Chapter 20 Laboratory: Quantitative Analysis 357CUTIOA nS
Sodium hydroxide is corrosive. Wear splash goggles,
gloves, and protective clothing.z
Before we begin the analysis, we need to decide what sample
sizes are appropriate for our quantitative tests.
We know that a 500 mg vitamin C tablet nominally contains
500 mg of ascorbic acid, and we expect the stated value to be
reasonably close to the actual value. The gram molecular weight
of ascorbic acid is 176.14 g/mol, so at nominal value one 500 mg
tablet should contain about 0.0028+ moles of ascorbic acid. One
mole of ascorbic acid reacts stoichiometrically with one mole of
sodium hydroxide, so we’ll require about 0.0028+ moles of sodium
hydroxide to reach the equivalence point. Our 0.1000 M solution
of sodium hydroxide contains 0.1 mol/L or 0.0001 mol/mL, so we
should need about 28+ mL of titrant to neutralize the ascorbic
acid present in one 500 mg tablet.
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Weigh a clean, dry 125 mL Erlenmeyer flask and record its
mass to 0.01 g on line A of Table 20-1. - Add one 500 mg vitamin C tablet to the flask, reweigh
the flask, and record the combined mass of the flask and
POCEDURER
tablet to 0.01 g on line B of Table 20-1. Subtract the empty
mass of the flask from the mass with the vitamin C table,
and enter the mass of the vitamin C table to 0.01 g on line C
of Table 20-1.- Use the graduated cylinder to transfer about 50 mL of
distilled water to the 125 mL Erlenmeyer flask and swirl
the flask gently to break up the tablet. - Place the 125 mL flask on the hotplate and heat it gently
(do not boil) for 5 to 10 minutes with occasional swirling
to dissolve the vitamin C from the sample. It’s normal
for a small amount of binder and other components to
remain undissolved. Set the flask aside and allow it to cool
to room temperature. - Rinse the 50 mL burette with a few mL of 0.1 M sodium
hydroxide, and allow it to drain into a waste container. - Transfer at least 40 mL of 0.1 M sodium hydroxide to
the burette. Drain a couple mL into the waste container
and make sure that there are no bubbles in the burette,
including the tip. Record the initial volume as accurately
as possible, interpolating between the graduation marks
on the burette, and record that volume on line D of
Table 20-1. - Add a few drops of phenolphthalein indicator solution
to the flask and swirl the contents until they are
thoroughly mixed. - We calculated that we should need about 28 mL of
sodium hydroxide titrant, so begin by running 25 mL or
so of titrant into the receiving flask with constant swirling.
As you near the endpoint, a pink color will appear in the
flask where the titrant is being added, but will disappear
quickly as you swirl the flask. When you reach this point,
slow the addition rate to a fast drip, and continue swirling.
When you reach the point where the pink color persists
for a few seconds, begin adding the titrant dropwise, and
swirl the flask to mix the contents thoroughly after each
drop. You’ve reached the endpoint when the pink color
persists for at least 30 seconds. Record the final volume
on the burette on line E of Table 20-1. Subtract the initial
volume of titrant from the final volume, and record the
volume of titrant used on line F of Table 20-1.
Using the actual molarity of your nominal 0.1 M sodium
hydroxide titrant, calculate the number of moles of
sodium hydroxide required to reach the endpoint and
enter that value on line G of Table 20-1.
Sodium hydroxide reacts stoichiometrically with ascorbic
acid at a 1:1 mole ratio, so the number of moles of
ascorbic acid present in the sample is identical to the
number of moles of sodium hydroxide needed to reach
the endpoint. Calculate the mass of ascorbic acid present
in the sample by multiplying the number of moles of
sodium hydroxide required by the gram molecular mass
of ascorbic acid (176.14 g/mol) and enter that value on
line H of Table 20-1.
10.
11.
12.
Scurvy is now almost unheard of in the developed world,
because our regular diets include more than sufficient vitamin
C to prevent scurvy. But many people take vitamin supplements
that provide much higher dosages of vitamin C than the
recommended daily minimum.
In this lab session, we’ll use acid-base titration with
standardized 0.1000 M sodium hydroxide titrant to determine
the actual amount of ascorbic acid (C 6 H 8 O 6 ) present in a
nominally 500 mg vitamin C tablet. One mole of ascorbic acid
reacts with one mole of sodium hydroxide to yield one mole
of sodium ascorbate and one mole of water, according to the
following balanced equation:
C 6 H 8 o 6 + Na oH→ C 6 H 7 o 6 Na + H 2 o
Using phenolphthalein indicator provides a sharp endpoint for
the reaction. The solution remains colorless while ascorbic acid
is in excess. When sodium hydroxide is even slightly in excess,
the pH of the solution rapidly rises to a point above the color
change range of phenolphthalein, and the solution quickly turns
bright pink.