Calibration data for a bromide ion-selective electrode (ISE) was collected and r

Question

Calibration data for a bromide ion-selective electrode (ISE) was collected and recorded below. The potent al of the ISE was measured against a saturated calomel electrode (SCE). All solutions were buffered at a pH of 7.56 A linear calibration curve can be constructed from this data as a plot of potential (in mV) vs log[Br^-]. Determine the slope and y-intercept of such a plot of the calibration data given. Two unknown solutions were tested for (Br^-) using the same bromide ISE. The potential reading for unknown 1 was -15.6 mV and the potential reading for unknown 2 was 124.8 mV. Using the calibration curve you constructed above, determine the concentration of Br- in each unknown solution.

Explanation / Answer

If you plot log[Br-] vs potential a linear graph will be obatined for which slope =-61.181 and intercept =-55.929

using the value of slope and intercept, the linear equation can be constructed.

y =-61.181 x-55.929

where y=potential in mV, x =log[Br-]

calculation for unknown 1:

y = -15.6 mV

substituting the value of y , calculate x.

x = (-15.6+55.929)/-61.181 =-0.6592

log[Br-] = -0.6592

[Br-] = 0.22 M

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Calculation for unknown 2

x = (124.8+55.929)/-61.181 =-2.95

log[Br-] = -2.95

[Br-] = 0.0011 M

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