Thermo Scientific Orion 320, User Manual

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2-5 Thermo Scientific Orion 320, 350 and 370 PerpHecT
®
LogR
®
Meter User Guide
Obtaining accurate pH values depends on both the correct measurement
of standard and sample temperatures and on the compensation for any
variations in these temperatures . This thermal compensation has generally
been accomplished either manually (by measuring sample solution with a
thermometer and then entering the measured temperature on the meter),
or automatically (by placing a separate temperature probe together with a
pH electrode in the solution) .
The temperature of a solution affects pH measurements in two ways:
1 . The pH of buffers and samples actually changes with temperature . For
buffers, this change is known and is built into the PerpHecT meters .
2 . The slope of the calibration, which the meter uses to convert the
raw millivolt readings of the electrode to pH values, changes with
temperature . At 25 °C, a 10 degree change in temperature changes the
slope by 3 .4% .
How important is direct temperature compensation to pH measurement?
To see the effects of temperature, look up your approximate sample
temperature on the left side of Table .1 . Read across the top to your
approximate pH value . The value shown at the intersection is the error
a 1° C change in temperature will cause in pH measurement without
compensation . Accurate temperature compensation is essential to accurate
pH measurement .
Table .1 .– . .pH .Error .due .to .1° .C .Temperature .Change .(in .pH .units) .
Temp.
(° C)
pH 6 or
pH 8
pH 5 or
pH 9
pH 4 or
pH 10
pH 3 or
pH 11
pH 2 or
pH 12
10 0.004 0.007 0.011 0.014 0.018
20 0.003 0.007 0.010 0.014 0.017
30 0.003 0.007 0.010 0.014 0.017
40 0.003 0.006 0.010 0.013 0.016
50 0.003 0.006 0.009 0.012 0.015
60 0.003 0.006 0.009 0.012 0.015
70 0.003 0.006 0.009 0.012 0.015
80 0.003 0.006 0.008 0.011 0.014
90 0.003 0.006 0.008 0.011 0.014
LogR Temperature
Measurements