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A photo of 15ml of distilled water accompanied with 3 drops of both universal indicator and bromothymol blue were mixed together and taken (white background was put into the back of the beake). The color of the resultant liquid in RGB is 140, 177, 185. The color space is sRGB IEC61966-2.1. Taken with an iPhone 14 camera. How would you find the exact number of pH level that the very distilled water measures? I understand that you need documentation to the color space and camera and figuring out many more stuff, but I couldn't put them together into math or any other tangible form and solve it

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    $\begingroup$ Welcome to Chemistry StackExchange! You could certainly, in principle, make a simple colorimetric pH meter with your equipment, namely a universal indicator and a camera. However, the crucial ingredient you are missing is the calibration. You would have to prepare multiple solutions with known pH, then "measure" they color by taking a photo. With this, you would create a sort of calibration curve to be used for determination of pH of some other solution. Moreover, there is a big issue with using phone cameras, because they usually do automatic ... $\endgroup$
    – Domen
    Jan 13 at 11:52
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    $\begingroup$ ... brightness/exposure/white point control, which means that you might not get reproducible measurements. You would have to use the "manual mode", and keep the camera parameters the same for all measurements. Long story short, given just one RGB color point, you unfortunately don't have any good chance of determining the pH. $\endgroup$
    – Domen
    Jan 13 at 11:52
  • $\begingroup$ Thank you so much, okay, so I have some calibrated distilled water images an their pHs, but don't know how to do the graphing thing, there are of course lots of math going on but I don't know anything about them. Anyways, what do I do next? $\endgroup$
    – Jimmy Zeng
    Jan 18 at 12:58

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AFAIK, the universal indicator already contains bromthymol blue. Why to add it explicitly?

RGB value is not enough to determine pH due multiple factors.

Aside of that, such a measurement is useless, as pH is set by the used indicators that are all weak acids or bases.

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  • $\begingroup$ Can you further elaborate on why a measure ment is useless and what are those "used indicators". I'm really just a beginner, don't know much, so correct me if I'm wrong $\endgroup$
    – Jimmy Zeng
    Jan 13 at 8:58
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    $\begingroup$ Try to localize, what exactly you do not understand, as the answers to both questions are already present above. // By measurement of pH as in your question, you do not measure pH of distilled water, but of diluted indicator solution. It is like if you measured temperature of water by thermometers much more massive than the water sample. // Used indicators = indicators in the universal indicator mixture (link) + bromothymol blue. $\endgroup$
    – Poutnik
    Jan 13 at 9:25
  • $\begingroup$ Tap water would be different as there is usually present CO2/HCO3- pH buffer. $\endgroup$
    – Poutnik
    Jan 13 at 10:21

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