An ion selective electrode for SO 4 2 ion has measured response for known calibr

Question

An ion selective electrode for SO42 ion has measured response for known calibration standards and an unknown, as follows:

Concentration, M

Electrode response, mV

Standard 1:    2.59 x 103

5.796

Standard 2:    4.73 x 104

+ 15.101

Standard 3:    1.55 x 105

+57.114

Unknown

+22.140

(a) Determine the equation of the line for electrode response based on the calibration standards. You may use either a graphical method or an algebraic method. If you choose graphical, attach graph and spreadsheet.

(b) Determine the concentration of the unknown sample.

Concentration, M

Electrode response, mV

Standard 1:    2.59 x 103

5.796

Standard 2:    4.73 x 104

+ 15.101

Standard 3:    1.55 x 105

+57.114

Unknown

+22.140

Explanation / Answer

let the equation be y = mx + c

here

conc is on x axis and eletrode response is on y axis

we know that

slope (m) = y2 - y1 / x2 - x1

so

slope = 15.101 + 5.796 / 4.73 x 10-4 - 2.59 x 10-3

slope = 20.897 / -2.117 x 10-3

slope = -9871

so

the equation becomes

y = -9871x + c

now use the third set to get the constant (c)

57.114 = -9871 x 1.55 x 10-5 + c

c = 57.267

so

the equation of the line is

y = -9871x + 57.267

b)

now

for the unknwon

y = 22.14

so

22.14 = -9871x + 57.267

x = 3.56 x 10-3

so

the cocentration of the unknown sample is 3.56 x 10-3 M

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