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

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

Thank you for your attention!