Standards for test kits

[michaelg]

Randy,

Our club is working on a presentation on test kits. We have tried to make a few standards of known concentration without much luck (2 of us work in labs so have acess to stuff). For calcium we used CaCl2, which immediately realized as the test kits measure calcium hardness. We used the seachem reference soln, and the only kit that was even close was the seachem kit- thus we were looking for an independant source. For pH, I just used some tris based buffers around the lab, and they worked fine for that purpose. For alk, we used seachem buffer (powdered) and dilluted it out, but this was difficult due to the measurements (container states use 1 tsp to raise X gallons about 0.5) not real accurate for what we wanted to do. Any suggestions on what we can use as more accurate standards- especially for calcium as this was the one with the greatest discrepancy between brands.

Thanks,

Michael

[Randy Holmes-Farley]

What's wrong with calcium chloride? It sounds like a good choice to me.

[michaelg]

From looking at the test kits, I was confused as they say they measure calcium hardness (and hardness is usually a term associated with carbonates for our purposes) or doesn't it matter once the solution is made for a while and exposed to air. I think one of the problems may have been that a dihydrate salt was used, not anhydrous calcium chloride (which the calculations were based upon). The measurements were real low. I will recalculate and try it again some time over the next couple of days.

[Randy Holmes-Farley]

The old time usage of the word hardness is very confusing.

Hardness is a measure of the divalent cations in the water (calcium, magnesium, sometimes strontium). It does not have anything to do, chemically, with carbonate or alkalinity.

Unfortunately, in the old days hardness was measured in units of calcium carbonate equivalents (and still is in some kits like the Hach kit), and that got many people thinking that the carbonate was somehow involved. It is not.

So, IMO, calcium chloride is a reasonable standard. Whether it is a hydrate or not will make a big difference in the percentage calcium in the product. It comes anhydrous, and as the dihydrate, the hexahydrate, and as a mixed hydrate of uncertain amounts of water.

[simonh]

To fully evaluate a test kit in the saltwater environment wouldn't it be necessary to include some of the known interferences to see how well the test kit deals with them?

For example, wouldn't magnesium be an interference for a calcium test kit. So any standard should include this to see how well the kit deals with this and gives accurate results when used on aquariums? or am I getting too complex?

[Randy Holmes-Farley]

Simon:

I agree that if your purpose is to do a comparison test of kits, or to do suitability tests on kits, you'd want to be testing them in saltwater.

However, if the purpose is just to ensure that a kit hasn't gone bad, or that an aquarist is using it right or some such thing, a simple standard that gives a known response might be all that's needed.

[michaelg]

This is the difficulty of the tests themselves as they use different dillutions of salt water- and some like lamotte can be done with fresh water. We did comparisons on fresh made instant ocean as well but the range of values that we got was incredible- far beyond what one would expect (we had multiple kits for several brands as well). Results within a given brand were consistent, but between brands was vastly different (+/- 100ppm or so) thus we are trying to determine which if any of them can be used with any sense of confidence (or maybe it is like the swing arm hydrometers where they are not accurate, but do tend to be consistent).

[simonh]

Aren't you able to get a sample of the IO water you used for all the tests analysed in the laboratory using a standard method suchas Craig Bingman used in his salt mix analysis. Then compare all the test kits to the 'true' result ?

[Experimenter]

Just to add a note to this thread, I participated in Dr. Ron's water parameters study. I tested my water at the time I took the samples and found some interesting results. For calcium, I will have to double check this, but with a Salifert test kit (which I thought I found relatively easy to use) I got measures that were roughly 100 points higher than the results I got from Ron. My beautiful 480 or so Ca reading was actually 380!

Ron has these data, so maybe others kits were more acurate. This has made me much more skeptical of these kits. They may be reliable in the sense of test-retest (same kit testing same water minutes apart), but maybe they are not all that valid. I think it is time we had a more comprehensive study of these kits in use by actual aquarists. I suspect most people would probabaly throw away all but absolutely essential (whatever that means) kits. Hell, I even get different salinity/specific gravity measures from the plastic hydrometer, an electronic probe, and a refractometer. My guess is since I have to set the probe and the refractometer, I may be making mistakes.

John

[tatuvaaj]

For a small report of Ca tests, click here:
http://www.lars-sebralla.de/ma_ca_test06.html

I think that if you are getting results that are +/- 100 mg/L either your test kit is defective or there is some serious operator error (doubtful, since these tests are extremely simple to make).

[New Reefer]

Interesting subject...

While we're on it, could you clever people come up with some recipes that us Plebs can use to actually make some standard solutions - I'm thinking of the more common tests, such as calcium, alkalinity, nitrate, phosphate, nitrite, ammonia/um, and perhaps pH of 8.2

If these standards were easy to make, and reasonably accurate, it would give one a much greater level of confidence in one's test-results.

Thanks in anticipation...

Hennie

[Randy Holmes-Farley]

Hennie:

Do you mean things that you mix from scratch? Or standards that you can buy? For the former, you may need a good balance and access to chemcials. For the latter, all you need is money.

[New Reefer]

Hi Randy,

I actually mean the "do it at home" type.

As an avid shooter and reloader, I have a balance capable of weighing to a resolution of 0.1 grain (about .01 gram). These scales are not all that expensive, and if accurate enough I'm sure many serious hobbyists would be prepared to buy one.

I'm also willing to buy a pipet or two, or even a buret, if really needed. On the other hand, if a small syringe measuring with a resolution of 0.5ml (for a 5ml syringe), or 0.01ml (for a 1.0ml syringe) is accurate enough, I already have them :) (the 1.0ml one from a Red Sea test kit, the other one just a "common" one from the family doctor...

Getting the chemicals would be more of a hassle, but IMHO not impossible. If clubs / friends could share, it would be even better...

Would you be prepared to do the "designs"?

Thanks,
Hennie

[Randy Holmes-Farley]

Here's one way that I've found people can somewhat bend the rules that chemical companies have and get some of the chemicals that they need. If a club were to buy these, there'd be plenty to go around after dilution.

Most chemical companies won't sell to individuals. But what you may be able to get are "standards" that are already made as these seem to fall under the category of "supplies", and are not hazardous or likely ways to make drugs, etc...

So here's an example:

Cole Parmer sells standards for ion selective electrodes. They sell

0.1 M CaCl2 (4,008 ppm Ca++). Dilute by ten to get a perfect standard. It costs about $65 for 475 mL.

0.1 M NaNO2 (nitrite) and NaNO3 (nitrate). $75 and $57, respectively, for 475 mL. These are 1400 ppm nitrogen, so dilute as necessary.

0.1 M NH4Cl (ammonia). $49 for 475 mL. This is also 1400 ppm nitrogen, so dilute as necessary. Depending on the kit, you might need to adjust the pH on this.

0.1 M NaHCO3 (bicarbonate). $65 for 475 mL. This can be used as an alkalinity standard. It is 100 meq/L before dilution.

Note that these are standards, not ways to test the performance of a kit, as Simon suggested. For example, the calcium standard will have no magnesium. So I'd take these standards as a first pass: the kit should get these right, and if it doesn't, something is wrong. That doesn't necessarily imply, however, that it will be right on a true tank water sample.

You can also get similar standards for I- and possibly others.

[New Reefer]

Thanks, Randy - that's a start...

Unfortunately for me, I don't live in the US, so that supplier won't work :(

The good part, though, is that South African chemical supply companies are not so full of @#$% - I can easily order whatever chemical I want (within reason, of course...), and I'm sure that many US aquarists would be able to come up with *some* valid reason for ordering the chemicals as well. Alternatively, would it be possible to use "household" products, such as normal bicarbonate of soda, caustic soda, washing soda, etc?

Remember, we're not looking at analitical accuracy (at least, I'm not...). I realize that our test kits are not very accurate, and with the restricted "resolution" and errors inherent in these tests, a "plus-minus" 5% accuracy would be great, (or am I wrong?) - so, a "standard" accurate to 5% would also be OK, or not ???

Hennie

[Randy Holmes-Farley]

Hennie:

I don't mind coming up with recipes for those with access. Where do you want to start?

As to the accuracy, 5% should do for anything that we're concerned with, even if you had a kit that was more accurate than that.

[Muttling]

I seldom post here, but often lurk. PLEASE post your ultimate results as I am looking back as my Salifert Calcium test results and seeing a disturbing pattern.

[New Reefer]

Thanks again, Randy...

Where to start, umm...., perhaps with the current "hot" topic, Alkalinity and Calcium.

I'm currently "playing" with vinegar spiking, and it would be nice to have some confidence in my dKh and Ca tests..

Then, if possible, Phosphate, I suppose...

Regards,
Hennie

[Randy Holmes-Farley]

OK, here we go:

Alkalinity


Start with sodium bicarbonate (baking soda).

The molecular weight is 84 g/mole.

So 1 gram contains 1/84 = 0.0119 moles

There is 1 equivalent of alkalinity for each bicarbonate, so that gram contains 11.9 meq/g.

If you put 1 gram in a gallon of water (3.79 L), you will have a solution that is 11.9/3.79 = 3.1 meq/L.

Note that the pH of this solution may change as CO2 comes off (pH will rise), but that will not change the total alkalinity. This is a known phenomenon. The pH will be around 8.

If weighing 1 g is an issue for anyone, you can of course scale up to 5 g/5 gallons, or you can dissolve 5 grams in 1 gallon and dilute a sample of 1 part solution to 4 parts water to get to 3.1 meq/L. Other dilutions are also perfectly fine.

[Randy Holmes-Farley]

Calcium


Anhydrous calcium chloride is 36.1% by weight calcium.

If you dissolve 1 g in 1 L of water, you will have a solution that is:

0.361 g/1L = 361 mg/L = 361 ppm

[Randy Holmes-Farley]

Phosphate

This on will depend on what you can get. If you can get

Na2HPO4
synonyms:
sodium phosphate dibasic
sodium hydrogen phosphate
disodium hydrogen phosphate
dibasic sodium phosphate
disodium orthophosphate
DSP
phosphate of soda
secondary sodium phosphate

or

NaH2PO4
synonyms:
sodium phosphate monobasic
sodium biphosphate
sodium dihydrogen phosphate
acid sodium phosphate
monosodium orthophosphate
primary sodium phosphate


The first one is 66.9% phosphate by weight if it is anhydrous (sometimes called excissated sodium phosphate). For the dihydrate (sometimes called Sorensen's phosphate or Sorensen's sodium phosphate) it is 53.3% phosphate. For the heptahydrate it is 35.4% phosphate. For the dodecahydrate it is 26.5% phosphate.

The second one is 79.2% phosphate by weight if it is anhydrous. For the monohydrateit is 68.9% phosphate. For the dihydrate it is 60.9% phosphate.

OK, so you've got tons of choices!

I'll go through the calculation for a common one, sodium hydrogen phosphate heptahydrate (35.4% phosphate):

Dissolve 1 g 1 gallon (3.78 L) of water.
The phosphate concentration is
1g x 0.354/3.79 L =
93.7 ppm phosphate

Since that is way too high, you need to dilute.

Take 1 part of this water and 99 parts of pure water (by either volume or weight) and you have 0.94 ppm.

Dilute 1 part of this fluid to 4 parts pure water and you have

0.19 ppm phosphate

That sounds like a good standard. FWIW, I'd make these fresh, especially the last 2 dilutions. The first standard (93 ppm) might be OK to sit, but a little absorption onto the walls of your container, or a little consumption by microbes in the water might eat up all of the phosphate if you let it sit.