December 04, 2020, 01:01:54 PM
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Topic: A question about Beer's Law  (Read 368 times)

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Offline docteurdeno

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A question about Beer's Law
« on: October 12, 2020, 01:17:46 PM »
Is it possible to determine the pH of the salt-water solution after an acid-base reaction by only using a spectrometer? You can determine the absorption of the solution using a spectrometer, and then using Beer's law, the concentration of that solution. But to determine the pH, you need the concentration of hydronium ions in a solution. Is there a way to calculate hydronium ion solution without using any other equipment?

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Re: A question about Beer's Law
« Reply #1 on: October 12, 2020, 01:28:12 PM »
Directly - no. At least not using a visible light. Adding something that changes color in different pHs might help to make an indirect measurement.
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Offline docteurdeno

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Re: A question about Beer's Law
« Reply #2 on: October 12, 2020, 02:35:36 PM »
Directly - no. At least not using a visible light. Adding something that changes color in different pHs might help to make an indirect measurement.
Thank you for responding, but in my research project, I need to investigate Beer's law. Are you aware of another way to calculate pH using Beer's law? pH of any substance, it doesn't have to be a salt solution. I'm currently searching for methods to come up with a research question.

Offline Babcock_Hall

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Re: A question about Beer's Law
« Reply #3 on: October 12, 2020, 04:00:49 PM »
What have you found, so far?

Offline docteurdeno

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Re: A question about Beer's Law
« Reply #4 on: October 12, 2020, 04:26:13 PM »
What have you found, so far?
I've found methods on finding the percentage of a metal in an alloy using spectrophotometry. My teacher rejected this, as I would've had to use very concentrated acids (around 8M of HCl), and deemed it unsafe for experimentation in school.

I also found a very basic experiment on Vernier's experiments book. It only changes the concentration of NiSO4 and measures its absorbance.

I would prefer it if I could somehow make it related to acids and bases, I am interested in that topic.

Offline Babcock_Hall

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Re: A question about Beer's Law
« Reply #5 on: October 12, 2020, 04:32:45 PM »
I have encountered the opposite sort of question.  When pH is known and controlled via the use of buffers, absorbance can be used to find the pKa of a substance.

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Re: A question about Beer's Law
« Reply #6 on: October 13, 2020, 03:32:13 AM »
pH of any substance

pH is not a property of a substance, it is property of a solution.

As I mentioned earlier - doable. Take a substance that changes color when the pH changes (plenty of such substances, perfectly safe and present in every lab). Typically it will have two forms - one for low pH, one for high pH. You should be able to find two wavelengths which will let you find the ratio of both forms. The color change is rather narrow, not more than about 4 pH units, but if you are in the correct range and know the ratio finding pH is quite simple.
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Offline Corribus

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Re: A question about Beer's Law
« Reply #7 on: October 13, 2020, 02:41:35 PM »
Visible absorbance is basically a measure of color (not exactly, but close enough). In that sense, UV-Vis could be used as a way to "read" a solution analogue to a pH strip. The trick would be to select the correct combination of pH-active dyes, and bearing in mind that the measurement would not be very precise.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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