Question: Can DIY spectrometers be useful for pH determination and/or dissolved organic carbon of a water solution?

alejobonifacio is asking a question about spectrometry
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by alejobonifacio | March 02, 2022 15:12 | #30090


These questions have to do with certain things that I would like to achieve with DIY spectrometers. The first thing that I want to achieve is to determine the pH of a solution with purple cabbage juice. There is a research paper that tells that the pH could be accurately measured at 533 nm with absorbance values for pH values between 2 and 11, which is a wide range. I was doing some tests in the spectral workbench and noticed that for each wavelength there are 3 values, red, green and blue. Are there some ways to change these values to absorbance measurements? because, in the paper use this unit measurement.

The other thing that I would like to achieve is to determine the dissolved organic carbon (DOC) of a water sample also with a spectrometer. This is a good tool for community science because you need no reagents. The only thing that I don't know if it is possible with DIY spectrometer is to measure in the UV wavelength, because the DOC is measured at 254 nm.

I hope someone have some time to answer these questions.

Cheers!



6 Comments

This seems like a great application and very feasible. Indeed the color/sensitivity range of anthocyanin in cabbage juice is really huge. 533 is in the green range, right? I'm a little skeptical that just absorbance in one color is the best way to go; it sounds like they just had a spectrometer, and traditional scientific methodology is to find one wavelength and try to build knowledge within it. That's partially just how old spectrometers are built rather than it being the ideal approach. Cabbage juice changes color across a wide range of colors, so my intuition is that actually measuring the hue would be a better match. The danger with just one color is that we lose sensitivity when the hue change is occurring far away from the chosen wavelength -- maybe the dynamic range is worse at the extremes. That might kind of explain why they chose a color roughly in the middle of the range, to try to have no hues too far away from the measurement wavelength.

So that makes me think - what about just hue measurement using a controlled angle/color/intensity of light and a camera? Or just use the spectrometer and measure the relative hue shift by looking across the whole spectrum? Could you find the "median" hue of a spectrum? I could imagine a standard equation for this.

I was doing some tests in the spectral workbench and noticed that for each wavelength there are 3 values, red, green and blue. Are there some ways to change these values to absorbance measurements? because, in the paper use this unit measurement.

The way to do this is to develop a scale through testing. So pre-mix some pH solutions bracketing your ideal range - from 2 to 11, maybe... 5 different values? And scan them, then to copy the paper, you'd look at the intensity at 533. You should be able to establish a scale, and it may not be linear. But since the spectrometer is not measuring absolute units of brightness, you need to use this technique to scale your data to a set of reference values.

The other thing that I would like to achieve is to determine the dissolved organic carbon (DOC) of a water sample also with a spectrometer. This is a good tool for community science because you need no reagents. The only thing that I don’t know if it is possible with DIY spectrometer is to measure in the UV wavelength, because the DOC is measured at 254 nm (Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon | Environmental Science & Technology (acs.org)).

254 is really low. LEDs we've bought have gone only as low as 360. I have found some "UVC" bulbs for disinfecting stuff down lower. Just looking now, it seems like COVID has helped us, both LED and quartz bulbs:

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Hi @warren! maybe you have something to say about this... 😃

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Sorry, but I only have access to the abstract. The wavelength mentioned there is 633 nm. Is the correct wavelength 633 nm or 533 nm?

Regards

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I got one of the 254 nm UVC lamps that might be usable for an uv/vis instrument as a gift . It's advertised as for sterilization of surfaces. The one purchased was" fbfl for portable uv". It's a mercury lamp in sheep's wool. It's powered by three "AA" batteries. And the mercury lamp is the smallest I've ever seen. There are many precautions that need to be taken with UVC light, including eye protection. Mercury disposal(as well as mercury breakage) will also be an issue. The led version out there might be better. But still, be careful using any of them.

Definitely these can be dangerous for the eyes. The LEDs seem nice because they are a little less 1800s feeling than those tiny tiny mercury bulbs!! Energy usage and probably consistency of brightness as well, i'd guess? Though I don't know mercury lines in that range, do you think there are advantages to spectral narrowness too? LEDs are usually not as narrow range, but I know very little about deep UV LEDs.

I'm very curious about lead detection with deep UV. A long time ago I used a toothbrush sterilizer to try to see lead residue on a bullet casing and it didn't glow at all. But I do see the references (https://libanswers.cmog.org/faq/143932) to "icy blue" fluorescence in glass due to lead. I'll post a question about that too...

OK here i posted it separately: https://publiclab.org/questions/warren/03-02-2022/using-uvc-or-deep-uv-light-to-detect-lead-by-fluorescence

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On battery life- it wasn't very good.
Mercury (as in mercury lamps) have a lot of lines that go all through the uv/vis spectrum. For a while(as in late 1970s), it was used as a source for hplc(high pressure liquid chromatography) uv/vis detectors. Trouble is, you have to filter out the ones that aren't needed. And there are a lot.

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