Thanks for the response, does each test require a reagent? Or do some compounds that produce color not need reagent and ones that do, do need reagent? What do you test for and do all need reagent?
Sorry for the delay, missed that. All the tests I've run on this instrument are reagent-based. I have been simply using hobbyist kits but with several steps that help to make the test both more accurate and higher resolution.
The short answer is you get a reading in absorbance that is easily converted to ppm.
Abs = Slope * [NH3] + Intercept[NH3] = (Abs - Intercept) / Slope
The short answer is you get a reading in absorbance that is easily converted to ppm.
The longer answer is best shown by example. Here is an example of the procedure I've used for NH3 with the Red Sea test. I didn't do anything special here, just applied standard procedures from spectrophotometry to analyze the results of this test, instead of reading them by eye. I am sure the Hanna checkers use similar math, with built-in constants instead of calculating them from a series of dilutions. (I'm showing this only to discuss how it can be done in principle for any colorimetric test, not recommending that anyone needs to do so for ammonia)
First make a standard curve using a stock solution of ammonium sulfate, by preparing a series of dilutions at known concentrations. They look like this
Next measure their absorbance on the spec, in this case at 680 nm. Here is an example figure
You can calculate the slope and intercept* of this equation, expressed as
To test your sample, you can measure Absorbance on the spec then calculate the concentration in ppm asCode:Abs = Slope * [NH3] + Intercept
It all takes much longer to explain than to do; the math is straightforward so it's handled by a spreadsheet or app.Code:[NH3] = (Abs - Intercept) / Slope
(*the intercept should be and is very close to zero, but to account for any errors in the blank I always include this in the calculation)
One of these days I'm gonna get around to doing Phosphate, since this is one of the only tests where I'm actually interested in the precise numbers at low levels. In principle even my cheap spec is surely at least as sensitive as the Hanna checker and possibly more sensitive (I seem to remember that sensitivity is a function of path length with spectrophotometry, and isnt the Hanna less than 1 cm path length?)
The Hannah cuvette has a 10 mm inside diameter, so same path length as most commonly used. Interestingly the hanna phosphate checker uses a wavelength far from the maximum absorbance of the chemical test product. In fact, the checker wavelength is nearer a minimum absorbance, but it still works because the absorbance is so broad.
Oh, interesting... so there may be some gains in sensitivity from doing the test on a spec at the optimum wavelength. Thanks, that makes me interested in moving this off the back burner. I've been curious about PO4 levels in some of my low nutrient tanks, maybe its time to finally do the PO4 test on the spec.
The process I describe works for any colorimetric test. That means any test where you evaluate the level based on the color you see (e.g. if its darker green, its a higher level).
Didn’t you write an article on the Hanna Alkalinity Checker? @Randy Holmes-Farley has also commented on the Hanna alkalinity method as well. I vaguely remember that titration to pH 4.3 (?) provides an alkalinity value closer to reality than the Checker methodology.Hach and Hanna both market a colorimetric test for alkalinity (as CaCO3). I ordered a Hanna alkalinity Checker along with the nitrite reagents. I'll compare alk results from the Checker to the Hach titration method I'm currently using.
Alkalinity (Total) TNTplus Vial Test (25-400 mg/L CaCO₃), 25 Tests
Alkalinity (Total) TNTplus Vial Test (25-400 mg/L CaCO₃), 25 Testswww.hach.com
I know I made some comparisons of results obtained through use of the Hanna alkalinity Checker and the Hach titration method (to pH of 4.2) and they were favorable. Don't recall if I published this info, and the Advanced Aquarist site is down (again.) FedEx said I'd get the new Hanna alk Checker today, but it's 7:15 pm and I doubt they're being honest with me. I'll do another comparison as soon as the Hanna stuff arrives.
just wanted to thank you for posting this raw Abs vs Conc curve. It's great info.
For curiosity, a few times recently I've tested PO4 samples with some color to the end product so I checked the spectrum. Every time I get something like this.
