31
CALIBRATION - 5
Martek Instruments, Inc.
5.3
pH C
ALIBRATION
Measuring pH in ultrapure water requires
specially designed electrodes. Unlike nor-
mal water applications, ultrapure water has
very low ionic activity making it difficult to
measure pH accurately. The Mark 22 can use
a variety of pH electrodes from different
manufacturers. However, the following cali-
bration procedures apply to the ultrapure pH
sensor offered by Martek Instruments.
The Martek pH sensor should be cali-
brated in buffer solutions that are traceable
to NIST standards for accuracy and reliabil-
ity.
Care should be taken to insure that fresh,
uncontaminated buffer solutions are used.
Buffer cups should always be rinsed with
distilled water and dried before each use and
used buffers should always be discarded.
Finally, the temperature of the buffer so-
lution must be measured and the appropri-
ate pH value versus temperature inputted
into the Mark 22. You can use either the
Martek Temperature Sensor or a laboratory-
grade thermometer to determine buffer tem-
perature. Most buffer solutions list the vari-
ous temperature variations on the side of the
buffer containers. If the values are not listed,
consult the manufacturer or distributor for
this information.
Most importantly,
care
must be taken to ensure that the solution is
properly grounded.
If not, pH measurements
can be erratic. Using the pH Adapter Plug
(P/N 180-51A) is a good way to ensure accu-
rate temperature measurement as well as
proper grounding of the solution.
5.3.1
pH Zero Adjustment
Connect the pH sensor to the Mark 22
using the pH adaptor cable (See Section 3.2).
Place the pH sensor in pH buffer 7.00 so-
l u t i o n .
Press the
SETUP
key and select the "CAL"
option from the
SETUP
menu. Next, select
the "pH" option. The display will prompt
with the following:
Press the
SELECT
key to choose "ZERO".
The display will prompt with the momen-
tary message:
Then will ask you to:
When the voltage reading on the display
has stabilized, Press the
SELECT
key. The
Mark 22 will prompt with the following dis-
p l a y :
and the "7" will be flashing. Input the pH
value as a 4-digit number (make sure you use
the correct pH value corresponding to the
temperature of the pH buffer and that you
input a 0 before single-digit pH values). You
can alter the value of the flashing digit by
pressing the arrow keys up or down. Move
forward to the next digit by pushing the
SE-
LECT
key. Move back to a previous digit by
pushing the
EXIT
key. Once the zero point
has been entered, the Mark 22 will automati-
cally respond with the following display:
ZERO SPAN
pH
PLACE SENSOR IN
LO pH STANDARD
HIT KEY W/STABLE
-0.1412 VOLT
INP 4 DIGIT STD.
LO pH = 07.00
