Essay on Electrochemistry and Ion Concentrations

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Words: 2151
Pages: 9
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Group: Brandon G., Sammy C., Alex S., Stacie , Andrew B.
TA: Sarah Melnick Water Project
(A Comparison of Ion Concentration and Park Age)

INTRODUCTION The objective of this project was to compare the content of the ions in drinking water samples collected from different parks of varying ages around Tucson. It was hypothesized that the water fountains in the older parks have the highest concentration of ions because the pipes which provides the water would likely be worn down more. This means that the water is then able to dissolve more ions from the inside of the pipes increasing concentration that in turn comes out of the fountains. The samples used were all from parks built in different years. These years include 1947, 1954, 1960, 1976, and 2005. There were three different methods to testing the water samples for the dissolved ions. The first method used was the complexometric titration test. The purpose of the test was to find out the concentration of M2+ ions in each sample. This test involved using EDTA as a base. This test allows for determining how hard the water is, the concentration of multivalent ions in other words (Ca2+ and Mg2+ in this case). In this experiment the indicator used was calmagite and as the base was added the color slowly turned from pink to blue indicating the end point. The second experiment that was performed was the ion-exchange chromatography. This test was used to find the total molarity of all positively charged ions within the sample. This was done by passing the water sample through a resin column during which the water traded metal ions with H+ ions. The resulting filtrate’s H+ concentration was then measured through acid base titration. This time the titration used NaOH as a base and phenolphthalein as an indicator. The third experiment that was performed was the ion selective electrodes which was used to find the concentration of Na+ and Ca2+ ions. The pH was also measured to find the H3O+ concentration. Firstly a calibration curve was made for Ca2+ and Na+ which were found by performing serial dilutions using the standards NaCl and CaCl2. The electrochemical potential, or Ecell, of each dilution was then measured and an Ecell vs -log [Ca2+ or Na+] graph was made. The resulting linear graph is the calibration plot and gives the Nernst equation for the selected ion. A specific ion calibrated ISE probe was used on each water sample to determine its potential which then using the corresponding Nernst equation was used to find the concentrations of calcium and sodium in the water samples. Lastly the concentrations of magnesium and potassium needed to be found. These were found simply by using the formulas [Mg2+] = [M2+] - [Ca2+] and [K+] = [PC] -2[M2+] - [Na+].

RESULTS Grams of EDTA to mass:
(0.01mol/L)x(1L)x(372.24g/mol)=3.7224g

[EDTA] dilution needed for complex titration:
C1V1=C2V2
(0.118mol)(10mL) = (C2)(40mL)
C2=0.00295mol

Moles of M2+:
M2+ (mols) = [EDTA] x EDTA (L)
M2+ (mols) = (0.00295mol/L)x(0.01250L)
M2+ (mols) =3.6875x10-5 mols [M2+]:
(moles of M2+)/(L of H2O)
(3.6875x10-5 mols)/(0.020 L of H2O)
M2+ = 0.00184375M Moles of positive charge, PC mols:
(NaOH L)x(NaOH M) = PC (mol)
(0.01675 L) (0.002802879 M) = PC (mol)
PC (mol) = 4.69x10-5 mols [PC]:
(PC (mol)) / (H2O L) = [PC]
(4.69x10^-5) / (0.01L) = [PC]
[PC] = 0.00469 M Grams of CaCl2:
CaCl2 (g) = (CaCl2 molar mass) x (CaCl2 (M)) x (H2O (L))
CaCl2 (g) = (110.99 g/mol) x (0.1 mol/L) x (0.1 L)
CaCl2 (g) = 1.11 g Finding [Na+] using Nernst Equation:
Ecell(mv)=-54.95(-log[Na+])+213.3
61.5mv=-54.95(-log[Na+])+213.3
[Na+]=0.00172778

Finding [H3O+]: pH = -log[H3O+]
[H3O+] = 10-pH
[H3O+] = 10-7.24
[H3O+] = 5.7544x10-8 M [Mg2+] = [M2+] - [Ca2+]
[Mg2+] = 0.00126M - 0.00105M
[Mg2+] = 0.000210 [K+] = [PC] - 2[M2+] - [Na+]
[K+] = 0.00633M - 2(0.00126M) - 0.00445M
[K+] = -0.00064M

Year
[H+] (M)
[Na+] (M)
[Ca2+] (M)
[K+] (M)
[Mg2+] (M)
[M2+] (M)
[PC] (M)
1947
2.51 x 10-7
0.00445
0.00105
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