Question

Calibration data for a bromide ion-selective electrode (ISE) was collected and recorded below. The potential 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. pBr. Determine the slope and y-intercept of such a plot of the calibration data given.

[Br-](M) Potential (mV)

0.300 -21.2

0.0300 36.7

0.00300 96.0

0.000300 154.7

0.0000300 225.6

a. slope=

b. y-intercept=

Two unknown solutions were tested for [Br–] using the same bromide ISE. The potential reading for unknown 1 was -6.8 mV and the potential reading for unknown 2 was 172.6 mV. Using the calibration curve you constructed above, determine the concentration of Br– in each unknown solution.

c. Unknown 1 [Br-]=

d. Unknown 2 [Br-]=

Answer #1

If we plot log[Br^{-}] vs potential a linear graph will
be obtained for which:

Slope = -57.9 and Intercept = -51.42

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

y = -57.9x - 51.42

Where y = potential in mV, x =log[Br^{-}]

**a.** Slope = -57.9

**b.** y-intercept = -51.42

**c.** Calculation for unknown 1:

y = -6.8 mV

Substituting the value of y , calculate x.

-6.8 = -57.9x - 51.42

x = -0.77

log[Br^{-}] = -0.77

[Br^{-}] = 0.17 M

**d.** Calculation for unknown 2:

y = -172.6 mV

Substituting the value of y , calculate x.

-172.6 = -57.9x - 51.42

x = -2.09

log[Br^{-}] = -2.09

[Br^{-}] = 0.008 M

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