Ion Selective Electrode Tutorial 1 Answers Final PDF

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Ion-Selective Electrodes Tutorial 1 Answers 1. a) Calibration graph should be as follows:

dy

dx

b) The Nernst equation is: E = constant +

2.303 RT log10 [X]Sample zF

Therefore plotting E versus log10[X] gives a straight line: y = mx + c where y = E, c = constant, x = log 10[X] and the gradient m = 2.303RT/zF (in V) or m = 2303RT/zF (in mV) Expected slope from Nernst equation (2303RT/zF): Analyte H+ F− Ca2+

Slope /mV +59.1 −59.1 +29.6

The slopes you determine from your graph should be similar to those calculated from the Nernst equation (i.e. H+ ≈ +59 mV, F− ≈ −59 mV and Ca2+ ≈ +30 mV); however, the exact value obtain will depend on accuracy of the graph. To determine slopes graphically draw a triangle (see example on graph above) measure the width of the triangle (dx) and the height of the triangle (dy) the gradient is given by dy/dx, remember if the line is sloping downwards the gradient is negative. b) A ten-fold increase in analyte concentration causes H+ probe to change by +59.1 mV, F− probe by −59.1 mV and Ca2+ probe by +29.6 mV. Change in probe output for monovalent cation twice that of divalent cation. Change in probe output for monovalent cation has same magnitude as for monovalent anion but output increases for cation and decrease anion. c) Nernstian response: probe output follows Nernst equation.

Hyper-Nernstian response: probe exhibits a larger change in output (mV) than would be expected for “Nernstian response” Sub-Nernstian response: probe exhibits a smaller change in output (mV) than would be expected for “Nernstian response” 2. a) The advantages of using ion-selective electrodes for chemical analysis are: 

Inexpensive and simple to use in the field and laboratory

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Wide concentration range for analysis, from a few ppm to thousands of ppm (10−5 to 10−1 M)

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Unaffected by sample colour or turbidity

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Ideal for monitoring i.e. environmental pollution, water quality etc.

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Ideal for following changes in ion concentration i.e. rates of reaction or nutrient uptake etc.

The limitations of using ion-selective electrodes for chemical analysis are: 

Ionic strength: must be similar for all solutions analysed

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Potential drift: can arise from changes in the reference liquid junction potential over time; periodic recalibration and reversing the order standards are measured in can help

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Temperature: must be the same for all solutions analysed (to within ±2°C)

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Selectivity: it is possible for other ions to interfere with readings

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Accuracy and precision: generally ±10% but can achieve ±2% (1σ) with careful analysis under ideal conditions

For more detailed information see chapters 2, 6, 7 and 8 in “A Beginners Guide to Ion-Selective Electrode Measurements” by Chris C Rundle BSc, PhD. (Nico2000 Ltd, London, UK.) Last Update: 5 May 2014. http://www.nico2000.net/Book/Guide1.html accessed 6th October 2014 b) Examples of ion-selective electrodes applications include: Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents, and natural waters. Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material, fertilisers and feedstuffs. Food Processing: NO3, NO2 in meat preservatives. Salt content of meat, fish, dairy products, fruit juices, brewing solutions. F in drinking water and other drinks. Ca in dairy products and beer. K in fruit juices and wine making. Corrosive effect of NO3 in canned foods. Detergent Manufacture: Ca, Ba, F for studying effects on water quality. Paper Manufacture: S and Cl in pulping and recovery-cycle liquors. Explosives: F, Cl, NO3 in explosive materials and combustion products. Electroplating: F and Cl in etching baths; S in anodising baths. Biomedical Laboratories: Ca, K, Cl in body fluids (blood, plasma, serum, sweat). F in skeletal and dental studies. Education and Research: Wide range of applications. From chapter 2 in “A Beginners Guide to Ion-Selective Electrode Measurements” by Chris C Rundle BSc, PhD. (Nico2000 Ltd, London, UK.) Last Update: 5 May 2014. http://www.nico2000.net/Book/Guide1.html accessed 6th October 2014

3. An unknown sample gave a response of 45.6 mV (averaged from 4 repeat readings) using a fluoride ion-selective electrode. Determine the fluoride concentration and standard deviation for the unknown sample. Calibration data and the line of best fit are shown below. Conc / mM 2 4 8 16 32

Log10(Conc) x 0.301 0.602 0.903 1.204 1.505

E /mV y 98.4 79.7 59.4 38.8 22.9

Line of best fit: y = −63.7478x + 117.41

4. An unknown sample gave a response of 135.2 mV (based on 1 reading) using a calcium ion-selective electrode. Determine the calcium concentration and standard deviation for the unknown sample. Calibration data and the line of best fit are shown below. Conc / mM 1 2 5 10 20

Log10(Conc) x 0.000 0.301 0.699 1.000 1.301

E /mV y 100.8 110.1 120.3 130.6 138.8

Line of best fit: y = 29.1925x + 100.8469

5. An unknown sample gave a response of 32.6 mV (averaged from 6 repeat readings) using an amphetamine ionselective electrode. Determine the amphetamine concentration and standard deviation for the unknown sample. Calibration data and the line of best fit are shown below. Conc / mM 0.1 0.2 0.5 1.0 2.0 4.0

Log10(Conc) x -1.000 -0.699 -0.301 0.000 0.301 0.602

E /mV y 28.5 45.2 64.5 85.1 99.0 116.2

Line of best fit: y = 54.8208x + 83.10558...