how to measure alkalinity in water samples –some …
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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
WATER SAMPLES IN FOREST MONITORING 2012
In total ca. 1000 samples/year of pH>5 → measurement of alkalinity
1. INSTRUMENT
2009: Mettler Toledo T50 Excellence Titrator for the analysis of total alkalinity in
open field, throughfall, stemflow samples and soil solutions from intensive
monitoring plots implemented with the FutMon project
From the beginning two-end point titration method was optimized on the
instrument following the ICP Forests Manual recommendations
Foto: Mettler Toledo
2. SAMPLE
approx. 50 g of sample is weighed on an analytical balance into a 100 ml plastic
titration vessel
REMARKS:
Øsimpler operation than pipetting manually
Ølimited volume necessary for analysis - convenient when the total volume of
the sample is small
3. SENSOR
REMARKS:
Øthis 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
Øsamples are stabilized to room temperature before titration
Øelectrode is conditioned for a few hours in buffer solution pH 4,01 before starting
analysis
Øcalibration 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 30 sec
DG115-SC
Combined glass pH electrode with a fixed ground-
glass sleeve junction for direct pH measurement and
acid/base titrations in aqueous medium (also food
and beverages) Especially suitable for media with low
ion concentration (e.g. low ion conductivity water)
Foto: Mettler Toledo
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 the precision of the
analysis
REMARKS:
Øonly after more than 3 years of regular use air bubbles in the tubing system
started to appear during the titration and the piston of the burette had to be
replaced
5.1 TITRANT
0,01 mol/L HCl prepared from 0,1 mol/L HCl Merck TitriPUR (dilution with Millipore
DI water)
REMARKS:
Øtiter determination is performed according to ISO standard 9963-1 to pH 4,5 with
blank correction because it was a pre-programmed method template in the T50
titrator, accurate and easy to execute
Øeach newly prepared titrant is standardized with 2,5mL water solution of
1 g/L Na2CO3 (2 repetitions) and blank value is determined from titration of DI
water (2 repetitions)
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)
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 µµµµl/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)
Ø these parameters were optimized in 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 CaCO3
8.1 METHOD CHARACTERISTICS
LOQ value: determined from blank sample
(DI water)
LOQ = 10 x s = 8,14 µeq/L
8.2 METHOD CHARACTERISTICS
Accuracy based on WRT 2010, 2011 and 2013:
WRT 2010SAMPLE 2
(THR)
SAMPLE 3
(THR)
SAMPLE 4
(SW)SYN 6 SYN 7 SYN 8
measured value 16 29 82 31 76 142
expected value 25 37 86 35 78 141
deviation from expected
value-35,3% -21,5% -4,7% -10,8% -2,4% 0,5%
WRT 2011SAMPLE 1
(BOF)SYN 5 SYN 6
measured value 46 769 67
expected value 41 840 62
deviation from expected
value11% -8,4% 7,4%
WRT 2013SAMPLE 3
(STF)
SAMPLE 4
(SW)SYN 5 SYN 6
measured value 81,27 26 815 85,7
expected value 78,27 21 796 83,2
deviation from expected
value3,8% 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
Tolerable Limits
40% if Alk<100 µeq/L
25% if Alk>100 µeq/L
WATER SAMPLES IN FOREST MONITORING 2012
In total ca. 1000 samples of pH>5 → measurement of alkalinity