Lecture #20 Dissolved Carbon Dioxide: Closed Systems II & Alkalinity (Stumm & Morgan, Chapt.4 ) Benjamin; Chapter 5.4 & 7

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1 Updated: 6 ctober 2013 Print version Lecture #20 Dissolved Carbon Dioxide: Closed Systems II & Alkalinity (Stumm & Morgan, Chapt.4 ) Benjamin; Chapter 5.4 & 7 David Reckhow CEE 680 #20 1

2 Alkalinity Alkalinity: ability of a water to neutralize strong acids a form of Acid Neutralizing Capacity (ANC) Interpretation in most natural waters: Alk tot = [HC 3- ] + 2[C 3 ] + [H - ] - [H + ] Net deficiency of protons with respect to C 2 Alk = 0 for a pure solution of carbon dioxide; therefore, C 2 does not add alkalinity: C 2 (aq)+h - =HC 3 - Alk tot = (α 1 + 2α 2 )C T + [H - ] - [H + ] Measurement by titration with a strong acid back to the ph of a pure C 2 solution (about 4.5) David Reckhow CEE 680 #20 2

3 Acidity Acidity: abilility of a water to neutralize strong bases a form of Base Neutralizing Capacity (BNC) Interpretation in most natural waters Acy tot = 2[H 2 C 3 ] + [HC 3- ] + [H + ] - [H - ] Net excess of protons with respect to C 3 Acy = 0 for a pure solution of carbonate; therefore, Na 2 C 2 does not add acidity: Na 2 C 2 + H + =HC Na + Acy tot = (2α 0 + α 1 )C T + [H + ] - [H - ] Measurement by titration with a strong base back to the ph of a pure C 3 solution (about 10.7) David Reckhow CEE 680 #20 3

4 Acidity & Alkalinity (cont.) Summation Alk tot + Acy tot = ([HC 3- ] + 2[C 3 ] + [H - ] - [H + ]) + (2[H 2 C 3 ] + [HC 3- ] + [H + ] - [H - ]) = 2[H 2 C 3 ] + 2[HC 3- ] + 2[C 3 ] =2C T therefore, you can determine C T from the two titrations Since Alkalinity is not affected by addition of C 2 it is considered a conservative substance in open systems e.g., loss of C 2 to the atmosphere does not affect alkalinity either David Reckhow CEE 680 #20 4

5 ther Alkalinity Species In sea water we use: Alk tot = [HC 3- ] + 2[C 3 ] + [B(H) 4 ] + [HP 4 ] + [H 3 Si 4 ] + [MgH - ] + [H - ] - [H + ] Species pka Average Conc. (M) Equilibria Chemical species which may contribute to alkalinity Carbonates 10.3/6.4 1x10-3 C3 + 2H + = HC3 - + H + = H2C3 Silicates 9.8 2x10-4 H3Si4 + H + = H4Si4 rganics 3 to 10 1x10-4 R-C - + H + = R-CH Borates 9.2 1x10-6 B(H)4 - + H + = B(H)3 + H2 Ammonia 9.2 2x10-6 NH4H + H + = NH4 + + H2 Iron 6.0/4.6 2x10-6 Fe(H) H + = Fe(H)2 + + H + = Fe(H) +2 Aluminum 8.0/5.7 2x10-6 Al(H) H + = Al(H)3 + H + = Al(H) /5.0 Al(H) H + = Al(H) H + = Al +3 Phosphates 7.2 7x10-7 HP4 + H + = H2P4 - Hydroxide x10-7 H - + H + = H2 Copper 9.8/7.3 1x10-7 Cu(H) H + = Cu(H) + + H + = Cu + + H2 Nickel 6.9 2x10-8 Ni(H)2 + H + = NiH + Cadmium 7.6 1x10-8 Cd(H) + + H + = Cd H2 Lead 6.2 1x10-8 Pb(H) + + H + = Pb H2 Sulfides 7.0 variable HS - + H + = H2S Zinc 6.1/9.0 variable Zn(H)2+ 2H + = Zn(H) + + H3 + = Zn H2 David Reckhow CEE 680 #20 5 See also, Table IX in Faust & Aly, 1981

6 Methyl range used as a colorimetric indicator of the final alkalinity titration endpoint (-) S N N H + N CH 3 CH 3 Yellow changes color at about ph 4.5 where all carbonates are as H 2 C 3 f=2 (-) (-) S S H N N (+) H N N CH 3 N CH 3 N (+) CH 3 CH 3 Red David Reckhow CEE 680 #20 6

7 Phenolphthalein used as a colorimetric indicator of alkalinity and acidity first endpoint changes color at about ph 8.3 ph signifies loss of H - and where all carbonates are as HC 3 - at f=1, and g=1 (-) H H C H C (-) C + 2 H H - 2 C (-) Colorless Red David Reckhow CEE 680 #20 7

8 Alkalinity procedures (cont.) calculations Equ t = Equ s V t N t = V s N s N s = V t N t /V s Sliding endpoint depending on concentration Alkalinity Potentiometric Colorimetric (mg/l) (ph) (from greenish blue to) light blue & lavender light pink red David Reckhow CEE 680 #20 8

9 Alkalinity: Chemical Interpretation At the phenolphthalein endpoint (Alk ph ), the following has occurred: H + + H - H 2 H + + C 3 HC 3 - Then at the methyl orange endpoint (Alk mo ): H + + HC 3- H 2 C 3 C 2 + H 2 Units: equ/l or more commonly, mg/l as CaC 3 1 equ/l = 50,000 mg/l as CaC 3 David Reckhow CEE 680 #20 9

10 Types of Alkalinity Speciation based on carbonate system Alk H = 50,000[H - ] = 50,000(10 phi-14 ) Alk HC3 = 50,000[HC 3- ] Alk C3 = 100,000[C 3 ] David Reckhow CEE 680 #20 10

11 Scheme for Alk determination If Alk ph > 0.5* Alk mo Alk H = 2*Alk ph - Alk mo Alk C3 = 2(Alk mo - Alk ph ) Alk HC3 = 0 If Alk ph 0.5* Alk mo Alk H = 0 Alk C3 = 2*Alk ph Alk HC3 = Alk mo - 2*Alk ph Where: Alk ph = 50,000V ph N t /V s Alk mo = 50,000V mo N t /V s David Reckhow CEE 680 #20 11

12 Acid Titration Curve for a Water Containing Hydroxide and Carbonate Alkalinity ph H + +H - =H 2 1 st Equivalence Point 2 nd Equivalence Point H + +C 3 =HC3 - Α A Titrant Volume (ml) David Reckhow CEE 680 #20 12 B Β H + +HC 3 - =H2 C 3 V ph Vmo

13 Acid Titration Curve for a Water Containing Carbonate and Bicarbonate Alkalinity ph Α Y[C 3 ] + Z[HC3 - ] 1 st Equivalence Point (Y + Z)[HC 3 - ] 2 nd Equivalence Point (Y)V s /N t (Y + Z)V s /N t V ph V mo Titrant Volume (ml) David Reckhow CEE 680 #20 13 Β C (Y + Z)[H 2 C 3 ]

14 Alkalinity & titrations (cont.) Relationship between chemistry, titration and buffer intensity See Stumm & Morgan, Figure 4.1 (pg. 154) Impact of C T on titration endpoints Refer to Benjamin, Figure 5.10 Also: Stumm & Morgan, Figure 4.3 (pg.157) and Pankow s Figure 9.2 (pg. 169) Conservation of Alkalinity Stumm & Morgan, Figures 4.7 and 4.10 (pgs. 167 and 177) David Reckhow CEE 680 #20 14

15 Pure H 2 C 3 : f=0 0-1 PBE Solutions H + H HC 3 - C 3 Log C David Reckhow CEE 680 #20 ph 15

16 Pure HC 3- : f=1 0-1 PBE Solutions H + H H 2 C 3 C 3 Log C David Reckhow CEE 680 #20 16 ph

17 Pure C 3 : f=2 0-1 PBE Solutions H + H H 2 C 3 HC 3 - Log C David Reckhow CEE 680 #20 ph 17

18 Stumm & Morgan Figure 4.3; pg. 157 David Reckhow CEE 680 #20 18

19 To next lecture David Reckhow CEE 680 #20 19