Problems related to measurements of low values of total alkalinity

Transcription

1 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, Croatia, September 2013 Problems related to measurements of low values of total alkalinity Rosario Mosello, Aldo Marchetto & Gabriele Tartari C.N.R. Institute of Ecosystem Study, Verbania (Italy) r.mosello@ise.cnr.it

2 General aspects Total alkalinity In WRT and in most of the intercomparison exercises, low values of alkalinity resulted among the variables affected by the highest errors Following ICP Forests manual, alkalinity measurement is mandatory in deposition samples with ph>5.0 In this samples alkalinity is important to check of the ion balance.

3 General aspects

4 General aspects Total alkalinity: definition Alkalinity of a water is its acid-neutralising capacity. It is a measure of an aggregate property of water and can be interpreted in terms of specific substances only when the chemical composition of the sample is known. It is the sum of all the titrable bases. TA = [HCO 3- ] + [CO 3= ] + [OH - ] [H + ] + [A - org ]+ [ B- inorg ] where concentrations are expressed in eq L -1, A - org are organic compounds and B - inorg are inorganic bases which may accept protons (borate, phosphate, silicate, etc)

5 General aspects In most of the freshwater and in atmospheric deposition alkalinity is mainly dependent on the inorganic carbon equilibrium. In the ph range of atmospheric deposition ( ) the prevailing form of inorganic carbon are H 2 CO 3* and HCO - 3 so it is normally assumed that TA= [HCO 3- ] C in. Tot H 2 CO 3 * + CO 2 Forms of inorganic carbon (Ct = 1) HCO 3 - CO 3 = pk 1 = 6,38 pk 2 = 10, Atmospheric deposition range ph

6 Acid Neutralizing Capacity (ANC) In literature also ANC is used for assessing effects of air pollution. ANC is defined as the potential of a solution to neutralise additions of strong acids to a given level. From the chemical point of view ANC and TA are exactly the same. ANC = TA = [HCO 3 - ] + [CO 3= ] + [OH - ] [H + ] + [A - org]+ [ B - inorg] ANC (TA) value is sometime calculated from the ion balance: ANC = ([Ca 2+ ]+ [Mg 2+ ] + [Na + ] + [K + ] + [NH 4 + ]) - ([Cl - ] + [SO 4 2- ]+ [NO 3 - ]) ANC = N base cations - N strong acid anions ANC, calculated as the difference between base cations and acid anions, is widely used as a measure of freshwater acid status, and as an indicator of biological conditions.

7 Calculations of Acid Neutralizing Capacity (ANC) However, since ANC is calculated as the residual of a large number of individual ion determinations, it is potentially sensitive even to relatively small analytical errors. It is recommended a more reliable measurement of ANC (TA), based on titration

8 Total alkalinity: methods of analysis Total alkalinity is determined through an acidimetric titration (it does exist as well a conductometric method). The measured value may vary significantly with the analytical technique used and with the selected end-point. The problem is the detection of the equivalence point of the titration, i.e. the condition: [H + ] = [ HCO 3- ]

9 8.0 ph Titration curve H + µeq/l Inflection point ph ml HCl added HCl 0.05 N (ml)

10 Indicators: Acidimetric titration of alkalinity Determination of the equivalent point by: Different indicators change color at different ph! Potentiometry: One end-point (ph 4.3, 4.5,.) Detection of the inflection point Two end-points (normally differing by 0.3 ph units ) Gran s titration

11 Indicators ph range Methyl orange red-yellow Bromophenol blue yellow-blue Methyl red red - yellow Bromocresol green yellow-blue Disadvantages: high error associated due to: 1) Not coincidence between the ph range of variation of colour and ph range of the equivalence point ( ); 2) range of ph of the colour change; 3) different sensitivity of the operator to detect the colour change.

12 8.0 ph One end point titration, potentiometry. H + µeq/l Inflection point ph ml HCl 400 ph ml HCl 4.0 ph ml HCl added HCl 0.05 N (ml)

13 One end point titration, potentiometry. Correction of alkalinity values Correction to be applied to the ph H+ conc. alkalinity concentration Real equivalent point µeq L One end point to µeq L -1 ~29 µeq L -1 One end point to µeq L -1 ~47 µeq L -1 Henriksen (1982) correction for one end point (ph = 4.5) titration: TA (µeq/l) = (TA ) (TA ) 0.5

14 8.0 ph Detection of the inflection point First derivative H + µeq/l Inflection point ph ml HCl added HCl 0.05 N (ml)

15 H + µeq/l Two end-point titration ph 4.2 (H + 64 µeq/l) ph 4.5 (H + 32 µeq/l) V 0 = V 1 - (V 2 - V 1 ) V 0 = 2 V 1 - V inflection point V 0 V added HCl 0.05 N (ml) 1 V

16 Gran s titration for sample with ph> F1 Gran 10 6 F1 Gran = (ml sample volume + ml HCl added ) 10 -ph V 0 is obtained by linear regression: F1 Gran = a +b ml HCL 500 inflection point added HCl 0.05 N (ml) V

17 Gran s titration for sample with ph<5 F1 Gran 10 6 Negative V 0 is obtained from extrapolation of linear regression V 0 = - 0,051 ml added HCl 0.05 N (ml) negative alkalinity = strong acidity

18 Conclusions The measurement of low values of alkalinity are reliable if: 1) the following analytical methods are used: Gran titration; two end point titration ( ph = 0.3 u e.g ); titration at ph 4.5 and correction for the extra acid added. 2) the ordinary AQC are adopted, e.g.: ordinary maintenance of electrodes and titrator; periodic calibration of the titrant acid (HCl 0.01 N); 260 use of quality control charts. 250 Example for NaHCO 3 standard solution µeq/l /8/06 5/9/06 18/9/06 22/9/06 25/9/06 26/9/06 3/10/06 10/10/06 11/10/06 19/10/06 25/10/06 2/11/06 8/11/06 16/11/06 21/11/06 27/11/06 28/11/06 5/12/06 7/12/06 18/12/06 19/12/06 3/1/07 8/1/07 10/1/07 11/1/07 18/1/07 22/1/07 26/1/07 12/2/07

19 Conclusions A chemical variable is normally identified by: name of the variable, unit of measurement, numerical value calcium, mg L -1, 0.25 In some case it is important to specify the analytical method used: total alkalinity, µeq L-1, 52 one end-point ph 4.5 acidimetric titration In the case of alkalinity this is of the main importance because of the relevant differences existing among the different analytical methods used. Alkalinity values stored in the ICPF European database, as well as other chemical variables, should be comparable.

20 Thank you for your attention

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22 WRT 2009 exercise 30 Alkalinity 20 SYN 5 ph 6.36 Alk 33 µeq/l SYN 6 ph 6.56 Alk 79 µeq/l SYN Alkalinity SYN5 SYN6 2 0 No. of labs 44-2 DQO No. measures 40-4 Within target (40 % = 2SD) 25 No. outliers 15 Not measured SYN5