A Ca 2+ ion-selective electrode was calibrated in metal ion buffers with ionic strength fixed at 0.50 M. The electrode is also sensitive to Mg2+. 1a)What is the general expression for the response of this electrode, including the Mg 2+ slectivity coefficient. What is the purpose of fixing the ionic strength of the calibration points? 1b.) Derive the equation for the response of the electrode to Ca 2+ and Mg 2+ from the data below. [Ca2+] (M) [Mg2+] (M) mV 1.00 x 10-6 0 -55.4 2.5 x 10-4 0 +17.2 1.00 x 10-6 3.7 x 10-3 -41.1 1c.) In region rich in limestone deposits, [Ca2+] in streams is about 1.0*10-5 M. A Ca2+ ion selective electrode can be used for continuous monitoring of [Ca2+] in streams. High levels of Mg2+ can interfere with this measurement. From answer in part (b), calculate the maximum [Mg2+] that can be tolerated to ensure less than a 10 % error in the measured [Ca2+].

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13 Dec 2019

A Ca 2+ ion-selective electrode was calibrated in metal ion buffers with ionic strength fixed at 0.50 M. The electrode is also sensitive to Mg2+. 1a)What is the general expression for the response of this electrode, including the Mg 2+ slectivity coefficient. What is the purpose of fixing the ionic strength of the calibration points?

1b.) Derive the equation for the response of the electrode to Ca 2+ and Mg 2+ from the data below.

[Ca^2+] (M) [Mg²⁺] (M) mV

1.00 x 10^-6 0 -55.4

2.5 x 10^-4 0 +17.2

1.00 x 10^-6 3.7 x 10^-3 -41.1

1c.) In region rich in limestone deposits, [Ca2+] in streams is about 1.0*10-5 M. A Ca2+ ion selective electrode can be used for continuous monitoring of [Ca2+] in streams. High levels of Mg2+ can interfere with this measurement.

From answer in part (b), calculate the maximum [Mg²⁺] that can be tolerated to ensure less than a 10 % error in the measured [Ca2+].

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CHEM 319, QUANTITATIVE ANALYSIS LA8ORATORY EXERCISE à¸®6 QUANTIFYING FLUORIDE USING AN ION SELECTIME ELECTRODE PRE-LAD READING Read Harris pages 303-306 311-317 (on-selective electrodes) Lean how to prepare samples for analysis from dmicut to wark with starting materials Learn how to prepare calibration curves from ISE data EXPERIMENT OVERVIEW For this lab, you will determine the fuoride content in commercial mouthwashes using an ion selective electrode. This lab uses the direct potentiometry method, which assumes that matrix interferences are not a problem for determining the concentration of fuoride in toothpaste. To combat possible matrx problems, the total ionic strength adjustment buler (TISAB) solution used in the experiment contains 12 gand ensures that cations of meta's such as iron and copper in the sample will not complex the fuoride on. The TISAB will also adjust and stablize the lonic strength of the measured solutions lonic strengt can be a large factor affecting accuracy in ion selective electrode (ISE) measurements MATERIALS Total ionic strengh adjustment bulffer (TISAB) Fluoride stock solution (500 ppm wv) Two commercial mouthwashes (one control and one unknown) Fluoride lon selective electrode 10 mL volumetric fasks Sar bars 20-mL vials 1. Prepare five 10 mL standard solutions (2 ppm. 5 ppm, 20 ppm. 50 ppm, and 100 ppm)each containing 50%w/v ofthe TISAB andthe residual volumne comprised of D' water Transfer these standards to abeled 20-mL viais. 2. In a 10mL vourmetric flask, prepare a sample that contains 1.00 mL mohash and 50% vv TISAB The solution will be quite foamy, so take care not to add the TISAB and Dil water too vigorously. You may also want to add the TISAB and some of the DI water prior to adding the mouthwash (be sure that you do not overshoot the mark, though). Mix well. Repeat for the other mouthwash sample. Transfer these samples to labeled 20-m vials 3. When all of your samples are prepared, add a small magnetic stir bar to the sample to be measured Put the vial onto strplate and carefully lower tuonde eectode the solton Record the potential (in mV). Remove the stir bar and wipe it dry with a Kimwlpe. Repeat for each standard and Prepare a calibration curve from your data (see page 313 of your textbook about how to do so) and use it to determine the concentration of your unknown and control samples. From these caiculated concentrations, determine the % wiv of fuoride in each sange not, that this reported as Neper 100 mL solution. You should also report the mg of NaF per 10 mL dose The waste generated from this lab can be safely washed down the drain. Data Determincd Table I of Solutions AB ( 50%) 0 rem Table 2: Data Determined pem 0 ppm 0 ppen 100 ppm 0 mV 5.3 mV 1 mV 18.0 mV 0 mV 74 mV

Show transcribed image text CHEM 319, QUANTITATIVE ANALYSIS LA8ORATORY EXERCISE à¸®6 QUANTIFYING FLUORIDE USING AN ION SELECTIME ELECTRODE PRE-LAD READING Read Harris pages 303-306 311-317 (on-selective electrodes) Lean how to prepare samples for analysis from dmicut to wark with starting materials Learn how to prepare calibration curves from ISE data EXPERIMENT OVERVIEW For this lab, you will determine the fuoride content in commercial mouthwashes using an ion selective electrode. This lab uses the direct potentiometry method, which assumes that matrix interferences are not a problem for determining the concentration of fuoride in toothpaste. To combat possible matrx problems, the total ionic strength adjustment buler (TISAB) solution used in the experiment contains 12 gand ensures that cations of meta's such as iron and copper in the sample will not complex the fuoride on. The TISAB will also adjust and stablize the lonic strength of the measured solutions lonic strengt can be a large factor affecting accuracy in ion selective electrode (ISE) measurements MATERIALS Total ionic strengh adjustment bulffer (TISAB) Fluoride stock solution (500 ppm wv) Two commercial mouthwashes (one control and one unknown) Fluoride lon selective electrode 10 mL volumetric fasks Sar bars 20-mL vials 1. Prepare five 10 mL standard solutions (2 ppm. 5 ppm, 20 ppm. 50 ppm, and 100 ppm)each containing 50%w/v ofthe TISAB andthe residual volumne comprised of D' water Transfer these standards to abeled 20-mL viais. 2. In a 10mL vourmetric flask, prepare a sample that contains 1.00 mL mohash and 50% vv TISAB The solution will be quite foamy, so take care not to add the TISAB and Dil water too vigorously. You may also want to add the TISAB and some of the DI water prior to adding the mouthwash (be sure that you do not overshoot the mark, though). Mix well. Repeat for the other mouthwash sample. Transfer these samples to labeled 20-m vials 3. When all of your samples are prepared, add a small magnetic stir bar to the sample to be measured Put the vial onto strplate and carefully lower tuonde eectode the solton Record the potential (in mV). Remove the stir bar and wipe it dry with a Kimwlpe. Repeat for each standard and Prepare a calibration curve from your data (see page 313 of your textbook about how to do so) and use it to determine the concentration of your unknown and control samples. From these caiculated concentrations, determine the % wiv of fuoride in each sange not, that this reported as Neper 100 mL solution. You should also report the mg of NaF per 10 mL dose The waste generated from this lab can be safely washed down the drain. Data Determincd Table I of Solutions AB ( 50%) 0 rem Table 2: Data Determined pem 0 ppm 0 ppen 100 ppm 0 mV 5.3 mV 1 mV 18.0 mV 0 mV 74 mV

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[Ca^2+] (M)	[Mg²⁺] (M)	mV
1.00 x 10^-6	0	-55.4
2.5 x 10^-4	0	+17.2
1.00 x 10^-6	3.7 x 10^-3	-41.1