lect 8 Flashcards
what are the dissolved solids in natural waters?
levels of dissolved solids vary widely
-ground water can be very high, e.g. >1000 ppm
Ca + HCO3- concentrations in rivers and lake do not parallel ocean levels
-fresh water dissolves ancient rocks containing CaCO3
-whereas oceans precipitate CaCO3 in the form of marine organism exoskeleton
what are topics to discuss in this lecture?
A. alkalinity
B. hardness
C. soft water (softening of hard water)
i. deionized water
what is alkalinity?
alkalinity: the capacity of water to accept H+ ions
-commonly defined as concentration of all bases in solution, usually HCO3- and CO2 and OH (these are major sources of alkalinity)
-usually given in term of mg/L CaCO3 equivalent
what are the minor sources of alkalinity?
ammonia, phosphates, silicates, and conj. bases of boric acid and organic acids
is pH a good indicator of alkalinity?
pH is sometimes used as an indication of alkalinity
PROBLEM: pH a poor measure of alkalinity because:
-weak carbonate bases more abundant than hydroxide ions
-many natural waters not in equilibrium with the atmosphere
what is one of the measurements of alkalinity?
- can measure either “phenolphthalein alkalinity”
what is another measurement of alkalinity?
“total alkalinity”
what are the units of alkalinity?
-moles H+ per litre (how much acid used to neutralize all bases) (also called eq/L)
-in terms of CaCO3 in mg/L (more common)
what is the influence of alkalinity on CO2 solubility?
-natural water has alkalinity of 1.00 x 10^-3 eq/L which comes from bicarbonate, carbonate and hydroxide concentrations
-pure water (alkalinity 0) if partial pressure of CO2=370 ppm
what are the difference between pure water vs. alkaline water
-most natural waters have pH=6->8.5, where HCO3- predominates
-natural water has alkalinity [alk] of 1.00 x 10^-3 eq/L (alkaline solutes in 1L of water will neutralize 1.00 x 10^-3 mol acid)
-note that subterranean waters can have very high alkalinity due to exposure to very high [CO2] generated by microbial oxidation of organic matter
what is hardness?
primarily due to Ca and Mg alkaline earth elements. Other elements e.g. Fe do contribute hardness
in fresh water, hardness and dissolved solids due to dissolution of rock:
what is calcium sulphate (Hardness)?
-calcium sulphate imparts ‘permanent hardness’ because it cannot be removed by boiling
-CaSO4 is less prone to leave scale in hot water pipes
what is calcium carbonate (hardness)?
this means water with CO2 dissolved in it will dissolve carbonate rocks
-also explains why kettles develop scale, but coffee makers dont
is there a difference between surface water vs. ground water?
- lower temp underground (5 celcius) gives higher CO2 solubility in ground water
- P(CO2) underground higher than at sea level pressure (up to 0.01atm) Note: this is greater than surface P(CO2)
- ‘temporary hardness’ can be removed by heating, precipitating CaCO3
-driving off of CO2(g) when boiling leads to eq shift to right…. (also leads to build up to scale in hot-water pipes, boilers, and kettles)
what is soft water?
soft water has low concentrations of Ca and Mg, and low concentrations of anions, e.g. carbonates, therefore soft water has low alkalinity (pH 7-8.5)
-naturally found in areas with granite bedrock
-HCO3- concentrations are 10^-4 to 10^-3 M in soft water, >10^-3 in hard water
-low alkalinity in soft water areas means low buffering capacity for acid rain (recall soil in ON have greater acidity than soils in SK-sewage sludge land spreading)
why would you want to soften water? (artificial water softening)
a) add lime (Ca(OH)2) or soda ash (Na2CO3) to precipitate Ca
b) can use ion exchanger resins with negative ion exchange sites
how does adding lime help with artificial water softening?
-soda ash added if hardness is mainly due to CaSO4
-this method is used industrially; it has the advantage of low cost
how does using an ion exchanger resins with negative ion exchange sites do with artificial water softening?
what is the graph of cation exchange resin?
what is the graph of anion exchange resin?
common exchange resins include:
-polystyrene with divinylbenzene or sulphonyl groups
-zeolites are porous sand-like materials which contain cation exchange sites (SiO2 and AlO4 units)
-Al in the zeolites provides the exchange sites and the polymer provides the support for the zeolites
what is the production of deionized water?
deionized water produced by using 2 ion exchangers in series
are the dissolved solids in seawater a different situation?
-major reservoir of soluble ions carried down by rivers
-contains major components by mass:
CaCO3, CaSO4, 2H20, NaCl 1:5: 250
we know there are large amounts of CaCO3/CaSO4/NaCl in seawater…. how?
-when seas dry up, evaporates form, made of gypsum (CaSO4), NaCl, or mixture of Na and Ca chlorides
-CaCO3 incorporated into exoskeletons of marine organisms
how do we determine equilibrium concentration of dissolved solids?
using Ksp
issues to deal with regarding apparent high concentrations of CaCO3 in seawater:
-ionic strength
-complexation
what is ionic strength?
ionic strength: measure of total concentration of ions in solution
-problem: ions in solution form an “ionic atmosphere”“double layer” around ions of interest, decreasing apparent charge
what is complexation?
-ions can form complexes with ions of opposite charges (counter ions), or with neutral species
-ions can form complexes with specific counter ions at measurable concentrations
what is the definition of activity and standard states?
what is the complexation with metal ions?
-in the case of metal ions, complexation occurs with the metal ion is bound to one or more ligands (h2O) as illustrated by the equilibrium
the reactions above show (complexation and chelation)
-complexation
-a ligand (CN-) binding to a metal ion, a reversible process
-formation of a complex (ion), or coordination compound
-CN- ion in the above example is a unidentate ligand
-ligation occurs with chelating agents that can bind in more than one place
what are some examples of formation constants?
what are hydrated metal ions as acids?
what are ligands?
-unidentate: a ligand with one binding site. e.g. amine, carboxylate ion, monophosphate phenoxide ion
-multi-dentate: a ligand with 2 or more binding sites (chelating agents)
-examples of chelating agents: ethylenediaminetetraacetic acid (EDTA), polyphosphates, ethylenediaminedisuccinic acid, nitrilotriacetate (NTA), humic acids nitrilotris (methylenephosphonic) acid (NTMP)
what is the graph of chelating agents (multi-dentate)?
what are polyphosphates and phosphonates in water?
what is the bonding and structure of metal complexes?
what are humic and fulvic acids?
