1. Field Methods of Monitoring Aquatic Systems
Unit 5 – pH, Acidity and Alkalinity
Copyright © 2008 by DBS

2. Title
Pure water is neither acidic or basic because it contains equal concentrations of hydroxide and hydronium ions

3. Role of pH in Water Quality
Brønsted-Lowry definition
Acid is a proton donor
HCl + H2O → H3O+ + Cl-
Base is a proton acceptor
NH3 + H2O → NH4+ + OH-
Acidic: H+ > OH- Basic: OH- > H+

4. pH Scale pH = -log10 [H+] [H+] = 10-pH
Typically 0 – 14 (can go beyond this)
[H+] = [OH-] = 1.0 x 10-7 moles L-1 (pH = 7, neutral)
For each change of one pH unit [H+] changes x10
Or pH = -log10 [H3O+]

5. pH of Common Substances
Battery acid
0.3
Lemon juice
2.4
Urine
Rainwater
Blood
Bleach
10.5
Ammonia
11.5

6. Typical pH Values
Reeve, 2002

7. Rainwater
Unpolluted rain water is slightly acidic due to dissolved CO2 (NO2 and SO2), pH ~ 5.6
H2O(l) + CO2(g) ⇌ H2CO3(aq)
⇌ H+(aq) + HCO3-(aq) ⇌ 2H+(aq) + CO32-(aq)
Gas
Natural
Anth.
CO2
NO2
SO2

8. PA Acid Deposition
Aerochem Metrics wet/dry precipitation collector

9. Question
We must always hold an objective view. If you look for it there is a positive side of the existence of acid rain. What could this be?
Acid rain cleans the atmosphere of pollutants

10. Alkalinity
Measure of the ability of a water body to neutralize acidity
Dissolution of limestone and other minerals produces alkalinity
e.g.
CaCO3 ⇌ Ca2+ + CO32-
CO32- + H2O ⇌ HCO3- + OH-
Water supply with high total alkalinity is resistant to pH change
Two samples with identical pH but different alkalinity behave differently on addition of acid
Different capacity to neutralize acid
Mineral
Composition
Calcite
CaCO3
Magnesite
MgCO3
Dolomite
CaCO3.MgCO3
Brucite
Mg(OH)2

11. Alkalinity
Measurement of the buffer capacity (resistance to pH change)
e.g. Carbonate neutralization reaction
CO32- + H+ ⇌ HCO3-
Bicarbonate neutralization reaction
HCO3- + H+ ⇌ H2O.CO2 ⇌ H2O + CO2
Hydroxide neutralization reaction
H+ + OH- ⇌ H2O
Alkalinity = [OH-] + [HCO3-] + 2[CO32-] – [H+]
Units are mg L-1 CaCO3 or mEq L-1 (regardless of species)
Acid titration to change the pH to 4.5 (methyl orange end-point)
If pH < 4.5 there is no acid neutralizing capacity i.e. no need to measure alkalinity

12. Biological and Chemical Effects
of acidification of waters
Sensitivity of fish populations
Salmon populations decrease below 6.5
Perch below 6.0
Eels below 5.5
Increases solubility of metals
Toxic Al3+ and Pb 2+ release
Particuarly from soils (aluminosilicates)
Increases weathering of minerals and crustaceans

13. Water Quality Public Health Service Act accepted level 6.5-8.5
Public health concern is corrosion and leaching of toxic metals (Pb, Cu, Zn, Fe) from metal pipes

14. Measuring pH Electrochemical Colorimetric
Remove sample from refrigerator ~30 mins prior to analysis
Measure on unfiltered samples
Samples may be stored for 24 hrs at 4 °C prior to analysis

15. Electrochemical Analogy:
Electrochemical potential - known pH liquid inside the glass H+ sensitive membrane electrode vs. unknown outside
Circuit is closed through the solutions - internal and external - and the pH meter
Electrodes generate a voltage directly proportional to the pH of the solution
pH 7 potential is 0 V
< 7 +ve V, > 7 –ve V
Analogy:
Battery where +ve is measuring electrode, -ve is reference electrode

16. Slows leak but gets contaminated (shorter life-span)
Flowing
Internal KCl slowly flows to the outside through the junction (salt bridge)
Must be refilled!
Gelled
Slows leak but gets contaminated (shorter life-span)
Source:

17. Thin Glass Membrane Aluminosilicate (Al2SiO5)
Kegley description is incorrect, not controlled by H+ but Na+

18. Electrochemical Potential
Nernst equation
Ecell = constant – pH (at 25 °C)
Calibrated with buffer solutions of known pH
Straight line plot of Ecell vs. pH

19. red → yellow yellow → blue
Colorimetric
Indicator
Color (acidic → basic)
pH Range
Malachite green
yellow → green
Thymol blue
red → yellow yellow → blue
Methyl orange
red → yellow
3.2 – 4.4
Bromocresol green
Yellow → blue
Methyl red
Red → yellow
Bromothymol blue
Cresol red
Yellow → red
Phenolphthalein
Colorless → pink
Thymolphthalein
Colorless → blue
Alizarin yellow
Acid-base indicator solution or indicator paper
Indicators are large organic molecules that change color depending on pH
e.g, cresol red is yellow < 7.0 and red > 8.8 and various shades in between

20. Measuring Total Alkalinity
Remove sample from refrigerator ~30 mins prior to analysis
Measure on unfiltered samples
To unfiltered sample add strong acid of known concentration, ( M H2SO4) titrate to pH 4.5
CaCO3 + H2SO4 → H2CO3 + CaSO4
Net ionic: CO H+ → H2CO3
Range mg CaCO3 L-1
Rainwater < 10
Surface water < 200
Groundwater > 1000 (due to MO decomposition)

21. Indicator Methyl Orange end-point ~4.5 Difficult to see
More precise indicator is a bromocresol green/methyl red mixture
5.2 – green-blue
5.0 – light blue with lavender grey
4.8 – light pink with blue cast
4.6 light pink
From Boehnke, D.N.

22. Question
What is the total alkalinity for a sample requiring mL of M H2SO4?
L x mol L-1 = x 10-4 mol H2SO4
Mole ratio is 1:1
2.125 x 10-4 moles H2SO4 = x 10-4 moles CaCO3
2.125 x 10-4 mol CaCO3 x g / mol = 2.13 x 10-2 g = 21.3 mg
21.3 mg CaCO3 = 213 mg CaCO3 L-1
0.100 L

23. Units Units are mg L-1 CaCO3 or mEq L-1 (regardless of species)
mEq L-1 = mg L-1 CaCO3 divided by 50
CaCO3 + 2H+ ⇌ H2CO3
mg x 1 mmol x 2mEq = mEq
L 100 mg mmol L
mg x 1/50 = mEq
L L

24. Field Method / High-Throughput Labs
Hach Titrator
Cartridge based system
100 mL cylinder
250 mL beaker
Source:

25. Text Books
Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil. Wiley-VCH.
Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker.
Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods. CRC Press.
Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis Publishers.
Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books.
Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative methods. Taylor & Francis.
Reeve, R.N. (2002) Introduction to environmental analysis. Wiley.
Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the Examination of Water and Wastewater, 20th Edition. Published by American Public Health Association, American Water Works Association and Water Environment Federation.