YSI 60 - Manual (Page 24)

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According to the Nernst equation as shown above, the slope of the plot of pH vs. millivolts is
directly proportional to the absolute temperature in degrees Kelvin. Thus, if the slope of the plot
is experimentally determined to be 59 mv/pH unit at 298 K (25 C), then the slope of the plot at
313 K (40 C) must be (313/298) * 59 = 62 mv/pH unit. At 283 K (10 C), the slope is calculated
to be 56 mv/pH unit ((283/298) * 59). Determination of the slope of pH vs. mv plots at
temperatures different from T
c
is thus relatively simple. In order to establish the intercept of the
new plot, the point where plots of pH vs. mv at different temperatures intersect (the isopotential
point) must be known. Using standard pH determination protocol, the YSI Model 60 software
assigns the isopotential point as the mv reading at pH 7 and then calculates the intercept using
this assumption. Once the slope and intercept to the plot of pH vs. mv are assigned at the new
temperature, the calculation of pH under the new temperature conditions is straightforward, and
is automatically carried out by the software.
Number of Calibration Points
When calibrating the YSI Model 60, you have the choice of 1-point 2-point, or 3-point
calibration. Perform a 2 or 3 point calibration at least once per day for accurate results.
Select the 1-point option only if you are adjusting a previous calibration. If a 2-point or 3-point
calibration has been performed previously (at least once per day), you can adjust the calibration
by carrying out a 1-point calibration at pH 7 (or pH 6.86). This calibration procedure adjusts
only the pH offset and leaves the previously-determined slope unaltered.
Select the 2-point option to calibrate the pH probe using only two calibration standards. In this
procedure, the pH sensor is calibrated using a pH 7 (or pH 6.86) buffer and one additional
buffer. A two point calibration procedure (as opposed to a 3-point procedure) can save time if
the pH of the sample is known to be either basic or acidic. For example, if the pH of a sample is
known to vary between 5.5 and 7, a two point calibration with pH 7 and pH 4 buffers is
appropriate. Three point calibration with an additional pH 10 buffer will not increase the
accuracy of this measurement since the pH is not within this higher range.
Select the 3-point option to calibrate the pH probe using three calibration solutions. In this
procedure, the pH sensor is calibrated with a pH 7 (or pH 6.86) buffer and two additional
buffers. The 3-point calibration method assures maximum accuracy when the pH of the media to
be monitored cannot be anticipated.
8.2 Temperature
The YSI Model 60 system utilizes a thermistor which changes predictably in resistance with
temperature variation. The algorithm for conversion of resistance to temperature is built-in to the
Model 60 software, and accurate temperature readings, in degrees Celsius, are provided
automatically. No calibration or maintenance of the temperature sensor is required.