Water Chemistry

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Water Chemistry
1. Units for expressing dissolved substance
concentrations 2. Measuring concentrations of
dissolved solids
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Units
mg/L -- mass per volume Suppose you dissolve 5
mg of NaNO3 in 1 L of water. What is the
concentration? Concentration of what?
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Concentration of NaNO3
5 mg/L
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Concentration of nitrate (NO3)
What fraction of NaNO3 is nitrate? Na atomic
weight 23 N atomic weight 14 O atomic
weight 16
NaNO3 2314(316)85 NO3 14(316)62 Fractio
n of NaNO3 that is NO3 62/85 0.729
So, 5 mg/L of NaNO3 0.729 3.65 mg/L of NO3
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Concentration of nitrogen (N)
14 mg N / 62 mg NO3 0.226 0.226 3.65 0.824
mg N / L or 0.824 mg NO3-N /L
n.b. -- NO3-N means nitrogen in the form of
nitrate, NOT nitrate with another nitrogen atom
hooked on!
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Milliequivalents / Lexpresses ionic strength
Suppose you had two solutions of cations. One
had 10 mg/L of Ca and the other Had 10 mg/L of
Na. How much anion will each react with?
Depends on equivalent weight.
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So, the 10 mg/L calcium solution would react with
more anion than would the 10 mg/L sodium solution.
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We can use mEq concentrations to check our data.
Anions and cations should balance if we have
included everything.
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Example calcium 15 mg/L (15 mg/L) / (20.05
mg/mEq) 0.750 mEq/L
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ppm parts per millionmass per mass
Convert 10 mg/L of Fe to ppm How much mass is 1
L of water? Usually 1.000 g/mL (or 1000
g/L) So, 10 mg Fe / 1000 g water or, 10 mg Fe /
1,000,000 mg water or, 10 mg Fe / million mg
water or, 10 ppm Fe
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So, ppm mg/L
Usually!
Suppose you had a water sample from a desert lake
with a total dissolved solids (TDS) concentration
of 7,000 mg/L. If you add 10 mg of sodium to a
liter of this solution (and there was none there
before), what is the sodium concentration in ppm?
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10 mg Na 1 L of water 10 mg/L That L of
water weighs 1000 g (1 million mg), but it also
contains 7000 mg of TDS. So, 10 mg Na /
1,007,000 mg 9.9 ppm
This is unusual. Also note that the density of
water changes with temperature but not enough to
change concentration calculations except under
extreme conditions.
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µg/L 1/1000th of a mg/L ppb 1/1000th of a
ppm µEq 1/1000th of a mEq
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How measure dissolved solid concentrations?

• Weigh solid, dissolve in measured amount of water
  i.e., make the solution
• Evaporate and weigh whats left
• Conductivity
• Titration
• Spectrophotometry
• Chromatography

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• Suppose you had a water sample with 120 mg/L TDS
  (world average), but you didnt know this and
  wanted to find the TDS concentration.
• Filter 25 ml of sample
• Put in weighed (tared) crucible and dry at 105oC.
• Weigh

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25 ml 120 mg/L 3 mg 71.64 g
(crucible) 0.003 g (solids) 71.64 g
This method only works for very high
concentrations.
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3. Measure conductivity
The ability of water to conduct electricity
depends on the presence of dissolved ions. Thus
conductivity is an indicator of dissolved solids.

Conductivity is measured as the reciprocal of the
resistance between two electrodes 1 cm2 and 1 cm
apart. It is usually measured in µS
(microsiemens, after a British engineer of the
1800s) or more properly µS/cm. Since
conductivity is affected by temperature, we
usually use specific conductance, the
conductivity if the solution was at 25oC.
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4. Titration
Alkalinity the acid buffering capacity of
water (Note it is NOT how alkaline or acid a
solution is. Some acidic waters may have high
alkalinity) To measure alkalinity, we titrate a
known volume of solution with a known
concentration of acid until it reaches a certain
pH (8.3 then 4.4). The more acid it takes to
lower the pH to 4.4, the greater the alkalinity.
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In natural waters alkalinity usually depends on
the carbonate buffering system (lots more on this
later!), and we usually express alkalinity in
units of carbonate
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Hardness the soap neutralizing power of water.
It used to be measured by titration with a soap
solution. Now we have a simpler and more
accurate titration.
Hardness generally reflects the presence of Ca
and Mg and heavy metals. Since Ca is usually
the most abundant, hardness is usually expressed
in units of CaCO3. In natural waters,
alkalinity and hardness are usually related, but
not always. They do not measure the same thing.
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• Spectrophotometry
• light
• atomic absorption
• flame emission
• plasma emission
• mass spectrophotometry

• Chromotography
• HPLC (high pressure liquid chromotography)
• GC (gas chromotography)
• IC (ion chromotography)

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• Light spectrophotometry
• Combine target chemical with another chemical
  that produces a color
• Measure the intensity of the color

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Example measuring phosphate concentration
(actually SRP, soluble reactive phosphate)

• Convert the phosphate to phosphomolybdate
• Reduce with ascorbic acid in the presence of
  antimony forms a blue complex that adsorbs
  light at a wavelength of 882 nm
• Measure the color intensity

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Since the PO4 Mo complex absorbs only in the
882 nm range, the better we focus on this range,
the better our accuracy. With a colorimeter,
this is done with a filter. With a
spectrophotometer this is done with a prism and a
grating.
prism
slit
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Rotate the prism to get desired wavelength. The
narrower the slit the higher the precision
i.e., less interference Spec20 20 nm Really
good specs 1 nm
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Standard Curve

• We have to have a way to convert absorbance to
  concentration
• Make a set of solutions with known PO4
  concentrations
• Measure their absorbance
• Make a graph of absorbance versus concentration
• Use this graph (or regression equation) to get
  concentration of samples

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Lake Morphology

• How do we describe the physical characteristics
  of a lake?

Make a list
Slide of lake