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GATE 2026 IN – Question 26

Sensors and Industrial Instrumentation · Temperature and analytical sensors · 1 mark · Multiple choice

A glass pH electrode behaves linearly with an output change of $60\ \mathrm{mV}$ per unit change in pH. The measurement reference is set such that a solution of pH 6 produces an output of $60\ \mathrm{mV}$. The pH of a solution that results in an output of $-90\ \mathrm{mV}$ is ________.

  1. 3.5
  2. 3
  3. 8
  4. 8.5

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Correct answer: (D) 8.5

Explanation

The output of a glass pH electrode falls as pH rises, so the sensitivity is $-60$ mV per pH unit. The reference point is pH 6 giving 60 mV, so

$V=60-60(\mathrm{pH}-6)$ mV.

For $V=-90$ mV: $-90=60-60(\mathrm{pH}-6)$, so $60(\mathrm{pH}-6)=150$ and $\mathrm{pH}-6=2.5$.

$\mathrm{pH}=8.5$, **D**.

Note: the wording gives only the slope magnitude. Taking the output to rise with pH would give pH 3.5 (option A). The stored key follows the usual falling-voltage convention of the glass electrode.