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Calibration of a glass electrode gave a reading of 142.3 mV with a 0.05 m potassium hydrogen phthalate buffer standard ( pH=4.015 ), and a reading of −61.3 mV with a 0.08 m HEPES, 0.08 m NaHEPES, 0.08 m NaCl buffer standard ( pH=7.454 ), both measured at 30 ∘C. What is the observed slope (mV/pH unit) of the calibration curve?

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IthaloAbreu

Answer:

-59.2 mV/pH unit

Explanation:

For a good calibration, the curve must be a line, which has equation

y = mx + b

Where m is the slope of the line, and can be calculated by:

m = Δy/Δx

In this case, y is the voltage (mV) and x the pH, so:

m = (142.3 - (-61.3))/(4.015 - 7.454)

m = -59.2 mV/pH unit

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