HOW TO MEASURE ALKALINITY IN WATER SAMPLES SOME PRACTICAL INFORMATION

Transcription

1 HOW TO MEASURE ALKALINITY IN WATER SAMPLES SOME PRACTICAL INFORMATION Analysis of Total Alkalinity with Mettler Toledo T50 Excellence titrator - measurement settings and procedure Zuzanna Cieśla Anna Kowalska

2 WATER SAMPLES IN FOREST MONITORING 2012

3 WATER SAMPLES IN FOREST MONITORING 2012 In total ca samples/year of ph>5 measurement of alkalinity

4 1. INSTRUMENT Foto: Mettler Toledo 2009: Mettler Toledo T50 Excellence Titrator for the analysis oftotal alkalinity in open field, throughfall, stemflow samples and soil solutions from intensive monitoring plots implemented with the FutMon project From the beginningtwo-end point titrationmethod was optimized on the instrument following the ICP Forests Manual recommendations

5 2. SAMPLE approx. 50g of sample is weighed on an analytical balance into a 100 ml plastic titration vessel REMARKS: simpler operation than pipetting manually limited volume necessary for analysis-convenient when the total volume of the sample is small

6 3. SENSOR DG115-SC Combined glass ph electrode with a fixed groundglass sleeve junction for direct ph measurement and acid/base titrations in aqueous medium(also food and beverages) Especially suitable formedia with low ion concentration (e.g. low ion conductivity water) REMARKS: this sensor was chosen instead of the DG117-Water electrode for precise measurements in rain water with integrated temperature sensor and fast response because of the much quicker response and stable signal samples are stabilized to room temperature before titration electrode is conditionedfora few hours in buffer solution ph 4,01 before starting analysis calibration with two Mettler Toledo buffer solutions ph: 4,01 and 7,00 stirring speed 30%(rpm???) (if too fast not stable signal of the electrode) stirring time before starting measurements 45 sec (longer than default time) acquisition time min 15(default 2)-max 30sec Foto: Mettler Toledo

7 4. BURETTE 10 ml -It was the standard size of the burette cylinder delivered with the T50 titrator but as the addition of the titrant can be set very precisely on the titrator the large volume of the burette does not have a strong effect on theprecision of the analysis REMARKS: only after more than 3 years of regular use air bubbles in the tubing system started to appear during thetitration and the piston of the burette had to be replaced

8 5.1 TITRANT 0,01 mol/l HCl prepared from 0,1 mol/l HCl Merck TitriPUR (dilution with Millipore DI water) REMARKS: titer determination is performed according to ISO standard to ph 4,5 with blank correction because it was a pre-programmed method template in the T50 titrator, accurate and easy to execute eachnewly prepared titrant is standardized with 2,5mLwater solution of 1 g/lna 2 CO 3 (2 repetitions) and blank value is determined from titration ofdi water (2 repetitions)

9 5.2 TITRANT Titer value : determinations from Nov-2009 to Oct-2010(1 year!) min = 0,982 max = 1,016 small difference (c.a. +/-2%, so influence on the alkalinity result relatively not so significant)

10 6. TITRATION PARAMETERS Width of the control band 0,5 ph(i.e. starting from 5,0 if titration to 4,5): when the measurement curve reaches the control band the titrator slows down the titrant addition to approach the end-point with the minimum dosing rate Max dosing rate 0,3 ml/min(quite fast to shorten the analysis time) Min dosing rate 10 µl/min(to be not to fast after reaching control band, in other case dose of titrant may be to high and overlaps the end-point) these parameters were optimizedin our method because the pre-defined settings were not appropriate and the detection of the 2 end-points was too fast and not accurate. 7. QC MATERIAL 100 µeq/l prepared from AccuStandard Alkalinity standard 1000 μg/ml CaCO 3

11 8.1 METHOD CHARACTERISTICS LOQ value: determined from blank sample (DI water) LOQ = 10 x s = 8,14 µeq/l

12 8.2 METHOD CHARACTERISTICS Accuracy based on WRT 2010,2011 and 2013: WRT 2010 SAMPLE 2 (THR) SAMPLE 3 (THR) SAMPLE 4 (SW) SYN 6 SYN 7 SYN 8 measured value expected value deviation from expected value -35,3% -21,5% -4,7% -10,8% -2,4% 0,5% WRT 2011 SAMPLE 1 (BOF) SYN 5 SYN 6 measured value expected value deviation from expected value 11% -8,4% 7,4% Tolerable Limits 40% if Alk<100 µeq/l 25% if Alk>100 µeq/l WRT 2013 SAMPLE 3 (STF) SAMPLE 4 (SW) SYN 5 SYN 6 measured value 81, ,7 expected value 78, ,2 deviation from expected value 3,8% 22% 2,4% 3% Standard deviation from QC material in 2012: Mean 107,26 s 5,94 CV% 5,53 Expected value 100 µeq/l

13 WATER SAMPLES IN FOREST MONITORING 2012 In total ca samples of ph>5 measurement of alkalinity

14 Thank you for your attention!