YSI ProDSS Meter Calibration and Maintenance
K:\Streamkeepers\Monitoring\Eqpt & supplies\Calibration-Maintenance\SOPs\YSI ProDSS maint-
cal SOP's
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Titration Steps:
NOTE:
dilute the chemicals going into the sink during the process with a continuous stream of
tap water to prevent damage to plumbing.
1. Put on the plastic apron, face shield and Nitrile/Latex gloves.
2. Pour off the water seal and invert the bottle several times to mix the floc.
3. Remove the glass stoppers and put sample bottles in sink.
4. Using a pipette, draw and add 2 mL of sulfuric acid to each sample bottle. Keep the tap water
running.
5. Rinse the pipette and put the cap securely on the acid bottle.
6. Put the stoppers back on the bottles and invert them several times over the sink until the
precipitate has completely dissolved.
7. To transfer the liquid to the 203 mL volumetric flask, place the flask on top of BOD bottle
number 1 and invert over sink. Once the flask is full, quickly remove the BOD bottle. The
liquid should appear slightly convex on the top of the flask. Transfer the sample to the #1
Erlenmeyer flask. Repeat for samples # 2 and 3.
8. Carefully uncover the sodium thiosulfate burette (a purple cover on it for protection and
storage.)
9. Ensure there is enough thiosulfate solution to complete the three trials in the burette. Add
solution to the reservoir if necessary.
10. Empty any of the sodium thiosulfate left in the burette.
11. Turn the stopcock valve so the handle faces the burette body (directly away from you toward
the wall).
12. Tighten the valve on the rubber bulb and pressurize the reservoir. Pump the rubber bulb gently
to raise the solution, drain, refill and drain twice more to rinse, then fill the burette until the
sodium thiosulfate escapes from the top nipple.
13. Turn the stopcock 90 degrees and loosen the valve on the bulb to allow the excess solution to
return to the reservoir.
14. Drop a stir bar in the first sample flask and place it centered on the magnetic stirrer. Place the
stirrer and flask centered under the burette valve to ensure drips fall cleanly into flask.
15. Turn the stirrer on the lowest setting.
16. Turn the stopcock valve handle toward you (there is a guide mark) and begin adding the
sodium thiosulfate drops. You can add drops somewhat quickly at first but slow down when
the sample starts turning a pale yellow.
17. Now, SLOWLY add one drop at a time until the sample turns very pale yellow and stop
titrating. (It helps to place white paper between the burette and sample bottle.)
18. Add 1 to 2 mL of the starch solution into the flask using the squirt bottle until the sample turns
a dark purple.
19. Continue to titrate the sample, adding drops VERY SLOWLY until the purple color just
disappears and the sample is clear. Record the volume of solution used for each sample, to the
nearest 0.1 mL. If ANY doubt about the end point, you can check the titration of the sample.
20. Check the end point by adding a drop of potassium bi-iodate into the flask with a dropper that
delivers 20 drops per mL, identical to the burette. If the end point is correct, the purple color
should reappear. If more than one drop is required, then the end point was overrun.
