of the disodium salt (Na2H2Y•2H2O) and 0.1 g MgCl2 in 1 liter of water. Store
in a plastic bottle. A small amount of sodium hydroxide may be added if there
is any difficulty in dissolving the EDTA. Try not to exceed 3.8 g of
Na2H2Y•2H2O because much more than this dissolves only with difficulty. The
EDTA solution should be filtered using suction filtration. See TA for the apparatus.
10 ammonia buffer. The stock buffer solution has been prepared for you, and the
appropriate quantity is dispensed directly into your titration flask from the
“Repipet” repetitive dispenser located in hood #7. The buffer should
be added immediately before
standard. Obtain from the TA approximately 0.7 g of predried
analytical-reagent-grade CaCO3. Accurately weigh a 0.25-g sample by difference
into a 100-mL beaker. Add about 25 mL water and then add dilute HCl dropwise
until the sample dissolves, then add 2 drops more. Mild heating will speed
dissolution if necessary. Transfer quantitatively to a 250-mL volumetric flask
(rinse well with water) and carefully dilute to the mark. (eye dropper!) Mix
thoroughly. Because this Ca2+ standard solution is used to standardize the EDTA
titrant, it must be prepared very carefully so you know its exact molarity, and
therefore the exact (to ± 0.1 mg) mass of CaCO3 weighed out.
aliquot contains one-tenth of the CaCO3 weighed out to prepare the standard
starting next sample. Add 7-8 mL of pH 10 buffer, 15 mL of water, and 3 drops
of Eriochrome Black T and titrate immediately with EDTA until the light red
solution turns a LIGHT SKY BLUE.
carefully) because ammonia will evaporate and thus the pH of the solution will
change. In general, the faster the titrations are performed the better the
results will be (as long as the endpoint is not overshot due to the speed). It
is advantageous to perform a trial titration to locate the approximate endpoint
and to observe the color change. In succeeding titrations, titrate very rapidly
to within about 1 or 2 mL of the endpoint, then titrate very carefully
(dropwise) to the endpoint. (See Note 1 at the end of the report.)
volume of EDTA used in the titration of each aliquot.
all agree very closely. If not, titrate additional aliquots until
agreement is reached, and any spurious values can be rejected with confidence
volumetric flask to the mark with water. Mix thoroughly.
erlenmeyer flasks. Add 15 mL of water, 9-10 mL of pH 10 buffer, and 2 drops of
Eriochrome Black T immediately prior to titrating a sample.
solution turns blue. (See note at the end of the report.)
the total sample. Remember that each aliquot represents one-tenth of the total
of concentrated ammonia. Slowly and carefully add 400 mL deionized water. This
should be sufficient for over 120 titrations