You are measuring Ca2+ in drinking water using an ISE. You diluted 1 mL of the d
ID: 937217 • Letter: Y
Question
You are measuring Ca2+ in drinking water using an ISE. You diluted 1 mL of the drinking water with 19 mL ddH2O. You have standards of Ca2+ in ddH2O. You add 1 mL of a commercial ISA reagent to 20 mL of each standard or the diluted sample. You then measure the ISE response of each sample using the voltmeter. The following data are obtained:
ID
mV
Ca2+ (?g/L)
Standard
22.07
0.12
Standard
45.11
1.08
Standard
75.92
11.5
Standard
100.34
178
Standard
116.97
1,206
Standard
161.55
20,045
Sample
67.83
?
What is the concentration of Ca2+ in the diluted drinking water (?g/L)? (ONE QUESTION, ALL INFORMATION GIVEN!)
ID
mV
Ca2+ (?g/L)
Standard
22.07
0.12
Standard
45.11
1.08
Standard
75.92
11.5
Standard
100.34
178
Standard
116.97
1,206
Standard
161.55
20,045
Sample
67.83
?
Explanation / Answer
For ISE measures, you can use the Nernst eq. as:
Emeasure = constant + (0.05916/z) * log ([A])
Where z is the charge of the ion and [A] is the molar concentration. we can plot the values you gave and get the value of the constant. Then, we can use the equation in order to get [A]. But first, check that you have the concentrations in mg/L and we need it in mol / L for use this. What I
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