Hold the titration flask(3) in one hand and the mini burette(6) in the other. Slowly push in the plunger
of the mini burette to expel the titration reagent drop by drop. A slight rotating or twisting motion
helps the plunger to move smoothly as it is pushed in.
As each drop of reagent falls into the flask(3), swirl the flask(3). Thoroughly mix the solution.
Continue adding sodium thiosulfate(4) until the brown iodine color has nearly disappeared. As you
near the point at which the color disappears, the solution will clear in the area around the drop and
reform as the solution is mixed.
If little oxygen was present in the water sample the solution will be only slightly brown, and as
little as only one drop of sodium thiosulfate titrant may be required at this step. On the other
hand if the oxygen content is high more than one mini burette full of titrant may be required.
With experience, drops of different sizes may be expelled from the mini burette depending upon
the angle at which it is held. If the mini burette is held vertically, the drops are large. As the
burette is held more in a horizontal position, the drop size becomes smaller. This is useful when
approaching the color endpoint and smaller increments of titration reagent are desired.
STARCH REAGENT SOLUTION
For test purposes only. Harmful if swallowed or inhaled. May irritate eyes, skin and mucous
membranes on contact. Do not ingest or allow contact with skin or eyes. Avoid breathing
vapors. In case of contact, flush eyes with water for at least 15 minutes. Get immediate medical
attention. Wash skin with soap and water. In case of ingestion, get immediate medical attention.
In case of inhalation, remove victim to fresh air.
Carefully set aside the mini burette(6) filled with sodium thiosulfate. Use the plastic dropping pipet(8)
to add 8 drops of starch solution(7). Swirl to mix. The test sample will turn blue.
Using the mini burette(6), continue adding drops of sodium thiosulfate titrant(4) to the test sample in
the flask(3). Add the titrant until the blue color just disappears. If no color change is noted by the
time the plunger tip(2, fig. 2-14) reaches the 20 mark on the burette, refill the burette(6, fig. 2-10)
from the titration reagent(4) bottle. Wipe off the end of burette(3, fig. 2-14), and continue adding
titration reagent (4, fig. 2-10) until the color change is observed.
Record the scale reading on the mini burette at the lowest point on the plunger tip(5, fig. 2-14).
Record the number of times if any that it was necessary to refill the mini burette.
If no additional tests are to be made, discard the titration solution left in the mini burette after the
level is recorded. Then thoroughly rinse the mini burette with deionized water before returning it
to the kit.