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Alkalinity and hardness

Alkalinity and hardness. Chemical Variables: Total Alkalinity. Total Alkalinity : the total amount of titratable bases in water expressed as mg/L of equivalent CaCO 3.

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Alkalinity and hardness

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  1. Alkalinity and hardness

  2. 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.

  3. 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?

  4. 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 biofiltrationcapacity. pH independent (What does this mean?) • Low alkalinity = low CO2 --> low natural productivity • Low alkalinity equals low pH.

  5. 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!

  6. 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

  7. 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-

  8. 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).

  9. 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).

  10. 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.

  11. 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.

  12. CO2 Level Affects Hemoglobin Saturation

  13. Chemical Variables: carbon dioxide • 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?

  14. The Effect of pH on Carbonate Buffering

  15. Chemical Variables: carbon 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

  16. 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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