Chapter 17 Laboratory: Photochemistry 311SBSTITUTIU oNS ANd modIfICATIoNS- Reasonably pure oxalic acid is available from hardware
stores as wood bleach powder and from auto parts
stores as radiator cleaner crystals. Zud cleanser
contains only 5% to 10% oxalic acid by weight, and so
should be used only as a last resort. To use Zud, mix
about 100 g of the cleanser with about 75 mL of warm
water. Strain the resulting goop through a coffee filter,
which yields a cloudy solution. Set the solution aside
to let the solids settle, which may take overnight, and
then carefully decant off 25 mL of clear liquid. This
liquid is a nearly saturated solution of oxalic acid. - Dr. Mary Chervenak adds that Bar Keeper’s Friend also
contains oxalic acid (as the dihydrate—the product is
available at Linens ’n Things, among other places). I
couldn’t tell the concentration from the MSDS, but I
think the powder is quite concentrated. - Not all drugstores carry tincture of iodine, because
much better antiseptics are available. You can
substitute a solution of 0.25 g of iodine crystals
dissolved in 25 mL of 50% to 70% ethanol. Iodine
crystals are sold by camping supply stores for water
purification. - The beaker is used only for mixing. You can substitute
any convenient small container.
dAR. Rm y CHERvENAk CommENTS:
Some interesting things about oxalic acid: oxalic acid and
oxalates are abundantly present in many plants, most
notably lamb’s quarters, sour grass, and sorrel (including
Oxalis). A lot of these plants are weeds that grow right
outside your doorstep and have been used, in the not-
so-distant past, as salad greens. Common foods that are
edible but that still contain significant concentrations
of oxalic acid include—in decreasing order—star fruit
(carambola), black pepper, parsley, poppy seed, rhubarb
stalks, amaranth, spinach, chard, beets, cocoa, chocolate,
most nuts, most berries, and beans. Oxalic acid (as
oxalate ion) reacts readily with metal ions, including Ca+2,
Fe+2, and Mg+2. The gritty “mouth feel” one experiences
when drinking milk with a rhubarb dessert is caused by
precipitation of calcium oxalate. The primary component
of kidney stones is calcium oxalate.how far the reaction has proceeded to the right. If the solution
remains orange, we know that it contains mostly reactants. If
the solution turns colorless, we know that it contains mostly
products. If the solution turns an intermediate color, we know
that the solution contains a mixture of reactants and products
proportionate to the degree of color change.
CUTIOA nS
Oxalic acid is a strong organic acid that is very toxic and an
irritant. Avoid breathing the dust or allowing it to contact
your skin. Ammonia is an irritant. Tincture of iodine is
an irritant and stains skin and clothing. (Stains can be
removed with a dilute solution of sodium thiosulfate.)
Wear splash goggles, gloves, and protective clothing.z
POCEDURER
This laboratory session has two parts. In Part I, we prepare
standard reference solutions of iodine. In Part II, we test the
effects of various types of light on the iodine/oxalate solutions
and compare our results against the standard reference solutions
we made in Part I. Setting up the experiment and completing
the procedures in Parts I and II should take about 30 minutes of
actual lab time.
PRTI: A pREREpA REfERENCE SAmpLES
We expect that some or all of the types of light we use will cause
the iodine/oxalate solution to react, partially or completely
converting the orange iodine solution to colorless iodide solution.
We can use the color of the resulting solutions to judge how far
the reaction has proceeded. An intense orange color tells us that
the reaction has proceeded little or not at all, and a colorless or
pale yellow solution tells us that the reaction has proceeded to
completion, or nearly so.Just eyeballing each sample gives us imprecise results. We can
judge that the reaction has proceeded to completion, mostly
to completion, only a bit, or not at all. For quantitative results,
we need a set of standard reference samples, with known
concentrations of iodine. By comparing the test samples with
these reference samples, we can judge the actual concentration of
iodine in the test samples (and therefore how far the reaction has
proceeded) with some degree of accuracy. (We’ll use the same
visual colorimetry procedures we used in Laboratory 7.5.)Follow these steps to prepare a set of standard reference samples
for comparison:- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Label six test tubes from 1 to 6.
- Measure 8 mL of water into test tube #1 and add 20
drops (1 mL) of the iodine solution to yield 9 mL of dilute