Conbine expert panel Zagreb April 2017 ORGANIC ACIDS IN DEPOSITION. Aldo Marchetto, Nils Koenig, Rosario Mosello

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1 ORGANIC ACIDS IN DEPOSITION Aldo Marchetto, Nils Koenig, Rosario Mosello

2 There are inconsistency in the manual: Pag 143: Pag. 147

3 RCOO - is a quite large category We measure DOC, and we do not know which compounds it exactly includes.

4 What are weak acids? Acids that are not completely dissociated in H+ and a base This definition depends from pk A and ph Ka= [H+] [X-]/[HX] pka = ph log ([X-]/[HX]) when ph = pka => [X-] = [HX] 50% dissociated when ph = pka + 2 => [X-] / [HX] = 100 only 1% dissociated when ph = pka -2 => [X-] / [HX] = 0.01 almost completely dissociated

5 In rainwater, as examples HCl pka = -2 completely dissociated, even at ph = 0! HCO 3 - pka = 6.35 completely dissociated at ph 4.35 (alkalinity titration end point : 4.3) NH 4 + pka = 9.24 start dissociating at ph > 7.24 In the DOC, we probably have two "groups" of "acids": one can be titrated at ph 4.3, i.e. they have pka >> 5.3 a second is not titrated, i.e. they have pka << 3.3 and obviously an intermediate group.

6 The latter (strong acids) (and part of the intermediate) is considered at page 147 of the manual: S an = Alk + SO4 * NO3 + Cl + RCOOand we use them for ion balance correction. data used by Mosello et al J.Limnol. 67: 1-14

7 Apparently, DOC contains relatively strong acids, with a formal charge of ca. 5 µeq / mg DOC Examples from Aquatic Humic Substances: Ecology and Biogeochemistry (ed. Dag Hessen,Lars Tranvik)

8 The former group of acids, which are relatively weak, are included in alkalinity measurement: TAlk = HCO 3- + CO OH - H + + RCOO - + A - At the ph typical of rainwater (ph = 4 to 8), CO3--, OHand A- are negligible alkalinity is formed by HCO 3- and a component related to DOC. Can we separe them?

9 HCO 3 = K CO2 pco 2 H i.e. log (HCO 3 )= pk log(pco 2 ) - ph When there are no organic acids (DOC close to zero) HCO 3- ~= Alk palk = - log(hco 3- ) and ph + palk = pk log(pco 2 ) = constant

10 When there are no organic acids (DOC close to zero) ph + palk = constant (red points in the plot, ICP Forests data 2014). Increasing DOC -> increasing weak acids -> the points are shifted left/down all ICPF data throughfall 2014 no QC

11 Assuming a relatively constant composition of DOC (which is probably not the case), it should be a relationship between DOC and RCOO - (weak). i.e. Alk HCO 3 - all ICPF data throughfall 2014 no QC

12 Assuming a relatively constant composition of DOC (which is probably not the case), it should be a relationship between DOC and RCOO - (weak). i.e. Alk HCO 3 - all ICPF data throughfall 2014 no QC

13 Oliver (1993) tried to formalize this relationship for humic and fulvic acids in humic water: RCOO K = m DOC a and Ka = a + b ph c K a +H ph2. In freshwater Oliver et al. (1993) m=10 µeq/mg DOC, a =.15, b = 1.41, c = Driscoll et al. (1994) m=15.1 µeq/mg DOC, a =.96, b = 0.90, c =

14 RCOO K = m DOC a and Ka = a + b ph c K a +H ph2. These equations were formulated on the basis of acid fresh water, by titration to ph 7 to obtain m, K a was determined by titrating a high concentrations (32 to 1000 mg/l) solutions of fulvic or humic acid with N 2 bubbling to exclude CO 2. Being acid solutions, no interference from HCO 3- was considered.

15 all ICPF data throughfall 2014 no QC... but we work on a different ph range, and very likely we have different substances, not humic or fulvic acids.

16 A second approach would be to measure dissolved inorganic carbon (DIC) and obtain HCO 3- from C inorg = HCO 3- + H 2 CO 3 and [H]*[HCO 3- ]/[H 2 CO 3 ] = Ks (pks = 6.38) and measuring ph we can calculate HCO - 3 from ph and DIC. NW-FVA data

17 A second approach would be to measure dissolved inorganic carbon (DIC) and obtain HCO 3- from C inorg = HCO 3- + H 2 CO 3 and [H]*[HCO 3- ]/[H 2 CO 3 ] = Ks (pks = 6.38) and measuring ph we can calculate HCO - 3 from ph and DIC. NW-FVA data

18 A second approach would be to measure dissolved inorganic carbon (DIC) and obtain HCO 3- from C inorg = HCO 3- + H 2 CO 3 and [H]*[HCO 3- ]/[H 2 CO 3 ] = Ks (pks = 6.38) and measuring ph we can calculate HCO - 3 from ph and DIC. NW-FVA data

19 Obtaining weak acids form DIC and alkalinity is possible also in soil solutions, where pco 2 is not known. However this will require to introduce new measurements and cannot be done on most previous data and there is a problem of CO 2 interference for low values of alkalinity.

20 Should we correct the manual? Pag 143: weak Pag. 147 strong

21 Do we need more information on organic acid? Do we need new measurements or a new elaboration of existing data?

Problems related to measurements of low values of total alkalinity

Problems related to measurements of low values of total alkalinity EU/UN ECE - International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests Working Group on QA/QC in Laboratories Meeting of the Heads of the Laboratories Zadar,