PH

You learned in general chemistry that pH is the negative log of the molar hydrogen ion concentration:

Ions in water are always surrounded by water molecules that partially cancel out the charge of the ion. The hydrogen ion, like all ions in solution, is surrounded by waters of hydration. For ease of writing, we normally represent the hydrogen ion as H3O+, suggesting one water of hydration, although in reality it can have 5 to 9. In fact, it is a cluster of water molecules with one extra proton.

The pH of a water solution is considered a master variable. By this, we mean that the pH of the solution can be the determining factor in a variety of parameters, especially chemical speciation, which we will spend considerable time discussing in Sections 2.5.1 and 3.2.2. Therefore, pH is one of the most common parameters measured for a water sample.

Although we commonly relate pH to hydrogen concentration [Eq. (2.1)], the pH of a water is almost always measured with an electrode, and electrodes measure activity instead of concentration. Activity is discussed in the next section.

Most natural waters have a pH between 5.5 and 9, but extreme pH values have been observed in natural settings such as geothermal water and eutrophic (organicrich) systems. When hazardous waste enters natural environments, any pH value is possible, given the vast amount of acidic and caustic wastes that the chemical industry produces. Other topics related to pH that you should review from general chemistry are buffer solutions and the Henderson-Hasselbach equation, which we will look at closely in Section 2.5.1.

2.4.2 Activity

You will find, as you take more chemistry, that what you learned in general chemistry was a simplification of reality. This is true for units of concentration. For example, molar units are only appropriate in what are called "ideal solutions," in which the molar concentration of an ion or compound is equal to its activity. Activity is a measure of the effective concentration of an ion, accounting for its interactions with other ions that can mask it. Therefore, an ideal solution is one containing very little dissolved salt. Activity (A), the concentration that the solution "sees," is expressed by

where g is the activity coefficient (almost always equal to or less than 1.00; for extreme conditions it can be greater than 1.00), and [C] is the molar concentration of the chemical or pollutant. The activity coefficient (g) is a direct function of ionic strength, or the amount of other salts in the solution, which helps explain why we must be concerned with activity instead of concentration. If you picture an ion in solution, it is not present as a free cation or anion, but is surrounded by water molecules (waters of hydration) and by ions of opposite charge, which serve to balance out the charge of the ion of interest. A representation of such a configuration is shown in Figure 2.5 for ions of CaSO4.

This balancing of charges tends to make the ion less active in solution than expected based on its concentration (by changing the ions mobility), and activity accounts for this canceling out of concentration. As noted above, the activity of a pollutant is equal to the activity coefficient times the molar concentration, and the activity coefficient is related to the salt content of the water. An activity coefficient of near 1.00 for dilute solutions yields an activity equal to the concentration. As the salt concentration increases, the activity coefficient decreases from 1.00, thus lowering the activity.

So, what is the practical reason for using activity rather than concentration? Remember that our end goal in pollutant fate and transport modeling is to be able to predict risk, and risk is based on toxicity. It has been almost universally found that water containing higher concentrations of nontoxic salts have lower toxicity for the same pollutant concentration. Toxicity is thus governed by activity instead of concentration.

So how can we quantify the effect of activity? As noted earlier, concentration is related to activity by the activity coefficient (g), which is related to the total ionic strength (|m). We can calculate the ionic strength of any water, if we know the anion/cation composition of the water, by

NO3-

Ca2+

SO42-

so42-

Ca2+

NO3-

Figure 2.5. Illustration of ions of Ca2+ and SO42- in aqueous solution.

m = 0.500 ((z2 + c2 Z22 + Q z2 + C4 Z42 + •••) = 0.500 £ cZ

where C is molar concentration and Z is charge.

Here is an example calculation: What is the total ionic strength of (a) 0.100M NaNOs and (b) 0.100M Na2SO4.

(a) m = 0.500 [(0.100 x 12) + (0.100 * 12)] = 0.100M

(b) m = 0.500 [(0.100 x 2 x 12) + (0.100 x 22)] = 0.300M

From this trend we can develop the general rules shown in Table 2.3.

For mixtures of ionic salts, the empirically derived extended Debye-Huckel equation can be used to estimate g:

where Z is the ionic charge you are calculating g for, m is the ionic strength, and a is the effective hydrated radius of the ion you are calculating g (found in Table 2.4). Note the trend implicit in Eq. (2.3). Small, highly charged ions bind solvent molecules more tightly and have smaller hydrated radii than do larger or less highly charged ions. Also note some generalizations: (1) As ionic strength (m) increases, the activity coefficient (g) decreases, (2) as the ionic charge (Z) increases, the activity coefficient (g) decreases, and (3) as the effective hydrated radius (a) decreases, the activity coefficient (g) decreases.

Example. Calculate the activity coefficient and activity of Ca2+ in 0.0200M CaCl2. m = 0.500 [(0.0200 x 22) + (0.0200 x 2 x 12)] = 0.0600 M

6 1 + 600V0.0600/305

Note the substantial difference between an activity of 0.00693 and a concentration of 0.0200 M. As noted, differences become more pronounced as the salt content increases. Waters where activity calculations are important include some ground-waters, inland salt lakes, estuaries, and oceans.

TABLE 2.3. Estimation of Ion Strength Based on Salt Composition

Electrolyte Molarity m

TABLE 2.4. Activity Coefficients and Hydrated Radii for Aqueous Solutions at 25°C (Data from Harris, 1999)

Ion Radius (pm)

0.001

Ionic Strength of Solution

0.005 0.01 0.05 Activity Coefficients

0.1

Charge ± 1

900

0.967

0.933

0.914

0.86

0.83

(C6H5)2CHC02-, (C3H7)4N+

800

0.966

0.931

0.912

0.85

0.82

(02N)3C6H20-, (C3h7)3nh+, ch3oc6h4co2-

700

0.965

0.930

0.909

0.845

0.81

Li+, c6h5co2-, hoc6h4co2-, cic6h4co2-, c6h5ch2co2-.

CH2=CHCH2C(V, (CH3)2CHCH2C(V, (CH3CH2)4N+, (C3H7)2NH2+

600

0.965

0.929

0.907

0.835

0.80

CFCHCCV, C13CC02-, (CH3CH2)3NH+, (C3H7)NH3+

500

0.964

0.928

0.904

0.83

0.79

Na+, CdCr, CKV, IO3", HCO3". H2PCXf, HS03". H2As04"

Co(NH3)4(N02)2+, CH3C02-, C1CH2C02-, (CH3)4N+,

(CH3CH2)2NH2+, H2NCH2CO2-

450

0.964

0.928

0.902

0.82

0.775

+H3NCH2C02H, (CH3)3NH+, CH3CH2NH3+

400

0.964

0.927

0.901

0.815

0.77

OH". F", SCN". OCN". HS-, CIO3-, CIO4-, Br03", I04", Mn04-,

HC02-, H2citrate-, CH3NH3+, (CH3)2NH2+

350

0.964

0.926

0.900

0.81

0.76

K+, cr, Br, r, cn_, no2", no3"

300

0.964

0.925

0.899

0.805

0.755

Rb+, Cs+, NH4+, H+, Ag+

250

0.964

0.924

0.898

0.80

(Continued)

TABLE 2.4. Activity Coefficients and Hydrated Radii for Aqueous Solutions at 25°C (Data from Harris, 1999) (continued)

Ion Radius (pm)

0.001

Ionic Strength of Solution

0.005 0.01 0.05 Activity Coefficients

0.1

Charge ± 2

Mg2+, Be2+

800

0.872

0.755

0.69

0.52

0.45

CH2(CH2CH2C(V)2, (CH2CH2CH2C(V)2

700

0.872

0.755

0.685

0.50

0.425

Ca2+, Cu2+, Zn2+, Sn2+, Mn2+, Fe2+, Ni2+, Co2+, C6H4(C02-)2,

H2C(CH2C02-)2, (CH2CH2C02-)2

600

0.870

0.749

0.675

0.485

0.405

Sr2+, Ba2+, Cd2+, Hg2+, S2~, S2CXr, W042~, H2C(C(V)2, (CH2C(V)2,

(CHOHCCV),

500

0.868

0.744

0.67

0.465

0.38

Pb2+, COr. S032~, Mo042~, Co(NH3)5C12+, Fe(CN)5N02~, C2042~,

Hcitrate2-

450

0.867

0.742

0.665

0.455

0.37

Hg22+, S042-, S2032-, S20<r, S2082-, Se04 2~, Cr04 2~, HP04 2~

400

0.867

0.740

0.660

0.445

0.355

Charge ± 3

Al1+, Fe,+, Cr1+, Sc3+, Y3+, In3+, lanthanides"

900

0.738

0.54

0.445

0.245

0.18

citrate1-

500

0.728

0.51

0.405

0.18

0.115

P043", Fe(CN)63-, Cr(NH3)63+, Co(NH3)63+, Co(NH3)5H20 3+

400

0.725

0.505

0.395

0.16

0.095

Charge ± 4

Th^, Zr4+, Ce4+, Sn4+

1100

0.588

0.35

0.255

0.10

0.065

Fe(CN)64-

500

0.57

0.31

0.20

0.048

0.021

" Elements 57-71 in the periodic table.

Source: J. Kielland, /. Am. Chem. Soc. 59, 1675 (1937).

" Elements 57-71 in the periodic table.

Source: J. Kielland, /. Am. Chem. Soc. 59, 1675 (1937).

As we continue in this textbook, we will almost always refer to concentration for simplicity, but remember that when you are dealing with a water with high salt content, it is better to work in activities.

2.4.3 Solubility

Solubility is defined as the maximum concentration of a chemical species that can be present in a solution at equilibrium. Since we will be dealing exclusively with water as our solvent, we are concerned with the aqueous solubility, given in moles/L or mg/L. We will divide our discussions into organic and inorganic pollutants. Although water is considered a universal solvent, it does not necessarily dissolve large quantities of all chemicals. Some organic pollutants, such as short-chained alcohols, acetone, acetonitrile, and a few other organic solvents, are miscible with water; that is, water and the solvent will completely mix in any proportion. Many other pollutants are incorrectly listed as insoluble in water, which really means that the solubility is very low. For example, the famous pollutants such as DDT and PCBs are listed as insoluble in many chemistry handbooks but are actually soluble in the ppb to ppm range. Their low solubilities contribute to these chemicals' toxicity to wildlife. In order to understand their toxicity, we must understand bioconcentration, or biomagnification.

Pollutants such as DDT and PCBs are hydrophobic; they do not like being dissolved in water. As noted above, their solubilities are in the ppb to ppm level in pure water. In the environment, they actively partition from (move out of) the water onto surfaces or into biological organisms, even at concentrations below their aqueous solubility. Microorganisms such as algae have a large surface area, and their hydrophobic cell surfaces will readily attract and sorb hydrophobic pollutants. Thus, these pollutants are concentrated into the algae or microbe cell mass. Organisms that feed on algae automatically concentrate more DDT and PCBs since they eat large quantities of algae over a longer lifespan. This bioconcentration of the pollutants continues up the food chain to the top predators, including birds of prey and humans. An example of bioconcentration in an ecosystem is shown in Table 2.5. As you see, the pollutant concentration in the water is only 0.00005 ppm, but it is bioconcen-trated to ~25ppm in biological species at the top of the food chain. This almost mil-lionfold increase illustrates the dangers of these extremely low levels of pollutants in the environment.

The experimental determination of the exact solubility of a hydrophobic or any low solubility pollutant is very difficult and is wrought with analytical errors. The lower the solubility, the more error in the results and disagreement between laboratory determinations. Some solubility values for the same pollutant disagree by a factor of 10 to 100; however, these procedures do allow more precise estimates of the relative solubilites of hydrophobic pollutants. As a result of the disagreement in literature values, a modeling method to determine absolute and relative solubilities of compounds is becoming more accepted today; solubilities are estimated from highly sophisticated calculations using the chemical parameter program SPARC, developed by the U.S. EPA (http://ibmlc2.chem.uga.edu/sparc/). This program can

TABLE 2.5. Bioconcentration of DDT in Long Island Food Web (USA) (Woodwell et al., 1967)

Organism DDT Residues (ppm)

Water 0.00005

Plankton 0.04

Silverside minnows 0.23

Sheephead minnows 0.94

Pickerel (predatory) 1.33

Needlefish (predatory) 2.07

Heron (feeds on small aquatic animals) 3.57

Herring gull (scavenger) 6.00

Osprey egg 13.8

Merganser (fish-eating duck) 22.8

Cormorant (feeds on large fish) 26.4

relatively accurately predict the aqueous solubility, vapor pressure, and Henry's law constant for any chemical with a known structure and melting point. Table 2.6 lists these parameters for several common pollutants that will be used in the modeling section of this textbook. This table and the SPARC model can be used in the modeling chapters to determine the source masses of pollutants in step and pulse models. The aqueous solubilities of different classes of pollutants are also compared in Figure 2.6. Note the wide range of pollutant solubilities.

While we will generally study organic pollutants in the aqueous phase, many sources of organic pollutants are pure liquids known as nonaqueous phase liquids (NAPLs). We discussed one example, from the Idaho National Engineering and Environmental Laboratory, in Chapter 1. The NAPL may be more dense than water and known as a dense nonaqueous phase liquid (DNAPL) or may be less dense than water and known as a light nonaqueous phase liquid (LNAPL). Neither of these liquids mix with water, but as water flows past the NAPL, compounds in the NAPLs slowly dissolve into the water phase. Although there are sophisticated models to predict the release rate of these "pools" of pollutants, one simple way to model them is to use the maximum aqueous solubility predicted from SPARC and use this as the input mass in a step transport model. We will discuss this in the groundwater modeling chapter.

The solubilities of inorganic pollutants (salts), at least as they are calculated in general chemistry, are much easier to determine than those of organics. You should remember the solubility product constant, or Ksp. Values of Ksp for several salts are given in Table 2.7. We will use an applied example from general chemistry to illustrate how solubilities are calculated from these constants. Note that in the following calculations we are concerned with the concentration of the dissolved metal ion in equilibrium with some solid phase.

Say that you have an industrial process that produces copper (I) bromide as a waste product. What will be the maximum Cu+ solubility if this solid waste is contacted by rainwater? The Ksp for CuBr is 5.3 x 10-9.

TABLE 2.6. Summary of Aqueous Solubility, Vapor Pressure, and Henry's Law Constants for Common Environmental Pollutants (Estimated by SPARC)

Classes of Pollutants

CAS #

Melting Point

MW

Solubility (mg/L)

VP (atm)

HLC (atm-L/mol)

C1-C2 Halocarbons

Trichlorofluoromethane

75-69-4

-111°C

137.4

2.413

1.228

0.0569

Dichlorodifluoromethane

75-71-8

-158°C

120.9

523.3

5.957

0.231

Chlorodifluoromethane

75-45-6

-157.4°C

86.4

4.005

6.223

0.0216

1,2-Dichloromethane

75-71-8

-158°C

120.9

523.3

5.955

0.2311

Trichloroethene

79-01-6

-84.7°C

131.388

913.9

0.1773

0.02549

Tetrachloroethene

127-18-4

-22.3°C

165.83

127.3

0.0511

0.06657

1.1.1 -Trichloroethane

71-55-6

-30°C

133.4

1.940

0.2032

0.01397

Methyl bromide

74-83-9

-93.7°C

94.94

23.970

1.831

0.003960

1,2-Dichloroethane

107-06-2

-35.5°C

98.96

7.903

0.1244

0.001558

1.1,2-Tetrachloroethane

79-72-1

-44°C

167.9

2.171

0.006077

0.004699

Hexachloroethane

67-72-1

187°C

236.74

12.95

9.931E-5

0.001816

Polychlorinated Biphenyls

2-Chlorobiphenyl

2051-60-7

34°C

188.66

5.769

9.179E-6

0.0003001

4-Chlorobiphenyl

2051-62-9

78.8

188.66

1.915

2.245E-6

0.0002212

2.2'-DCB

13029-08-8

58°C

223.10

0.7528

1.304E-6

0.0003865

4.4'-DCB

2050-68-2

149.3°C

223.10

0.07793

5.732E-8

0.0001641z

2.2'.3.3'-TCB

38444-93-8

123°C

291.99

0.008024

3.641E-9

0.0001312

2,2',5,5'-TCB

35693-99-3

84°C

291.99

0.03705

2.755E-8

0.0002171

2.3.4.5-TCB

33284-53-6

93°C

291.99

0.01478

5.235E-9

0.0001034

2.2'.4.5.5'-PCB

37680-73-2

26°C

326.43

0.02945

1.266E-8

0.0001403

2.2'.4.4'.6.6'-HCB

33979-03-2

110°C

360.88

0.0009090

2.563E-10

0.0001018

TABLE 2.6. Summary of Aqueous Solubility, Vapor Pressure, and Henry's Law Constants for Common Environmental Pollutants (Estimated by SPARC) (continued)

Classes of Pollutants

CAS #

Melting Point

MW

Solubility (mg/L)

VP (atm)

HLC (atm-L/m

Carboxyate Esters (Pyrethrins)

Pyrethrin I

121-21-1

Liquid at 25°C

328.45

3.303

8.517E-10

1.198E-7

Pyrethrins

8003-34-7

Liquid at 25°C

328.45

3.974

9.099E-10

1.141E-7

Pyrethrin II

121-29-9

Liquid at 25°C

372.46

12.37

4.053E-11

1.221E-9

Organophosphates (chemfinder.com)

Parathion

56-38-2

6.1°C

291.26

3.064

3.498E-8

3.324E-6

Trimethyl phosphate

512-56-1

-46°C

140.08

7.750.000

0.02638

1.279E-7

Triphenylphosphate

115-86-6

50.5°C

326.29

38.74

2.666E-11

2.246E-10

Paraoxon

311-45-5

NA" (assume liq.)

275.20

24.070

9.462E-8

1.082E-9

Methyl parathion

298-00-0

36°C

263.2105

58.51

3.033E-7

1.365E-6

Disulfoton

298-04-4

-25 °C

274.41

0.4989

1.959E-7

0.0001078

Diazinon

333-41-5

~120°C

304.35

1.658

3.351E-8

6.152E-6

Dichlorvos

62-73-7

-60°C

220.98

761.700

8.389E-4

2.434E-7

Acephate

30560-19-1

88°C

183.17

5397.000

2.006E-7

6.134E-13

Carbamates

Carbofuran

1563-66-2

151°C

221.26

44.88

2.203E-9

1.086E-8

Aldicarb

116-06-3

100°C

190.37

981.6

4.500E-7

8.723E-8

Carbaryl

63-25-2

145°C

201.22

17.59

9.532E-10

1.090E-8

Methomyl

16752-77-5

78°C

162.21

24.160

1.114E-6

7.479E9

Propoxur

114-26-1

87°C

209.24

262.5

6.175E-8

4.922E-8

Mexacarbate

315-18-4

85°C

222.29

218.4

8.370E-10

8.519E-10

Chlorinated Pesticides and Byproducts

p,p—DDT

50-29-3

108.5°C

354.49

0.003758

2.185E-10

2.061E-5

p,p'-DDE

72-55-9

89°C

318.03

0.003216

5.141E-10

5.074E-5

o,p'-DDD

53-19-0

76°C

p,p'-DDD

72-54-8

109.5°C

Lindane (HCB)

58-89-9

113°C

PCP

77-10-1

46.5°C

Kepone

143-50-0

350°C (decomp.)

Mirex

2385-85-5

485°C (decomp.)

cis- or alpha chlordane

5103-71-9

106°C

Heptachlor

76-44-8

95.5°C

Aldrin

309-00-2

104°C

Dieldrin

60-57-1

175°C

Endosulfan

115-29-7

106°C

Alachlor

15972-60-8

40°C

Aldicarb

116-06-3

100°C

Endrin

72-20-8

200°C (decomp.)

2,3,7,8-Tetrachlorodibenzo-p-dioxin

1746-01-6

295°C

Phenoxy Acid Herbicides

2,4-DB

106-93-4

9.9°C

Dalapon

75-99-0

166.5°C (decomp.;

Dicamba

1918-00-9

115°C

Dichlorprop

120-36-5

117.5°C

Dinoseb

88-85-7

40°C

MCPA

94-74-6

120°C

MCPP

93-65-2

93°C

2,4,5-T

93-76-5

153°C

2,4-D

94-75-7

140.5°C

Substituted Phenols

4-Methylphenol

106-44-5

35.5°C

4-Ethylphenol

123-07-9

45.0°C

3-Methylphenol

108-39-4

11.8°C

320.04 320.04 290.83 243.39 490.64 545.54 409.78 373.32 364.91 380.91 406.93 269.77 190.37 380.91 321.97

0.03473 0.01293 10.14 114.3 0.2108 2.516E-6 0.06442 0.1677 0.01839 0.1193 1.211 946.7 981.6 0.07949 5.573E-5

1.010E-9

3.133E-10

4.059E-7

4.934E-8

1.290E-10

1.640E-10

1.031E-8

1.208E-7

1.761E-7

4.688E-9

1.947E-8

1.251E-8

4.499E-7

3.125E-9

2.426E-12

9.305E-6

7.755E-6

1.164E-5

1.051E-7

3.002E-7

0.03556

6.556E-5

0.0002688

0.003494

1.498E-5

6.544E-6

3.566E-9

8.723E-8

1.497E-5

1.401E-5

187.86 142.97 221.04 235.07 240.22 200.62 214.65 255.48 221.04

7.266 69.300

278.6

325.4 2.120

377.5 494.8 63.47

257.7

0.01260

3.125E-4

7.437E-7

9.587E-8

1.305E-7

2.179E-8

1.559E-7

1.714E-9

1.315E-8

0.0003258

5.919E-7

5.901E-7

6.925E-8

1.479E-8

1.158E-8

6.766E-8

6.901E-9

1.128E-8

108.14 122.17 108.14

19.210 6.240 25.090

TABLE 2.6. Summary of Aqueous Solubility, Vapor Pressure, and Henry's Law Constants for Common Environmental Pollutants (Estimated by ° SPARC) (continued)

Classes of Pollutants

CAS #

Melting Point

MW

Solubility (mg/L)

VP (atm)

HLC (atm-L/mol)

2-Chlorophenol

95-57-8

9.8°C

235.07

325.4

9.587E-8

6.926E-8

Phenol

108-95-2

40.9°C

94.11

58.840

4.148E-4

6.633E-7

4-Chlorophenol

106-48-9

42.7°C

128.56

27.890

9.998E-5

4.609E-7

4-Hydroxybenzoic acid

99-96-7

214.5°C

138.12

3.681

4.056E-10

1.257E-11

3-Chlorophenol

108-43-0

32.6°C

128.56

38.860

1.307E-4

4.322E-7

4-Hydroxyacetophenone

99-93-4

109.5°C

136.15

3.201

2.207E-8

9.378E-10

2-Hydroxybenzoic acid

69-72-7

158°C

138.12

2.190

6.139E-6

3.486E-7

4-Nitrophenol

100-02-7

113.8°C

139.11

7.514

1.028E-7

1.903E-9

Triazines

Atrazine

1912-24-9

173°C

215.68

37.95

9.535E-9

5.419E-8

Eptam

759-94-4

Liquid at 25°C

189.32

152.4

3.357E-5

4.172E-5

Sutan

2008-41-5

Liquid at 25°C

217.37

18.35

1.502E-5

0.0001779

Vernam

1929-77-7

Liquid at 25°C

203.35

47.51

1.190E-5

5.096E-5

Tilam

1114-71-2

Liquid at 25°C

203.35

50.44

1.084E-5

4.372E-5

Propachlor

1918-16-7

ITC

211.69

648.7

1.547E-7

5.048E-8

Trifluralin

1582-09-8

49°C

335.28

2.121

2.858E-8

4.518E-6

Simazine

122-34-9

226°C

201.66

34.44

2.886E-9

1.690E-8

Propazine

139-40-2

213°C

229.71

12.24

8.861E-9

1.664E-7

Bromacil

314-40-9

158°C

261.12

913.7

2.437E-9

6.964E-10

Prowl

40487-42-1

56°C

281.31

1.044

7.028E-9

1.894E-6

Phthalate Esters

Di- or bis(2-ethylhexyl)

phthalate esters (DEHP)

117-81-7

-55°C

390.56

0.009507

1.046E-11

4.294E-6

Diethyl phthalate

84-66-2

-40.5°C

222.24

261.2

1.078E-6

9.169E-7

Dimethyl phthalate

131-11-3

5.5°C

194.19

2.383

5.391E-6

4.393E-7

Di-n-butyl phthalate Di-n-octyl phthalate

Polycyclic Aromatic Hydrocarbons

Naphthalene

Phenanthrene

Anthracene

Benzanthracene

Pyrene

Fluoranthene

Fluorine

Chrysene

Benzo(£)fluoranthene Benzo(£>)fluoranthene Benzo(ii)pyrene Benzo(e)pyrene

Aliphatic Hydrocarbons

C5 (n-pentane) C6 (n-hexane) C7 («-heptane) C8 («-octane) C9 (n-nonane) Cio(n-decane) C„ (undecane) C12 (dodecane) Cl3 (tridecane) C14 (tetradecane) C15 (pentadecane) C16 (hexadecane) C17 (heptadecane)

112-40-3 -9.66°C 629-50-5 -5.3°C 629-59-4 5.82°C 629-62-9 9.9°C 544-76-3 18.12°C 629-78-7 22°C

278.35 390.56

2.191 0.003374

1.711E-8 1.739E-12

1.631E-6 3.096E-6

128.17 178.23 178.23 228.29 202.26 202.26 166.22 228.29 252.31 252.31 252.31 252.31

39.06 0.9889 0.08508 0.06121 0.1055 0.2149 1.963 0.001461 0.003810 0.01237 0.003501 0.003605

1.197E-4

1.164E-7

8.126E-9

2.141E-10

6.148E-9

2.173E-8

7.973E-7

6.278E-12

1.879E-12

6.849E-12

5.871E-12

6.871E-12

0.0003926

2.098E-5

1.703E-5

7.987E-7

1.178E-5

1.280E-5

6.750E-5

9.808E-7

7.258E-7

9.185E-7

9.356E-7

1.149E-6

72.15 86.18 100.20 114.23 128.26 142.28 156.31 170.34 184.36 198.39 212.42 226.12 240.47

51.81 14.53 3.725 0.9318 0.2349 0.05677 0.02252 0.01496 0.01030 0.007339 0.005402 0.004100 0.003184

0.7393

0.2245

0.06942

0.02128

0.006343

0.001925

5.839E-4

1.455E-4

4.376E-5

3.256E-5

4.345E-6

1.443E-6

4.814E-7

1.029 1.331 1.867 2.608 3.463 4.824 6.749 9.555 13.35 18.76 26.35 37.12 49.08

TABLE 2.6. Summary of Aqueous Solubility, Vapor Pressure, and Henry's Law Constants for Common Environmental Pollutants (Estimated by SPARC) (continued)

Classes of Pollutants

CAS #

Melting Point

MW

Solubility (mg/L)

VP (atm)

HLC (atm-L/n:

C18 (octadecane)

593-45-3

28.2°C

254.50

0.002490

1.603E-7

61.35

C19 (nonadecane)

629-92-5

32.1°C

268.52

0.001998

5.748E-8

82.28

C20 (icosane)

112-95-8

36.8°C

282.55

0.001631

2.139E-8

110.3

Substituted Nitrobenzenes

2-Methylnitrobenzene

88-72-2

-10°C

137.14

764.6

2.299E-4

4.125E-5

3-Methylnitrobenzene

99-08-1

15.5°C

137.14

1.394

1.556E-4

1.529E-5

4-Methylnitrobenzene

99-99-0

51.6°C

137.14

869.1

8.833E-5

1.389E-5

2-Chloronitrobenzene

88-73-3

32.5°C

157.56

603.3

9.349E-5

2.442E-5

3-Chloronitrobenzene

121-73-3

44.4°C

157.56

900.3

1.139E-4

1.994E-5

4-Chloronitrobenzene

100-00-5

83.5°C

157.56

476.4

5.994E-5

1.982E-5

2-Acetylnitrobenzene

577-59-3

28.5°C

165.15

4.715

2.996E-6

1.050E-7

3-Acetylnitrobenzene

121-89-1(7)

81°C

165.15

2.953

4.792E-7

2.681e-8

4-Acetylnitrobenzene

100-19-6

81.8°C

165.15

3.008

4.279E-7

2.349E-8

2.4,6-Trinitrotoluene

118-96-7

80.1°C

227.13

144.3

1.759E-9

2.769E-9

2,4-Dinitrotoluene

121-14-2

71°C

182.14

376.5

5.431E-7

2.627E-7

2,6-Dinitrotoluene

606-20-2

66°C

182.14

186.5

5.453E-7

5.327E-7

Nitrobenzene

98-95-3

5.7°C

123.11

3.720

4.066E-4

1.346E-5

HMX

2691-41-0

281°C

296.16

4.519

4.235E-17

2.774E-15

RDX

121-82-4

205.5°C

222.12

69.45

1.055E-12

4.249E-12

Common Solvents

Pentane

Pentadecane

Benzene

72.15 212.42 78.11

51.81 0.005402 2.481

0.7383

4.345E-6

0.1355

1.029 26.35 0.004266

Toluene Styrene Pyridine

Methylene chloride

Chloroform

Carbon tetrachloride

Trichloroethylene p-Dichlorobenzene

Acetone

Methyl ethyl ketone

Acetonitrile

Methanol

Ethanol

Aniline

Nitrobenzene

Methyl tert-butyl ether

Gasoline Components

Benzene Ethylbenzene methyl tert-butyl ether heptane

2-methylpentane

Toluene

Toluene m-xylene o-xylene

108-88-3 -94.95°C 108-88-3 -94.95°C 108-38-3 -47.8°C

a NA, not available.

92.14 104.15

79.10 84.93

119.38 153.82

131.39 147.00

58.08

123.11

88.15

775.8 425.5 178.600 25.240 11.930 2.348

914.0

122.1 246.800 129.200 265.800

1.144.500 406.200 47.480 3.720 15.290

0.04133

0.008913

0.03064

0.5370

0.2088

0.1671

0.1773

0.001934

0.3051

0.1323

0.09967

0.1982

0.08656

7.075E-4

4.066E-4

0.5634

0.004909

0.002181

1.357E-5

0.001807

0.002090

0.01095

0.02549

0.002329

7.180E-5

7.382E-5

1.539E-5

5.549E-6

9.817E-6

1.388E-6

1.346E-5

0.003247

78.11 106.17 88.15 100.20 86.18 92.14 92.14 106.17 106.17

2.481 270.4 15.300 3.725 16.69 775.8 775.8 268.3 320.0

0.1355 0.01569 0.5634 0.06942 0.3243 0.04133 0.04133 0.01241 NA

0.004266 0.006160 0.003247 1.867 1.674 0.004909 0.004909 0.004909 NA

Gasoline Components Common Solvents Substituted Nitrobenzenes Aliphatic Hydrocarbons PAHs

Phthalate Esters Triazines Substituted Phenols Phenoxy Acid Herbicides Chlorinated Pesticides Carbamates Organophosphates Pyrethrins PCBs

C1 to C2 Halocarbons

Figure 2.6.

Aqueous Solubility in Log (mg/L)

Summary and comparison of aqueous solubilities of selected pollutants.

Solution

For every mole of CuBr that dissolves, one mole of Cu+ and one mole of Br- dissolve, so let [Cu+] = [Br-] = x. Substituting into the Ksp equation yields

or x = [Cu+] = 7.3 x 10 -5MCu+ Finally, to get solubility in ppm,

(7.3 x 10-5 M Cu + )(63.6 g/mol) = 4.6 x 10-3 g/L or 4.6 mg/L Cu + or 4.6 ppm

Now, let's make the problem a bit more complicated. Say that you have a hazardous waste sludge sample containing PbCO3, PbCl2, PbCrO4, PbF2, PbSO4, and PbS. Which form of lead waste will determine the maximum concentration of lead in any leachate that may come from the waste? What could be the maximum concentration of Pb2+ in the leachate?

TABLE 2.7. Solubility Products for Inorganic Compounds at 25°C (Handbook of Chemistry and Physics, 1980)

Solubility Product at Solubility Product at

TABLE 2.7. Solubility Products for Inorganic Compounds at 25°C (Handbook of Chemistry and Physics, 1980)

Solubility Product at Solubility Product at

Substance

Temperature Noted (°C)

Substance

Temperature Noted

Aluminum hydroxide

4x KT13 (15°)

Lead iodide

7.47 x 10-" (15°)

Aluminum hydroxide

1.1 X lCT15 (18°)

Lead iodide

1.39 x 10-8 (25°)

Aluminum hydroxide

3.7 x 1(T15(25°)

Lead oxalate

2.74 x 10-" (18°)

Barium carbonate

7 x 1CT9 (16°)

Lead sulfate

1.06 x 10~8 (18°)

Barium carbonate

8.1 x 10-" (25°)

Lead sulfide

3.4 x 10-28 (18°)

Barium Chromate

1.6 x lCT10 (18°)

Lithium carbonate

1.7 x 1CT3 (25°)

Barium Chromate

2.4 x lCT10 (28°)

Magnesium ammonium phosphate

2.5 x 10"13 (25°)

Barium fluoride

1.6 x lCT6 (9.5°)

Magnesium carbonate

2.6 x 10~5 (12°)

Barium fluoride

1.7 x icr6 (18°)

Magnesium fluoride

7.1 x 10-" (18°)

Barium fluoride

1.73 x 10~6 (25.8°)

Magnesium fluoride

6.4 x 10~9 (27°)

Barium iodate, Ba(IO,) 2H,0

8.4 x 10-" (10°)

Magnesium hydroxide

1.2 x 10-" (18°)

Barium iodate, Ba(IO,) 2H,0

6.5 x lCT10 (25°)

Magnesium oxalate

8.57 x 10~5 (18°)

Barium oxalate, BaC203 3.5H20

1.62 x lCT7 (18°)

Manganese hydroxide

4x 10~14 (18°)

Barium oxalate, BaC204 2H20

1.2 x lCT7 (18°)

Manganese sulfide

1.4 x 1(T15 (18°)

Barium oxalate, BaC204 0.5H,0

2.18 x lCT7 (18°)

Mercuric sulfide

4 x 10"53 to

Barium sulfate

0.87 x 1CT10 (18°)

2 x 10~49 (18°)

Barium sulfate

1.08 x 10-10 (25°)

Mercurous bromide

1.3 x 10~21 (25°)

Barium sulfate

1.98 x 10-10 (50°)

Mercurous chloride

2 x 10-18 (25°)

Cadmium oxalate CdC204 3H20

1.53 x 10-8 (18°)

Mercurous iodide

1.2 x 10-28 (25°)

Cadmium sulfide

3.6 x 10-29 (18°)

Nickel sulfide

1.4 x 10~24 (18°)

Calcium carbonate (calcite)

0.99 x 10-8 (15°)

Potassium acid tartrate [K+] [HC4H406~]

3.8 x 10~4 (18°)

Calcium carbonate (calcite)

0.87 x 10~8 (25°)

Calcium fluoride

3.4 x 10-" (18°)

Silver bromate

3.97 x 1(T5 (20°)

Calcium fluoride

3.95 x 10-" (26°)

Silver bromate

5.77 x 10~5 (25°)

Calcium iodate, (Ca(I03)2 6H,0

22.2 x 10-8 (10°)

Silver bromide

4.1 x 10-13 (18°)

Calcium iodate, (Ca(I03)2 6H,0

64.4 x 10-8 (18°)

Silver bromide

7.7 x 10~13 (25°)

TABLE 2.7. Solubility Products for Inorganic Compounds at 25°C (Handbook of Chemistry and Physics, 1980) (continued)

Substance

Solubility Product at Temperature Noted (°C)

Substance

Solubility Product at Temperature Noted

Calcium oxalate, CaC204 H20

1.78 X 10-" (18°)

Silver carbonate

6.15 Xl0~12 (25°)

Calcium oxalate, CaC204 H20

2.57 x 10-" (25°)

Silver chloride

0.21 x 10-10 (4.7°)

Calcium sulfate

2.45 x KT5 (25°)

Silver chloride

0.37 x 10-10 (9.7°)

Calcium tartrate, CaC4H406 2H,0

0.77 x KT6 (18°)

Silver chloride

1.56 x 10-10 (25°)

Cobalt sulfide

3 x 10-26 (18°)

Silver chloride

13.2 x 10-10 (50°)

Cupric iodate

1.4 x 10-7 (25°)

Silver chloride

215 x 10~10 (100°)

Cupric oxalate

2.87 x 10-8 (25°)

Silver chromate

1.2 x 10~12 (14.8°)

Cupric sulfide

8.5 x 10-45 (18°)

Silver chromate

9 x 10~12 (25°)

Cuprous bromide

4.15 xlO-8 (18-20°)

Silver cyanide [Ag+][Ag(CNr2]

2.2 x 10~12 (20°)

Cuprous chloride

1.02 x 10-6 (18-20°)

Silver dichromate

2 x 10~7 (25°)

Cuprous iodide

5.06 x 10~12 (18-20°)

Silver hydroxide

1.52 x 10~8 (20°)

Cuprous sulfide

2 x 10~47 (16-18°)

Silver iodate

0.92 x 10~8 (9.4°)

Cuprous thiocyanate

1.6 x 10-" (18°)

Silver iodide

0.32 x 10-16 (13°)

Ferric hydroxide

1.1 x 10"36 (18°)

Silver iodide

1.5 x 10-16 (25°)

Ferrous hydroxide

1.64 x 10~14 (18°)

Silver sulfide

1.6 x 10-49 (18°)

Ferrous oxalate

2.1 x 10~7 (25°)

Silver thiocyanate

0.49 x 10~12 (18°)

Ferrous sulfide

3.7 x 10-19 (18°)

Silver thiocyanate

1.16 x 10~12 (25°)

Lead carbonate

3.3 x 10~14 (18°)

Strontium carbonate

1.6 x 10~9 (25°)

Lead chromate

1.77 x 10~14 (18°)

Strontium fluoride

2.8 x 10-" (18°)

Lead fluoride

2.7 x 10-8 (9°)

Strontium oxalate

5.61 x 10~8 (18°)

Lead fluoride

3.2 x 10~8 (18°)

Strontium sulfate

2.77 x 10~7 (2.9°)

Lead fluoride

3.7 x 10~8 (26.6°)

Strontium sulfate

3.81 x 10~7 (17.4°)

Lead iodate

5.3 x 10~14 (9.2°)

Zinc hydroxide

1.8 x 10~14 (18-20°)

Lead iodate

1.2 x 10"13 (18°)

Zinc oxalate, ZnC204 2H20

1.35 x 10-" (18°)

Lead iodate

2.6 x 10~13 (25.8°)

Zinc sulfide

1.2 x 10~23 (18°)

Pollutant

Ksp

PbCOs

7.4 x 10-14

PbCl2

1.7 x 10-5

PbCrO4

2.8 x 10-13

PbF2

3.6 x 10-8

PbSO4

6.3 x 10-7

PbS

3 x 10-28

Solution. In general, this can be solved by looking at the compound with the highest Ksp value, since this will be the one most responsible for the solubility of Pb2+ (but for similar Ksp values you must also consider stoichiometry). In this case, PbCl2 will determine the overall solubility of Pb2+ from the waste.

For each mole of PbCl2 that dissolves, one mole of Pb2+ ion is released and two moles of Cl-. We will let x = Pb2+. For every Pb2+ there are two Cl-, so if x = Pb2+, Cl- = 2x. Substituting into the Ksp equation yields

Ksp = 1.7 x 10-5 = [x] [2x]2 = 4x3 x = 0.016 M Pb2+ in ppm: (0.016 M Pb2+)(207.2 g/mol) (1000 mg/g) = 3400 mg/L

There is one complicating variable concerning solubility, both for organic and inorganic pollutants. Numerous field investigations report "dissolved" concentrations of pollutants that exceed their known maximum solubilities. But how can this be? The answer is the presence of a second "dissolved" phase, usually in the form of colloidal inorganic or organic particles. There is no conclusive way to isolate free ion in the dissolved phase from these phases for measure

0 0

Post a comment