Drop Counter
4
012-08470G
9.
Observe the data appearing on the pH versus drop count
graph. After the equivalence point is reached, continue
collecting data until the pH curve flattens.
10.
Close the stopcocks.
11.
Stop data recording.
12.
Read the final volume of fluid in the syringe and calcu-
late the net volume of titrant that was dispensed.
13.
Read the final drop count on the graph.
14.
In the calculator window of the software or datalogger,
enter a calculation as shown below, but using your own
values of total volume dispensed and final drop count.
The calculation gives the volume of titrant dispensed
over time based on the number of drops and the average
volume per drop.
volume = [Drop Count (drops)] * 100/3050
In this example, “100” is the total volume dispensed (in
mL) and “3050” is the final drop count.
15.
In the graph, change the horizontal axis to the calculated
“volume”.
Tips
•
Except for opening and closing the stopcocks, do not
touch the syringe during the titration; otherwise, the
drop size may change significantly.
•
The drop rate must remain approximately constant
(about 2 drops per second) in order for the drop size to
remain constant.
•
Drop size depends on the exact position of the bottom
stopcock. Therefore, the average drop size will be dif-
ferent for every titration. For each titration, re-enter the
volume used and drop count values in the calculation.
Other Titrations
Titrations with different solutions can be performed using
the same equipment and procedure. Other probes (such as
conductivity or ORP) can be used in place of the pH probe.
Theory of Operation
The Drop Counter
uses an infrared light
source and a photo-
detector. Lenses
spread the light into a
“sheet” and refocus it
onto the photodetec-
tor. When a drop of
fluid passes through the Drop Counter, it partially blocks this
sheet of light, and the photodetector registers a momentary
decrease in light intensity.
The Drop Counter uses infrared light and ignores visible
light. On power up, it automatically adjusts the light level for
the best sensitivity. Use the Drop Counter away from direct
sunlight or other sources of infrared that may interfere with
it.
Troubleshooting
equivalence point
final drop count
Select volume calculation
for horizontal axis
Problem
Cause
Solution
Indicator light
flashes twice for a
single drop. Drop
Counter registers
more drops than
actually dispensed.
Fluid is splashing
back into the
drop window.
Adjust the position
of the beaker, Drop
Counter, or syringe
to eliminate splash
back
Indicator LED
remains on
constantly. Drop
Counter does not
register drops.
Lenses in drop
window are dirty
or wet.
Clean lenses with
water and dry with a
cotton swab or
tissue.
detector
collimating
lens
focusing
lens
sheet of light
drop
infrared light
source
