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Standard Methods for the Examination of Water and Wastewater

2320

2320 A
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Discussion
Alkalinity of a water is its acid-neutralizing capacity
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The measured value may vary significantly with the end-point pH used
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Alkalinity is significant in many uses and treatments of natural waters and wastewaters
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The
measured values also may include contributions from borates, phosphates, silicates, or other
bases if these are present
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Alkalinity measurements are used in the
interpretation and control of water and wastewater treatment processes
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Properly operating anaerobic digesters typically have supernatant alkalinities in the range of 2000
to 4000 mg calcium carbonate (CaCO3)/L
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Reference
1
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G
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E
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1963
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J
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Control Fed
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2320 B
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General Discussion
a
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Alkalinity thus depends on the end-point pH used
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1a
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The amount of standard acid required to reduce pH exactly 0
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Because this change in pH corresponds to an exact doubling of the hydrogen
© Copyright 1999 by American Public Health Association, American Water Works Association, Water Environment Federation

Standard Methods for the Examination of Water and Wastewater
ion concentration, a simple extrapolation can be made to the equivalence point
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End points: When alkalinity is due entirely to carbonate or bicarbonate content, the pH at
the equivalence point of the titration is determined by the concentration of carbon dioxide (CO2)
at that stage
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The pH values in Table 2320:I are
suggested as the equivalence points for the corresponding alkalinity concentrations as milligrams
CaCO3 per liter
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3 irrespective of the colored indicator, if any, used in the
determination
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3
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5
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Interferences: Soaps, oily matter, suspended solids, or precipitates may coat the glass
electrode and cause a sluggish response
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Do not filter, dilute,
concentrate, or alter sample
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Selection of procedure: Determine sample alkalinity from volume of standard acid
required to titrate a portion to a designated pH taken from ¶ 1b
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If using
color indicators, prepare and titrate an indicator blank
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Construct a titration curve for standardization of reagents
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e
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1e for selection of size sample to be titrated and normality
of titrant, substituting 0
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1N sulfuric (H2SO4) or hydrochloric (HCl) acid for the
standard alkali of that method
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02N H2SO4 from a 10-mL buret
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Sampling and storage: See Section 2310B
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2
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2
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Reagents
a
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05N: Dry 3 to 5 g primary standard Na2CO3
at 250°C for 4 h and cool in a desiccator
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5 ± 0
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Do not
keep longer than 1 week
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Standard sulfuric acid or hydrochloric acid, 0
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Standardize against 40
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05N
Na2CO3 solution, with about 60 mL water, in a beaker by titrating potentiometrically to pH of
about 5
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Cool to room temperature, rinse cover glass into beaker, and finish titrating to
the pH inflection point
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Use measured normality in calculations or adjust to 0
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1000N solution = 5
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c
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02N: Dilute 200
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1000N standard
acid to 1000 mL with distilled or deionized water
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00 mL 0
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00 mg CaCO3
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Bromcresol green indicator solution, pH 4
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e
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2) Dissolve 100 mg bromcresol green and 20 mg methyl red in 100 mL 95% ethyl alcohol or
isopropyl alcohol
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Metacresol purple indicator solution, pH 8
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g
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3 indicator
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Sodium thiosulfate, 0
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3i
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Procedure
a
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4b
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Potentiometric titration curve: Follow the procedure for determining acidity (Section
2310B
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5 or lower
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c
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Prepare sample and titration assembly (Section 2310B
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Titrate to the
end-point pH without recording intermediate pH values and without undue delay
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d
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02N standard
acid solution
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3 to 4
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Carefully add additional titrant to reduce the pH exactly 0
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5
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Potentiometric titration to end-point pH:

where:
A = mL standard acid used and
N = normality of standard acid
or

where:
t = titer of standard acid, mg CaCO3/mL
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b
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3 unit lower, and
N = normality of acid
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Calculation of alkalinity relationships: The results obtained from the phenolphthalein and
total alkalinity determinations offer a means for stoichiometric classification of the three
principal forms of alkalinity present in many waters
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It further presupposes the
incompatibility of hydroxide and bicarbonate alkalinities
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According to
this scheme:
1) Carbonate (CO32–) alkalinity is present when phenolphthalein alkalinity is not zero but is
less than total alkalinity
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3) Bicarbonate (HCO3–) alkalinity is present if phenolphthalein alkalinity is less than half the
total alkalinity
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Then, carbonate alkalinity equals twice the smaller
value
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When the smaller value is
(T−P), the balance (2P−T ) is hydroxide
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The mathematical
conversion of the results is shown in Table 2320:II
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4)
Alkalinity relationships also may be computed nomographically (see Carbon Dioxide,
Section 4500-CO2)
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Precision and Bias
No general statement can be made about precision because of the great variation in sample
characteristics
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In the range of 10 to 500 mg/L, when the alkalinity is due entirely to carbonates or
bicarbonates, a standard deviation of 1 mg CaCO3/L can be achieved
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The titration procedure of ¶ 4b was used, with an end point pH of 4
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The
standard deviation was 5 mg/L and the average bias (lower than the true value) was 9 mg/L
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6 The standard deviations were 8 and 5 mg/L,
respectively, with negligible bias
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The pooled standard deviation was 40 mg/L, with
negligible difference between the procedures
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References
1
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E
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M
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1955
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Anal
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27:851
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THOMAS, J
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J
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J
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1960
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J
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Water Works Assoc
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3
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S
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The mixed indicator bromocresol green-methyl red for carbonates in
water
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Eng
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, Anal
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13:466
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JENKINS, S
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& R
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MOORE
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A proposed modification to the classical method of
calculating alkalinity in natural waters
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Amer
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69:56
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WINTER, J
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& M
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MIDGETT
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FWPCA Method Study 1
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Federal Water Pollution Control Admin
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C
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SMITH, R
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Research Rep
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379, Council for Scientific and Industrial Research,
South Africa
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Bibliography
1982
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Publ
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Testing & Materials,
Philadelphia, Pa
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W
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J
...
C
...
E
...
S
...
1979
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In Techniques of
Water-Resources Investigation of the United States Geological Survey
...
S
...
C
...


© Copyright 1999 by American Public Health Association, American Water Works Association, Water Environment Federation

Standard Methods for the Examination of Water and Wastewater
Endnotes
1 (Popup - Footnote)
* APPROVED BY STANDARD METHODS COMMITTEE, 1997

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