On the other hand, the total alkalinity is measured by titrating the sample to pH 4.5 using bromocresol green as the indicator.. In this titration, a standard acid titrant is added to a
Trang 2SPECIF IC CLASSES OF SUBSTANCES
2
© 1997 by CRC Press LLC
Trang 5Alkalinity of water is a measure of its acid-neutralizing ability The titrable bases that contribute to the total alkalinity of a sample are generally the hydrox-ides, carbonates, and bicarbonates However, other bases such as phosphates, borates, and silicates can also contribute to the total alkalinity The alkalinity value depends on the pH end point designated in the titration The two end points commonly fixed in the determination of alkalinity are the pH 8.3 and pH 4.5 (or between 4.3 and 4.9, depending on the test conditions) When the alkalinity
is determined to pH 8.3, it is termed phenolphthalein alkalinity In such alkalinity titration, phenolphthalein or metacresol purple may be used as an indicator On the other hand, the total alkalinity is measured by titrating the sample to pH 4.5 using bromocresol green as the indicator Alkalinity may also be determined by potentiometric titration to the preselected pH An acid standard solution, usually 0.02 N H2SO4 or HCl, is used in all titrations
The procedure for potentiometric titration is presented in Chapter 1.6 In this titration, a standard acid titrant is added to a measured volume of sample aliquot
in small increments of 0.5 mL or less, that would cause a change in pH of 0.2 unit or less per increment The solution is stirred after each addition and the pH
is recorded when a constant reading is obtained A titration curve is constructed, plotting pH vs cumulative volume titrant added The volume of titrant required
to produce the specific pH is read from the titration curve
CALCULATION
where V is mL standard acid titrant used and N is normality of the standard acid
Since the equivalent weight of CaCO3 is 50, the milligram equivalent is 50,000 The result is, therefore, multiplied by the factor of 50,000 to express the alkalinity as mg CaCO3/L
Alkalinity, mgCaCO L =
mL sample
3/ V× ×N 50 000,
2.2
Trang 6
Bromide (Br–) is the anion of the halogen bromine, containing an extra electron
It is produced from the dissociation of bromide salts in water It may occur in ground and surface waters as a result of industrial discharges or seawater intrusion
Bromide in water may be analyzed by one of the following three methods:
1 Phenol red colorimetric method
2 Titrimetric method
3 Ion chromatography
While the first method is used for low level detection of bromide in the range 0.1 to 1 mg/L, the concentration range for the titrimetric method is between 2 and
20 mg/L The samples may be diluted appropriately to determine bromide con-centrations at higher range Ion chromatography is used to analyze many anions including bromide and is discussed in Section 1.8
PHENOL RED COLORIMETRIC METHOD
Bromide ion reacts with a dilute solution of sodium p -toluenesulfonchlora-mide (chloramine-T) and is oxidized to bromine which readily reacts with phenol red at pH 4.5 to 4.7 The bromination reaction with phenol red produces a color that ranges from red to violet, depending on the concentration of bromide ion
An acetate buffer solution is used to maintain the pH between 4.5 and 4.7 The presence of high concentration of chloride ions in the sample may seriously interfere in the test In such cases, the addition of chloride to the pH buffer solution or the dilution of the sample may reduce such interference effect Remove free chlorine in the sample by adding Na2S2O3 solution In addition, the presence
of oxidizing and reducing agents in the sample may interfere in the test
Pr ocedure
A 50-mL sample aliquot is treated with 2 mL buffer solution followed by
2 mL phenol red indicator and 0.5 mL chloramine-T solution Shake well after
2.3
© 1997 by CRC Press LLC
Trang 7Chloride (Cl–) is one of the most commonly occurring anions in the en vi-ronment It can be analyzed using several different methods, some of which are listed below:
1 Mercuric nitrate titrimetric method
2 Argentometric titrimetric method
3 Automated ferricyanide colorimetric method
4 Gravimetric determination
5 Ion-selective electrode method
6 Ion chromatography
Methods 1 and 3 are EPA approved (Methods 325.3 and 325.1-2, respec-tively) for chloride determination in wastewater For multiple ion determination, ion chromatography technique should be followed (see Section 1.8)
MER CURIC NITRATE TITRIMETRIC METHOD
Chloride reacts with mercuric nitrate to form soluble mercuric chloride The reaction is shown below for calcium chloride (CaCl2) as a typical example
The analysis may be performed by titrimetry using a suitable indicator
Diphenyl carbazone is a choice indicator that forms a purple complex with excess mercuric ions in the pH range of 2.3 to 2.8 Therefore, the pH control is essential
in this analysis Xylene cyanol FF is added to diphenyl carbazone to enhance the sharpness of the end point in the titration Nitric acid is used to acidify the indicator to the required low pH range
Other halide ions, especially bromide and iodide, are interference in this analysis Acidify alkaline samples before analysis Fe3+, CrO42–, and SO32– at concentrations above 10 mg/L are often used to interfere with the analysis
CaCl2+Hg NO( )3 2 ⇒HgCl2+Ca NO( )3 2
2.4
Trang 9
The formula of cyanate is CNO– It is a univalent anion formed by partial oxidation of cyanide (CN–)
Under neutral or acidic conditions, it may further oxidize to CO2 and N2 The analysis of cyanate is based on its total conversion to an ammonium salt
This is achieved by heating the acidified sample The reaction is shown below:
The concentration of ammonia (or ammonium) minus nitrogen before and after the acid hydrolysis is measured and the cyanate amount is calculated as equivalent
to this difference
Calculation
Thus, the amount of NH3–N produced from mg CNO–/L = A – B
where A = mg NH3–N/L in the sample portion that was acidified and heated, and
B = mg NH3–N/L in the original sample portion
Therefore, the concentration of cyanate as mg CNO–/L = 3.0 × (A – B)
[In the above calculation for cyanate, the concentration of ammonia–nitrogen was multiplied by 3 because the formula weight of CNO– is (12 + 14 + 16) or 42, which is three times 14 (the atomic weight of N, as ammonia–N).]
Pr ocedure
Add 0.5 to 1 mL of 1:1 H2SO4 to a 100-mL portion of sample acidifying to
pH 2 to 2.5 Heat it to boiling for 30 min Cool to room temperature and bring
up to the original volume by adding NH3-free distilled water
CN – →O CNO–
2 KCNO + H SO2 4+4 H O2 ⇒(NH ) SO4 2 4+2 KHCO3
2.5
Trang 10
CYANIDE, TOTAL
Cyanides are metal salts or complexes that contain the cyanide ion (CN–) These cyanides could be subdivided into two categories: (1) simple cyanides such
as NaCN, NH4CN, or Ca(CN)2 containing one metal ion (usually an alkai or alkaline-earth metal or ammonium ion) in its formula unit, and (2) complex cyanides such as K4Ce(CN)6 or NaAg(CN)2 containing two different metals in their formula unit, usually one is an alkali metal and the other a heavy metal The complex cyanide dissociates to metal and polycyanide ions The latter may further dissociate to CN– which forms HCN The degree and rate of dissociation
of complex cyanides depend on several factors, including the nature of the metal,
pH of the solution, and dilution Cyanide ion and HCN are highly toxic to human beings, animals, and aquatic life
Cyanide in water may be determined by the following methods:
1 Silver nitrate titrimetric method
2 Colorimetric method
3 Ion-selective electrode method
4 Ion chromatography
SILVER NITRATE TITRIMETRIC METHOD
Cyanide reacts with silver nitrate as shown below forming the soluble cyanide complex, Ag(CN)2–
When all the CN– ions in the sample are complexed by Ag+ ions, any further addition of a few drops of titrant, AgNO3, can produce a distinct color with an indicator that can determine the end point of the titration Thus, in the presence of
a silver-sensitive indicator, p-dimethylaminobenzalrhodamine, Ag+ ions at first com-bine preferentially with CN– When no more free CN– is left, little excess of Ag+
2 CN–+AgNO3⇒Ag(CN)2–+NO3–
2.6
© 1997 by CRC Press LLC
Trang 12CYANIDE AMENABLE TO CHLORINATION
This test is performed to determine the amount of cyanide in the sample that would react with chlorine Not all cyanides in a sample are amenable to chlori-nation While HCN, alkali metal cyanides, and CN– of some complex cyanides react with chlorine, cyanide in certain complexes that are tightly bound to the metal ions are not decomposed by chlorine Calcium hypochlorite, sodium hypochlorite, and chloramine are some of the common chlorinating agents that may be used as a source of chlorine The chlorination reaction is performed at a
pH between 11 and 12 Under such an alkaline condition, cyanide reacts with chlorine to form cyanogen chloride, a gas at room temperature, which escapes out Cyanide amenable to chlorination is therefore calculated as the total cyanide content initially in the sample minus the total cyanide left in the sample after chlorine treatment
Cyanide amenable to chlorination = Total CN– before chlorination – Total CN–
left after chlorination
Pr ocedure
Two 500-mL aliquots or the volume diluted to 500 mL are needed for this analysis Perform the test for total CN– in one aliquot of the sample following distillation
To the other aliquot, add calcium hypochlorite solution (5 g/100 mL) drop-wise while maintaining the pH between 11 and 12 with caustic soda solution
Perform a test for residual chlorine using KI-starch paper The presence of excess chlorine is indicated from the iodide-starch paper turning to a distinct blue color when a drop of the solution is poured on the paper If required, add additional hypochlorite solution
Agitate the solution for 1 h After this period, remove any unreacted chlorine
by adding one of the following: (a) 1 g of ascorbic acid, (b) a few drops of 2%
sodium arsenite solution, or (c) 10 drops of 3% H2O2, followed by 5 drops of 50% sodium thiosulfate solution Insure that there is no residual chlorine as indicated from no color change with KI-starch paper
2.7
© 1997 by CRC Press LLC
Trang 14Fluoride (F–) is a halogen ion that occurs in many potable and wastewaters
It may also occur in soils, sediments, hazardous waste, aerosol, and gas While
a low concentration of fluoride (below 1 ppm at controlled level in drinking water)
is beneficial for reducing dental caries, a higher content is harmful Fluoride in water may be determined by one of the following methods:
1 Colorimetric SPADNS method
2 Colorimetric automated complexone method
3 Ion-selective electrode method
4 Ion chromatography
Methods 1 and 2 are colorimetric techniques based on the reaction between fluoride and a dye Methods 3 and 4 are discussed in Chapters 1.9 and 1.11, respectively
COLORIMETRIC SP ADNS METHOD
In acid medium, fluoride reacts instantaneously with zirconyl-dye lake which is composed of zirconyl chloride octahydrate, ZrOCl2 ⋅8 H2O, and sodium 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene disulfonate (SPADNS), displacing the Zr2+ from the dye lake to form a colorless complex anion, ZrF62–
and the dye As a result, the color of the solution lightens as the concentration
of F– increases
Chlorine, carbonates, bicarbonates, and hydroxides are common interfer-ences The former is removed by adding a drop of 1% solution of sodium arsenite (NaAsO2) If the sample is basic, neutralize it with HNO3 Sample may be diluted
to reduce the interference effect
Pr ocedure
Prepare a standard calibration curve using fluoride standards from 0 to 1.5 mg
F–/L A 50-mL volume of standard solutions is treated with 10 mL of
zirconyl-2.8
© 1997 by CRC Press LLC
Trang 15HALOGEN ATED HYDROCARBONS
Halogenated hydrocarbons or halocarbons are halogen-substituted hydrocar-bons These substances contain carbon, hydrogen, and halogen atoms in the molecules They are widely used as solvents, dry cleaning and degreasing agents, refrigerants, fire extinguishers, surgical anesthetics, lubricants and intermediates
in the manufacture of dyes, artificial resins, plasticizers, and pharmaceuticals
Because of their wide applications, these compounds are found in the environment
in trace quantities and constitute an important class of regulated pollutants Most halogenated hydrocarbons are liquids at ambient temperature and pressure Some low-molecular weight compounds such as methyl chloride or vinyl chloride are gases; compounds of higher molecular weight, such as iodoform, are solids at ambient conditions Several halogenated hydrocarbons have been listed as priority pollutants by U.S EPA and their methods of analysis are documented (U.S EPA 1984-1992; Methods 601, 612, 624, 625, 501, 502, 503, 524, 8240, and 8260)
In this chapter, halogenated hydrocarbons are defined as halogen-substituted compounds of alkane, alkene, cycloalkane, and aromatic classes, but exclude polychlorinated biphenyls and chlorinated pesticides like BHC isomers Their methods of analyses are based on their certain physical properties, such as vol-atility, boiling point, and water solubility Halogenated hydrocarbons may be analyzed using any one of the following methodologies:
1 Purge and trap concentration (or thermal desorption) from the aqueous matrices (aqueous samples or aqueous extracts of nonaqueous samples or methanol/ace-tone extract of nonaqueous samples spiked into reagent-grade water), separa-tion of the analytes on a suitable GC column and their determinasepara-tion using a halogen-specific detector or a mass spectrometer.
2 Liquid–liquid extraction or liquid–solid extraction for aqueous samples (and soxhlett extraction or sonication for nonaqueous samples), followed by sample concentration, cleanup, and determination by GC or GC/MS Direct injection, waste dilution, or other extraction techniques, depending on the sample matri-ces, may be used.
In general, the purge and trap technique is applied to analyze substances that have boiling points below 200°C and are insoluble or slightly soluble in water
2.9
Trang 16
Hardness of a water sample is a measure of its capacity to precipitate soap The presence of calcium and magnesium ions in water essentially contributes to its hardness Other polyvalent ions, such as aluminum, also cause hardness Their effect, however, is minimal, because these polyvalent ions occur in water often
in complex forms and not as free ions As a result, they cannot precipitate soap Although calcium is not the only cation causing hardness, for the sake of con-venience, hardness is expressed as mg CaCO3/L Similarly, anions other than carbonate, such as bicarbonate, also cause hardness in water To distinguish the contributions of such anions from carbonates, hardness is sometimes termed as
“carbonate hardness” and “noncarbonate hardness.” This can be determined from alkalinity The relationship is as follows:
When the total hardness measured in the sample is numerically greater than the sum of both carbonate alkalinity and bicarbonate alkalinity, then
Carbonate hardness = carbonate alkalinity + bicarbonate alkalinity
and
Noncarbonate hardness = total hardness – carbonate hardness
or
Total hardness – (carbonate alkalinity + bicarbonate alkalinity)
When total hardness is equal to or less than the sum of carbonate and bicarbonate alkalinity, all hardness is noncarbonate hardness only and there is no carbonate hardness.
Hardness can be measured by either (1) calculation from the concentration
of calcium and magnesium ions in the ample, or (2) EDTA titration
2.10
© 1997 by CRC Press LLC