S ervice
M étrologie
etrologische
D ienst
CHEMICAL IMPURITIES IN WATER
TRIPLE POINT CELLS: CURRENT
PROGRESS ON THE SILICON EFFECT
Miruna Dobre,
Dobre Andrea Peruzzi
Motivation
A step forward has already been done for TPW improvement with
the CIPM decision to take into account the isotopic composition of
water .
The effect of chemical impurities is know the next step toward a
more accurate triple point of water.
The dissolution of borosilicate glass in water triple point cells is a
major source of impurities.
The effect on the triple point of the temperature is a depression
ranging, according to the published research, from 14 µK per year
(Hill, NRC, 1995) to 4 µK per year (Furukawa and Bigge, NIST,
1982 and Hill, NRC, 2001).
Workshop Euramet 732
2
The starting point…
G. F. Strouse (NIST) and M. Zhao (Fluke) studied the evolution of
impurities concentrations in TPW and their effect on triple point
temperature (Tempmeko ’07).
They concluded (on a 2 years measurements basis) that main
impurities dissolved from the borosilicate glass in water, were K,
Na, B and Si.
While concentrations of K, Na, B stabilized after 6 months, Si
concentration increased linearly with time.
In the same time, the borosilicate glass cells showed an average
drift of -13µK/year. The quartz cells used in the study showed no
drift (-2 µK/year).
Workshop Euramet 732
3
The starting point…
250
Molar Fraction, nmol·mol–1
200
K
Na
Si
B
150
100
50
0
Feb-05
Sep-05
Mar-06
Date of Measurement
Workshop Euramet 732
4
Oct-06
Apr-07
The starting point…
Calculated from chemical analyses
Freezing depression (µK)
1000
Measured
The hottest cell is
used as reference
750
500
250
0
1
2
3
4
5
6
Cells
K.D. Hill (NRC) – Tempmeko ‘99
Workshop Euramet 732
5
7
8
9
10
NMi-VSL triple point of water cell design
To investigate the effect of Si
on triple point temperature a
cell doped with 3 µmol/mol Si
is manufactured.
The design allows to separate
a bulb with cell water for
analysis.
The expected temperature
depression (Raoult law of
dilute solutions) is 300 µK
Workshop Euramet 732
6
The Si doped cell needs 5 days more than the
normal cell to stabilize
Workshop Euramet 732
7
The Si doped cell has a triple point lower by 4.7
mK than the normal cell
Freezing depression (mK)
5,5
5,0
4,5
4,0
3,5
1
Workshop Euramet 732
2
3
4
5
6
7
Days
8
8
9
10
11
12
The Si doped cell has a triple point lower by 4.7
mK than the normal cell
The first results with the doped cell shows an unexpected great
difference between the Raoult law estimation and the measured
values, 4700 µK instead of 300 µK.
Taking in acount that our Si was in fact a solution of ammonium
hexafluorosilicate (NH4)2SiF6 in water matrix and this might create
3 ions, 2 NH4+ and the SiF6, we will have three particles for one
atom of input Si.
The doped impurity effect will therefore be three times larger than
expected…but this is still not enough to explain the difference
Workshop Euramet 732
9
Summary
Available studies on impurities effects conclude that there is a
continuos dissolution of Si from the borosilicate cell envelop.
Raoult law seems not to predict well the total impurities effect on
triple point temperature.
A cell was doped with 3 µmol/mol Si in order to quantify the freezing
depression.
Cell measurements showed a much greater depression than
predicted by Raoult’s law of dilute solutions.
Further investigation, theoretical and experimental (other doped
cells with different impurities amounts)
Workshop Euramet 732
10