Aspects of Alkalinity

Transcription

1 Aspects of Alkalinity K. Molt Fachgebiet Instrumentelle Analytik 9. Juli 2007 K. Molt (IAC) Alkalinity 9. Juli / 25

2 The carbonate system Dissolved Inorganic Carbon: DIC = c CO2 + c HCO 3 + c CO 2 3 K. Molt (IAC) Alkalinity 9. Juli / 25

3 c CO2 : c HCO : c 3 CO 2 0.5% : 86.5% : 13% 3 K. Molt (IAC) Alkalinity 9. Juli / 25

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5 Carbonate Alkalinity Alkalinity keeps track of the charge of the ions of weak acids Simplified seawater with ph = 8.2 Consider 1 kg of simplified seawater with ph = 8.2 and C CO2 = 2 mmol kg 1. At T = 25 C and S = 35 we have c CO2 = 8 µmol kg 1, c HCO = 1.7 µmol kg 1, and 3 = 0.3 µmol kg 1 c CO 2 3 K. Molt (IAC) Alkalinity 9. Juli / 25

6 Carbonate Alkalinity Titration of the simplified seawater Carbonate Alkalinity: CA = c HCO + 2c 3 CO 2 3 Second equivalence point: c H + = c HCO + 2c 3 CO 2 + c 3 OH Proton condition The number of moles of HCl added to reach this point is equal to the initial carbonate alkalinity of our sample (2.3 µmolkg 1 ). K. Molt (IAC) Alkalinity 9. Juli / 25

7 Carbonate Alkalinity Titration of the simplified seawater The carbonate alkalinity measures the charge concentration of the anions of the weak acid present in the solution. The carbonate alkalinity here is the number of equivalents of strong acid required to neutralize 1 kg of seawater until the second equivalence point is reached. Operational definition! K. Molt (IAC) Alkalinity 9. Juli / 25

8 Carbonate Alkalinity Alkalinity as master variable Given the alkalinity and C CO2, carbonate system parameters can be determined. Example: T A = 2.3 mmol kg 1, DIC = 2.0 mmol kg 1. T A c HCO + 2c 3 CO 2 3 DIC c HCO + c 3 CO 2 3 T A DIC c CO 2 3 c CO 2 3 c HCO 3 = 300 µmol kg 1 = 1700 µmol kg 1 K. Molt (IAC) Alkalinity 9. Juli / 25

9 Practical Alkalinity Including borate and water alkalinity borate alkalinity = c B(OH) 4 water alkalinity = c OH c H + P A = c HCO + 2c 3 CO 2 + c 3 B(OH) + c OH c H + 4 Proton condition: c H + }{{} proton donor = c HCO + 2c 3 CO 2 + c 3 B(OH) + c OH 4 }{{} proton acceptors K. Molt (IAC) Alkalinity 9. Juli / 25

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12 Dickson s definition of alkalinity The current most precise definition of titration or total alkalinity was given by Dickson (1994): The total alkalinity of a natural water is defined as the number of moles of hydrogen ion equivalent to the excess of proton acceptors (bases from weak acids with a dissociation constant K , at 25 C and zero ionic strength) over proton donors (acids with K > in one kilogram of sample. K. Molt (IAC) Alkalinity 9. Juli / 25

13 Total alkalinity due to Dickson T A = c HCO + 2c 3 CO 2 + c 3 B(OH) + c OH 4 +c HP O c P O c H3 SiO 4 + c NH 3 + c HS c H +,F c HSO 4 c HF c H3 P O 4 Proton condition that defines the equivalence point: c H +,F + c HSO + c HF + c H3 P O 4 4 = c HCO + 2c 3 CO 2 + c 3 B(OH) 4 +c OH + c HP O 2 4 +c H3 SiO + c NH c HS + 2c P O 3 4 K. Molt (IAC) Alkalinity 9. Juli / 25

14 Determination of total alkalinity due to Dickson The sample is titrated with strong acid, usually HCl, and the ph is recorded as a function of acid added. Then TA is calculated by nonlinear curve fitting of the theoretical titration curve of the aqueous system with the assumed components to the actual data using all the titration points. This procedure is also called inverse calculation. K. Molt (IAC) Alkalinity 9. Juli / 25

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18 Total alkalinity and charge balance Conservative ions Ions such as Na +, K +, Ca 2+, Mg 2+, Cl, (SO 2 4 ) and NO 3 can be regarded as conservative in the sense that their concentrations are unaffected by changes in ph, pressure, or temperature (within the ranges normally encountered in the earth s surface and assuming no precipitation or dissolution of solid phases, or biological tranformations). K. Molt (IAC) Alkalinity 9. Juli / 25

19 Total alkalinity and charge balance Alkalinity of the ocean The origin of the alkalinity in the ocean has to do with the charge balance of the major conservative ions in seawater. The sum of the charges of the major cations Na +, K +, Ca 2+, Mg 2+ are not exactly balanced by the major anions Cl, SO 2 4. This small charge imbalance is responsible for the total alkalinity in the ocean and is mainly compensated for by the anions of carbonic acid. Total alkalinity is equal to the charge difference between conservative cations and anions. K. Molt (IAC) Alkalinity 9. Juli / 25

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22 Conservation of total alkalinity Total alkalinity will remain exactly conserved during changes of temperature and pressure Total alkalinity does not change when CO 2 is exchanged between water and air or when CO 2 is taken up or respired by algae. Total alkalinity is conserved during mixing K. Molt (IAC) Alkalinity 9. Juli / 25

23 Total alkalinity in the ocean Dependence from salinity The charge difference between conservative cations and anions varies with salinity. Therefore TA in the ocean is closely related to salinity. Salinity changes are due to precipitation, evaporation, fresh water input, and formation or melting of sea ice and causes correponding changes in total alkalinity. K. Molt (IAC) Alkalinity 9. Juli / 25

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25 Total alkalinity in the ocean Biogenic precipitation of CaCO 3 Marine organisms such as coccolithophorids, foraminifera, pteropods, and corals, and dissolution of calcareous shells or skeletons. Precipitation leads to a decrease in Ca 2+ concentration. Precipitation of 1 mol CaCO 3 reduces DIC by 1 mol and TA by 2 mol. These changes are independent of the carbon source (HCO 3, CO3 2 or even CO 2 ) used by the organisms for calcification. (As a result, the carbon source cannot be inferred from observations of changes in DIC and TA in seawater.) K. Molt (IAC) Alkalinity 9. Juli / 25