Water Chemistry

Dissolved Gases in Water

  • only a relatively few gasses are dissolved in natural waters under normal conditions; this is largely dependent on the amount of gas present in the air and how soluble it is; e.g.,

 

Air Composition

%

Solubility  (ml/L at 1 atm)

Total Solubility accounting for both

Nitrogen

78

    18.61

14.53

Oxygen

21

    38.46

  8.06

Argon

    0.9

    41.82

  0.39

Carbon dioxide

     0.03

1194.00

  0.39

 

    • others found in water as trace amounts are H2, CO, NO2 (nitrous oxide), O3 (ozone), CH4 (methane), NH3 (ammonia), SO2 (sulfur dioxide), krypton, and neon.

Oxygen (Dissolved Oxygen, DO)

  • includes only free dissolved oxygen, not that which is bound in compounds
  • one of the most fundamental parameters of a lake
  • >90% of aquatic organisms require it
  • is a vital player in biochemical reactions; e.g., under oxic conditions, phosphates are tied up in bottom muds; under anoxic conditions, phosphorous becomes soluble and circulates, triggering algal blooms.
  • solubility-exhibits an inverse nonlinear relationship:  as temperature increases, the amount of oxygen water can hold decreases.

oxygen solubility in fresh and sea water

         Hence, at 4C, water is saturated (100%) with oxygen, holding 10.92 mg/l.

o       Nota bene:  100% saturation does not mean that no more O2 can be held in solution.  I have measured DO >200%.  Does this mean that bubbles should be forming?  Not necessarily. Saturation here means that 10.92 mg/l can be held at equilibrium; if 200% is produced by intense photosynthetic activity, the extra amount will be lost (diffused) at the air/water interface.

Oxygen Solubility in Water at 760 mm Hg pressure

Temp. C

Oxygen (mg/L)

Temp. C

Oxygen (mg/L)

0

14.16

18

9.18

1

13.77

19

9.01

2

13.40

20

8.84

3

13.05

21

8.68

4

12.70

22

8.53

5

12.37

23

8.38

6

12.06

24

8.25

7

11.76

25

8.11

8

11.47

26

7.99

9

11.19

27

7.86

10

10.92

28

7.75

11

10.67

29

7.64

12

10.43

30

7.53

13

10.20

31

7.42

14

9.98

32

7.32

15

9.76

33

7.22

16

9.56

34

7.13

17

9.37

35

7.04

         is effected by altitude (air pressure); at higher altitudes there is less air pressure and less O2.  Saturation is usually figured for sea level pressure (760 mm = 1 ATM).  What is our altitude?

         on average, solubility decreases 1.4% with each 100 m increase in altitude.  More specifically:

0 600 m           = 4% decrease/300 m increase in altitude

600 1500 m     = 3% decrease/300 m increase in altitude

1500 3050 m   = 2.5% decrease/300 m increase in altitude

         pressure exerts a role, too; recall that pressure increases 1 ATM/10 m; at depth there is much more pressure on gases to stay in solution.  Hence, it is hard for bubbles to form against it.  Therefore it takes very high supersaturation rates (compared to surface pressure) to cause bubble formation.  (Think of a sealed vs. open bottle of beer.)

  • a nomogram can be used to determine degree of saturation; use a straightedge to connect the water temperature and DO.  Read the % saturation at the intersection of this line with the middle line.

 

Dissolved Oxygen % Saturation Nomogram

         at 10 meters, with a temperature of 10C, at surface pressure would hold (at 100% saturation) 10.92 mg, but you may find 15 mg/l.; compared to the surface it would be supersaturated, but at the depth and pressure its at, it may be less than saturated.

o       How can water be supersaturated?

         intense photosynthesis

         entrainment of air falling over a dam or spillway; high pressure of impact drives gases into solution; may lead to gas bubble disease, a problem in TVA dams

         affects fish if subjected for a few hours to >115% saturation; bubbles form in tissues; emboli collect in gills causing anoxia and death; also affects cladocerans.  Other biota, e.g., crayfish and stoneflies are hardier.

         effect of salinity.

o       oceanic salinity averages ~3.5% (35 );

o       at the same temperature, saltwater holds about 20% less O2 when saturated; e.g., at 0C freshwater is saturated at 14.2 mg/l, while saltwater is saturated at 12.0 mg/l.

Sources of gain/loss of O2 water

Gain

  1. dissolving at air/water interface
    1. a very slow process, would take years for any to reach 5 m;
    2. turbulence carries O2 from surface to depth; how far depends on how much of the lake is circulating; wind-driven waves and currents are critical.
    3. Note:  agitation and turbulence are a source of loss of O2 if waters are supersaturated.
  2. photosynthesis:           6 CO2  +  6 H2  C6H12O6 + 6O2
  3. inflow of oxygen rich water

Loss

  1. same as (1) above
  2. respiration by plants, animals, microbes

                       6 O2  +  C6H12O6    6 CO2  +  6 H2O

      3.   inflow of O2 poor water that may have a BOD/COD that uses up available O2.

      4.   increasing temperatures.

Oxygen Profiles

  • take oxygen levels at all depths (1 m intervals) to develop the profile.
  • oligotrophic lakes (oligo = few) are characterized by low nutrients and low productivity
    • little decomposition to use up O2, therefore usually high O2 in the hypolimnion.
    • usually lower O2 at the surface due to warmer temperatures
    • in the Great Lakes, Superior is the closest to oligotrophic.
    • orthograde profile

Orthograde Profile.  Dashed line represents temperature; solid line is D.O.

  • in eutrophic lakes (high nutrients, high productivity), enough things live, die, and respire that when they sink to the bottom, the decomposition may use all the O2.
    • the hypolimnion become anoxic (anaerobic)
    • in the Great Lakes, the most eutrophic is Erie.
    • clinograde profile develops soon after stratification in eutrophic lakes (by May 8 in Paint Creek Lake.)

Clinograde profile.  Dashed line represents temperature; solid line is D.O

    • most O2 loss is at the water/sediment interface
    • respiration use of O2 occurs throughout the water column; it is highest where most organisms are, in the photic zone.  There, however, O2 production is far in excess of use.
    • where does O2 production = O2 use?  The compensation point where ~2% of sunlight reaches (approximately the Secchi depth); therefore, at the bottom of the photic zone.
    • Recall that O2 can be used in non-biological, strictly chemical reactions, the chemical oxygen demand (COD).

Deviations from these profiles:

Heterograde curves (hetero = different)

  • caused by high/low concentration of DO at seemingly unlikely places
  • e.g., positive heterograde, caused by a metalimnetic O2 maximum-usually due to high concentration of photosynthesizers at metalimnion.  Why there?  Species adapted to low light and low temperature make use of more nutrients in the metalimnion compared to the epilimnion, e.g., the cyanophyte, Oscillatoria.

Positive Heterograde Profile. Dashed line represents temperature; solid line is D.O

         e.g., negative heterograde, caused by a metalimnetic O2 minimum-may be due to accumulation of dead/decomposing organisms caught at the density boundary.

Negative Heterograde Profile.  Dashed line represents temperature; solid line is D.O

River Longitudinal Profile

What caused this oxygen profile?

Diel cycle of epilimnetic oxygen content-how does oxygen content vary during a day?

Measurement of DO

Two dominant methods today are

  1. oxygen meter/probe
  2. Winkler method of O2 titration.

Oxygen meter

         O2 crosses a membrane, creates a current flow across electrodes; the more O2, the more current

        needle-type meters are read directly to avoid parallax error using a mirror

         requires agitation as oxygen is used up at the electrodes, so if not agitated, DO level will gradually drop.

         must be regularly calibrated

         only trust machines if they are trustworthy!

Winkler titration-a lab test (bench chemistry) that is the basis for Hach kit tests.

  1. collect discrete water samples from various depths with Kemmerer or van Dorn bottles.
  2. transfer to and overfill BOD bottles without turbulence;
  3. Winkler chemistry

         manganous sulfate (Reagent 1) reacts with the potassium hydroxide-potassium iodide (alkaline-iodide - Reagent 2) to produce a white flocculent precipitate of manganous hydroxide:

MnSO4 + 2 KOH   Mn(OH) 2 + K2SO4 (white) 

      • if there is any DO in the water a second reaction between the Mn(OH)2 and DO occurs immediately to form a brownish manganic oxide floc:

2Mn(OH)2 + O2   2MnO(OH)2 (brown)

      • a further reaction with potassium iodide releases an amount of iodine equivalent to the original oxygen.

2Mn(SO4) 2 + 4KI   2MnSO4 + 2K2SO4 + 2I2

      • if any bubbles are present up to this point, the sample must be discarded and you must begin again.
      • from this time on you are titrating the iodine, so the sample can no longer be contaminated by bubbles or atmospheric oxygen; hence, it can be worked up at a later time.
      • using sodium thiosulfate, titrate the solution from iodine color to clear; to aid in finding the endpoint accurately, starch may be added.  This will turn the mixture blue-black; when the color disappears, the endpoint has been reached.
      • high levels of nitrate, as well as hydrogen sulfide and reducing materials will interfere, yielding inaccurate results.
      • concentrations are generally expressed in millimolar, μMoles/kg, mg/L or ml/L (1 ml/L = 1.33 mg/L).

Reliability of your DO determinations

  1. only as good as your reagents are.  The critical one is the titrant (PAO or sodium thiosulphate), therefore, must standardize, i.e., compare your titrant to a sample of known amount.  Directions for this are in "Standard Methods", aka The Bible.

  2. many things can cause erroneous determinations

    • not your fault-interferences-azide removes nitrates which are common in polluted water; other interferences are iron and high suspended solids.  May cause too much or too little I2 to be released.

    • your fault-sloppy technique, such as dirty glassware or careless titration (missed endpoint).

  3. accuracy and precision

    • accuracy-how close to the real value?  STANDARDIZE!

    • precision-how reproducible are your results?  would you get the same answer if you titrated again; would someone else get the same answer using your sample?

Hardness

  • measure of the capacity of water to precipitate soap;

  • ions in water combine with soap to form insoluble precipitates; prevents sudsing until the combination is complete

  • causes the hard, white crust on kettles, etc. when the water is heated

  • caused mainly by calcium and magnesium, but can also be from Al, Fe, Sr, Mn, or Zn.  These are usually insignificant.

  • often expressed as equivalent CaCO3 concentration.

hardness = ____ mg/L as CaCO3

  • broken into temporary hardness and permanent.

  • temporary hardness is that which precipitates on heating; it is also carbonate hardness (mainly CaCO3 and MgCO3

  • permanent hardness is the part that stays in solution when boiled.