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    PH values follows a linear equation, y = mx + b, the 3.5 is the slope of the linear line. Typically a PH probe need to be calibrated with two buffer solutions, for example, at PH4 and another at PH9 or PH7, the line can then be draw and the slope can be determined. Commented Aug 5, 2022 at 6:31
  • A voltage of 0 volts means a pH value of 0, and a voltage of 4 volts means a pH value of 14. Not exactly, the measurement of PH value usually required to be amplified via an OP AMP, and since PH value can't be a negative value, the OP AMP is usually "raise" the reference voltage, say, to 2.5V, and then calibrate the probe for the OP Amp to output 0 voltage when PH is at 7.0 (Center of the PH range), so the PH value swing between +/-2.5V. Commented Aug 5, 2022 at 6:35
  • As I wrote, we have no information what sensor and external circuitry the OP uses. Therefore, I simply explained the factor. Anyway, your comments are welcome, as an potential edition of my answer. Just keep the meaning, please. Commented Aug 5, 2022 at 6:39
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    Re “the value ‘1024’ should be ‘1023’”: In theory, it should be 1024. This is clearly stated in the datasheet of the microcontroller: the value 1023 “represents the selected reference voltage minus one LSB”. In practice, you have to calibrate the ADC if you need this level of accuracy. I have measured a slope of ≈1026 on three Arduinos. Commented Aug 5, 2022 at 7:40
  • @EdgarBonet Thanks for this valuable insight! Commented Aug 5, 2022 at 7:51