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19.2 Calculating the ATC Slope of Solutions
The conductivity of all solutions changes as the temperature of the solution changes. The ATC
Slope of a solution is the amount that this Conductivity changes per degree Celsius. The ATC Slope
is dependant upon the chemicals dissolved in the solution, the concentration of those chemicals, and
the actual temperature of the solution. Consider the following examples…
•
A 0.02 Molar solution of KCl has a different ATC Slope to a 0.02 Molar solution of NaCl.
•
A 0.02 Molar solution of KCl has a different ATC Slope to a 0.01 Molar solution of the same
salt.
•
A 0.02 Molar solution of KCl at 25
O
C has a different ATC Slope to a 0.02 Molar solution of
KCl at 15
O
C.
To calculate the ATC Slope of a solution…
1. Set the ATC Slope for sample measurements to zero, with the following key sequence…
→
→
F4:Setup
→
→
F6:Next
→
→
F1:ATC Slope
→
→
F1:Sample
→
→
0.00
→
→
2. Ensure that the temperature function has been calibrated (see section 7.1). The conductivity
function does not need to be calibrated at this point.
3. Warm or cool the solution to approximately 5
O
C below the temperature at which you expect to
take sample measurements. Record the Conductivity and Temperature readings, once both have
become stable.
4. Warm the solution to approximately 5
O
C above the temperature at which you expect to take
sample measurements. Record the Conductivity and Temperature readings, once both have
become stable.
5. Apply the following formula, where C1 and T1 are the first readings taken, and C2 and T2 are
the second readings taken…
C2
Log C1
T2 - T1
ATC Slope = 100 x 10 - 1
6. Enter the result that is obtained as the ATC Slope of the solution.
(a) If this is the ATC Slope of the sample solution to be measured…
→
→
F4:Setup
→
→
F6:Next
→
→
F1:ATC Slope
→
→
F1:Sample
→
→
[result]
→
→
(b) If this is the ATC Slope of the standard solution to be used for calibration…
→
→
F4:Setup
→
→
F6:Next
→
→
F1:ATC Slope
→
→
F2:Standard
→
→
[result]
→
→
Note
To assist in calculating the ATC Slope the following information can be entered into a spreadsheet.
This example has been specifically written for Microsoft® Excel®, although the syntax should be
similar for other spreadsheet software.
A
B
1
Conductivity 1
<Enter first Conductivity reading>
2
Temperature 1
<Enter first Temperature reading>
3
Conductivity 2
<Enter second Conductivity reading>
4
Temperature 2
<Enter second Temperature reading>
5
ATC Slope
=100*((10^((LOG10(B3/B1))/(B4-B2)))-1)
Alternatively a ready-made file in can be downloaded from the Downloads page of the TPS Web
Site. Enter
http://www.tps.com.au/downloads/
into your browser to go there directly.