47
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(
m
c
/
S
µ
)
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)
m
c
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(
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m
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(
0
.
5
7
.
4
7
.
5
5
.
2
4
.
6
3
.
2
1
.
5
1
.
4
8
.
5
4
.
2
5
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6
2
.
2
2
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5
6
.
3
9
.
5
4
.
2
6
.
6
1
.
2
3
.
5
3
.
3
0
.
6
4
.
2
7
.
6
6
.
2
4
.
5
0
.
3
1
.
6
4
.
2
8
.
6
1
.
3
5
.
5
8
.
2
2
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6
5
.
2
9
.
6
8
.
3
6
.
5
6
.
2
3
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6
4
.
2
RESISTIVITY MEASUREMENT
Make sure the instrument and probe has been calibrated in conductivity mode before taking resistivity
measurements.
DIRECT MEASUREMENT
To measure the resistivity of a sample using the
Direct reading mode:
Note:
A temperature bath at 25.0 ±1 °C is required for this
measurement.
• Take the water sample from the stage 2 test and increase its
ionic strength for a pH measurement at 25 °C.
• Use 100 mL of Stage 2 water and add 300 µL saturated KCl to
the sample.
• Calibrate a pH sensor in pH 4.01 and pH 6.862 (or 7.01)
buffers.
• Thermally equilibrate the sample to 25.0 ±1 °C.
• Measure sample with the calibrated pH sensor.
• The pH of sample must be between 5.0 and 7.0 pH.
• Record the pH and round it to the nearest 0.1 pH.
• Find the measured pH and corresponding conductivity in the stage 3 table on the next page.
• Compare the conductivity value determined in stage 2 to the conductivity value found in the stage 3 table.
• If the stage 2 conductivity is lower than the conductivity from the table below, the sample has meet the
USP requirements. Otherwise, the water didn’t meet the USP requirements.