH is over 13dKH 0.5ml is enough, and I toss the rest out of convenience.

I have to say that at $5 each the API kit is every bit as accurate (for my purposes) using this method as the Salifert kit. And I like the color change in the API better.

Next round will be testing Ca using the same method.

 

[t4zalews]

I was interested by this theory and tested it myself. When using the salifert syringe I used about .35ml which would equal 9.1 using a 26x. Then I tested using the regular ATI dropper and had a KH of 9. I'd say this theory is pretty dead on.

As to whether or not its more accurate...very well could be. This is a nice method for people who are accuracy nuts. As long as I'm putting in at least 8 drops for color change I'm happy.

Either way...nice connection between the two. I appreciate the calculations.

 

[hilgert]

As to whether or not its more accurate...very well could be. This is a nice method for people who are accuracy nuts. As long as I'm putting in at least 8 drops for color change I'm happy.

It's the precision dropping which can lead to a more accurate test. Using the nuclear bomb drops from the API bottle always worried me...I could be 1 dKH off (or even 2) if the drops just "happened" to be the wrong size (perhaps a manufacturing defect in that particular bottle, etc.). With the syringe I get smaller, more precise sizes. That leads to a more accurate result.

I have also found the API color change, even with the small drops from the syringe (which I find are about 1/5 to 1/6 the size of the normal API nuclear bomb drops), is very dramatic, down to 1 or 2 tiny drops (or less than half of a normal API bottle drop).

So, for less than 50% of the cost I get the accuracy I want (through the precision of using the Salifert syringe) in a single-step process (reagent and dye in one solution).