Should I test these parameters with a spectrophotometer or with test kits?

[Swingline77]

First off, thanks for all the help I've gotten on this forum. It's really a valuable place. I have some questions regarding testing. My tank will be 90 gal with just gastropods, live rock, and plants (probably chaeto). I’m doing an amateur experiment.

I plan to test for the following:
Temp
pH
Salinity
Ammonia
Nitrite
Nitrate
Phosphate
Alk
Ca
Mg

Temp, pH, and salinity are straight forward. I have no questions regarding these.

For Ammonia and Nitrite, I only plan to test these to determine when cycling is complete, or if I’m having a major problem with the tank. I plan to get the cheapest tests possible, since accuracy isn’t important. When cycling, when ammonia and nitrate read “zero”, I’ll wait a week or two to make sure levels are truly at zero, then stock the tank. Do you see any problems with this approach?

As for the rest of the tests, things get tricky. There are three issues that I have to consider: how accurate I need my results to be, costs, and how much of a pain the test is to carry out. I have a vintage 1970s spectrophotometer that's worth about $40, but it still works. I can use if it makes sense.

If I use the spectrophotometer at all, there will be some fixed costs. For each additional parameter I test with the spectrophotometer, I’ll have some significant start up costs since I need to use reagent to test pure ionic compounds to get reference plots in order to interpolate my experimental results. For me, all this represents significant expense. Thereafter, once everything is up and running, the running costs of the spectrophotometer will be about 50% higher than they will be for reagent kits.

So, I can use the spectrophotometer, reagent kits, a combination of the above, or if I’m only going to run one test with “light readings”, I can buy a Hanna colorimeter.

I don’t know what to think about testing phosphate. On the one hand, I feel that if I use a cheap test kit and have bogus readings of zero, or some number which is lower than 0.1 ppm, and my chaeto stays alive, I should be fine. On the other hand, it might be important to have a fairly accurate reading of phosphate for the health of the chaeto, and to make absolutely sure my concentration is below 0.1 ppm. My gut reaction is that I can get away with an accurate reading.

As for nitrate, I'd guess a cheap test would be fine. As long as it's somewhat accurate, and can tell me when I'm below 5 ppm I'll be fine. Does this need to be tested accurately?

As for Alk, Ca, and Mg, I don’t know how important accuracy is.

So, regarding NO3, PO4, Alk, Ca, and Mg, considering the importance of accuracy and the hassle of doing the tests, do you think I should go with the spectrophotometer or the test kits?

Thanks, I really appreciate it.

 

[aKlevans]

For that kind of livestock, you really don't need to be testing calcium, alkalinity, or magnesium.

 

[JimWelsh]

Interesting question! I don't understand why running the spectrophotometer is 50% more expensive, but oh well.

You are correct that establishing calibration curves will be a necessity, although I'm not sure what you mean by "pure ionic compounds", but for most of these assays, you should prepare standards in a seawater matrix, and not use simple aqueous standards, e.g., sodium nitrate in RO/DI water.

Regarding phosphate, I don't know what sort of path length your spectrophotometer has, but for phosphate it needs to be pretty long. This is why the "pro" phosphate test kits typically use those really tall vials.

Alk, Ca, & Mg are typically titrations, and not well suited for spectrophotometer-based methods, although for alk I suppose you could try to reverse-engineer the Hanna checker method. I think it's adding enough acid to the sample to target a pH of 4.5 or so at the middle of the range, and has bromocresol green as the indicator. Comparing the height of the peaks at the two wavelengths for the acidic form vs. the basic form should enable you to calculate the actual pH of the resulting mixture, and you can develop a calibration curve based on that. The Hanna device is probably monochromatic (I'm guessing), so with the spectrophotometer you should be able to develop a more robust method.

If I had a working spectrophotometer at home, I'd be all over this, but that's just me. You may very well be better off just sticking with hobby test kits. I prefer Salifert for most everything except phosphate, where I prefer the Elos Phosphate Pro kit.

 

[Swingline77]

Interesting question! I don't understand why running the spectrophotometer is 50% more expensive, but oh well.

You are correct that establishing calibration curves will be a necessity, although I'm not sure what you mean by "pure ionic compounds", but for most of these assays, you should prepare standards in a seawater matrix, and not use simple aqueous standards, e.g., sodium nitrate in RO/DI water.

Regarding phosphate, I don't know what sort of path length your spectrophotometer has, but for phosphate it needs to be pretty long. This is why the "pro" phosphate test kits typically use those really tall vials.

Alk, Ca, & Mg are typically titrations, and not well suited for spectrophotometer-based methods, although for alk I suppose you could try to reverse-engineer the Hanna checker method. I think it's adding enough acid to the sample to target a pH of 4.5 or so at the middle of the range, and has bromocresol green as the indicator. Comparing the height of the peaks at the two wavelengths for the acidic form vs. the basic form should enable you to calculate the actual pH of the resulting mixture, and you can develop a calibration curve based on that. The Hanna device is probably monochromatic (I'm guessing), so with the spectrophotometer you should be able to develop a more robust method.

If I had a working spectrophotometer at home, I'd be all over this, but that's just me. You may very well be better off just sticking with hobby test kits. I prefer Salifert for most everything except phosphate, where I prefer the Elos Phosphate Pro kit.

I don't understand why running the spectrophotometer is 50% more expensive

As for the cost, the Hanna reagents are the same for their "checker" as well as for their spectrophotometer, which both use the colorimetric method of testing Alk via methyl orange (https://www.epa.gov/sites/production/files/2015-08/documents/method_310-2_1974.pdf). Reagents cost $8/25 tests ($0.32/test). The titration kits cost cost (Salifert) $0.17/test, and $6 (entire API test) for a test kit, which has an unknown number of tests.

I didn't think it was feasible to use the reagents test kits for any of the parameters with the spectrophotometer. Maybe I'll give this a try at some point. A nice thing regarding testing these various parameters is that the spectrophotometric methods often don't involve titration, e.g., the Calgamite method of testing Mg.

By the way, in looking at test kits, it seems that although some more expensive test kits might yield more precise results, or have clearly defined colors, the online consensus is that API is more consistent batch-to-batch in terms of accuracy. I don't know if this is true. Maybe someone can weigh in.

You are correct that establishing calibration curves will be a necessity, although I'm not sure what you mean by "pure ionic compounds", but for most of these assays, you should prepare standards in a seawater matrix, and not use simple aqueous standards, e.g., sodium nitrate in RO/DI water.

By "pure ionic compounds" I meant aqueous standards. I can get a container of lab grade Na(CO3)2 for $16. I don't know why standards for calibration should be prepared from seawater. I know that ionic species other than carbonate would add to buffering capacity. If I were to test with seawater, it seems that I'd need to send a freshly mixed sample of water with whatever brand salt I'm using to Triton, etc. for precise testing of carbonate concentrations. I would then mix this saltwater sample with aqueous standards of sodium bicarbonate of known and varied concentrations. I could then plot a curve. From this point on, if I were to test water made from salt mixture that has had kalkwasser added to it, I could get an accurate reading as to the amount of carbonate in solution. However, the big problem with all this is that the aquarium would not be holding freshly mixed water. It will contain water that has had buffers other than carbonate removed in unknown amounts as a result of precipitation and the metabolic activity of tank inhabitants.

From this perspective, am I wrong in thinking that I'd eliminate the error caused by changing concentrations of non-carbonate buffers in tank water by creating standards with dH2O and lab grade Na(CO3)2 rather than with saltwater?

Thanks for the detailed reply. I've had some replies to this topic on another forum, too. For now, I can get API tests cheaply enough. I think I'll go with these, combined with an Mg test of some other kind. The question of which Mg test to use is throwing me for a curveball at this point.
I've heard of issues involving accuracy of the Mg tests. IF, the salifert titration test is accurate, and this is a big if, I might start off with it since it's $20, as compared with $50 for a Hanna checker, and somewhat more for the spectrophotometer.

 

[Randy Holmes-Farley]

If the goal is just suitable measurements to keep a reef, I personally wouldn't bother to use the spectrophotometer. If you enjoy doing such things (like Jim ) then go for it.

I do think there will be a lot of issues of calibration curves for seawater, and method development unless you buy someone elses method and chemicals.

Nitrate, for example, typically has chloride interference and becomes an issue of calibration for a particular salinity if you want high accuracy.

 

[Swingline77]

Thanks. Nope, I don't want to make this more complicated than necessary (though it might not seem this way from the above posts). I'll just go w/ the test kits, and use either a kit or the "checker".