Alkalinity, Hardness, pH

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ALKALINITY AND HARDNESS
Chemical Variables:
Total Alkalinity
 Total Alkalinity: the total amount of titratable
bases in water expressed as mg/L of equivalent
CaCO3.
 “Alkalinity” is primarily composed of the following
ions: CO3-. HCO3-. hydroxides. ammonium. borates.
silicates. phosphates.
 Alkalinity in ponds is determined by both the quality
of the water and bottom muds.
 Calcium is often added to water to increase its
alkalinity. buffer against pH changes.
Total Alkalinity
 Total alkalinity = 200 mg/L. Good buffering capacity of
a water source.
 Freshwater 5 mg/L (soft water) to 500 mg/L (hard
water).
 Seawater ~ 115-120 mg/L.
 Seldom see pH problems in natural seawater.
 Alkalinity < 30 mg/L? Problem?
Total Alkalinity
Total Alkalinity (TA) level can be associated with several
potential problems in aquaculture:
 If TA< 50 mg/L: copper compounds are more toxic.
avoid their use as algicides (copper sulfate)
 Natural waters with less than 40 mg/L alkalinity as
CaCO3 have limited biofiltration capacity. pH
independent (What does this mean?)
 Low alkalinity = low CO2 --> low natural productivity
 Low alkalinity equals low pH.
Total Hardness
 Total Hardness: total concentration of metal ions




expressed in terms of mg/L of equiva- lent CaCO3.
Primary ions are Ca2+ and Mg2+. also iron and
manganese.
Total Hardness approximates total alkalinity.
Calcium is used for bone and exoskeleton formation
and absorbed across gills.
Soft water = molt problems.
bone deformities....
or minimally...clogged pipes!
CONVERSION OF WATER HARDNESS UNITS
Water Hardness Internatio Physical America & English
oe
Unit
nal
measures states
Definition
recomme mval/liter
PPM
nded
mmol/liter
French
of
German
odH
100mg CaCO3 28 mg CaO or 1 part CaCO3 1 grainCaCO3 10 mg/ CaCO3 10 mg/CaO
per 1000 ml 50 mg CaCO3 per million = 1 per gallon= per 1000 ml per 1000 ml
water
per 1000 ml
mg CaCO3
14.3 mg
water
water
water
per 1000 ml
CaCO3 per
water
1000 ml water
1 mmol/litre
1
2
100
7.0
10.00
5.6
1 mval/litre
0.5
1
50
3.5
5.00
2.8
1 PPM
0.01
0.02
1
0.070
0.10
0.056
1 Oe
0.1429
0.285
14.29
1
1.429
0.7999
1 Of
0.10
0.20
10.00
0.700
1
0.5599
1 OdH
0.1786
0.357
17.86
1.250
1.786
1
DEGREE OF HARDNESS EXPRESION
SOFT: < 1.6 mmol/l = 160 PPM = 9 odH
SLIGHTLY HARD: 1.6-3.2 mmol/l = 160-320 PPM = 9-18 odH
HARD: 3.2-4.6 mmol/l = 320-460 PPM = 18-26 odH
VERY HARD: ABOVE 4.6 mmol/l = ABOVE 460 PPM =
ABOVE 26 odH
Chemical Variables:
pH
 pH: the level or intensity of a substance’s acidic
or basic character.
 pH: the negative logarithm of the hydrogen ion
concentration (activity) of a substance.
 pH = -log(1/[H+]).
 Ionization of water is low (1x10-7 moles of H+/L
and 1x10-7 moles OH-/L).
 Neutral pH = similar levels of H+ and OH-
Chemical Variables:
pH
 At acidic pH levels. the quantity of H+ predominates.
 Acidic pH = pH < 7. basic = pH >7
 Most natural waters: pH of 5-10. usually 6.5-9;
however. there are exceptions.
 Acid rain. pollution.
 Can change due to atm. CO2 , fish respiration.
 pH of ocean water is stable (carbonate buffering
system. later).
Chemical Variables:
pH
 Other sources of change:
 Decay of organic matter.
 Oxidation of compounds in bottom sediments.
 Depletion of CO2 by phytoplankton on diel basis.
 Oxidation of sulfide containing minerals in bottom soils
(e.g.. oxidation of iron pyrite by sulfide oxidizing bacteria
under anaerobic conditions).
Chemical Variables:
Carbon Dioxide
 Normal component of all natural waters.
 Sources: atmospheric diffusion. respiration of
cultured species. biological oxidation of organic
compounds.
 Usually transported in the blood as HCO3 Converted to CO2 at the gill interface. diffusion
into medium.
 As the level of CO2 in the medium increases. the
gradient allowing diffusion is less.
Chemical Variables:
Carbon Dioxide
 This causes blood CO2 levels to increase. lowering
blood pH.
 With lower blood pH. carrying capacity of
hemoglobin decreases. also binding affinity for
oxygen to hemoglobin.
 This phenomenon is known as the Bohr-Root
effect.
 CO2 also interferes with oxygen uptake by eggs and
larvae.
CO2 Level Affects Hemoglobin
Saturation
Chemical Variables:
dioxide
carbon
 In the marine environment. excesses of CO2 are




mitigated by the carbonate buffering system.
CO2 reacts with water to produce H2CO3. carbonic
acid.
H2CO3 reacts with CaCO3 to form HCO3(bicarbonate) and CO32- (carbonate).
As CO2 is used for photosynthesis. the reaction shifts
to the left. converting bicarbonates back to CO2.
What large-scale implications does this have?
The Effect of pH on
Carbonate Buffering
Chemical Variables:
dioxide
 Concentrations of CO2 are
small. even though it is
highly soluble in water
 inverse relationship
between [CO2] and
temperature/salinity
 thus. CO2 solubility depends
upon many factors
carbon
Chemical Variable:
Carbon Dioxide
 CO2 is not particularly toxic to fish or
invertebrates. given sufficient D.O. is available.
 Maximum tolerance level appears to be around
50 mg/L for most species.
 Good working level of around 15-20 mg/L.
 Diel fluctuation opposite to that of D.O.
 Higher levels in warmer months of year.

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