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Potash from Wood Ashes
Plants synthesize the sugar β–glucose using carbon dioxide, water, and sunlight with the aid
of enzymatic catalysts. The glucose can then be polymerized into long chains called cellulose:
C6H12O6 → (C6H10O5) n + 6n H2O
glucose
cellulose
The number of glucose units (n) bonded together varies from several hundred up to 15,000. Cellulose
is the major structural component of the cell wall. It provides rigidity and support needed to
withstand the osmotic pressure differences between the inside and outside of the cell. In woody
plants the cellulose is distributed in a plastic-like phenol polymer matrix called lignin. This
composite material provides strength by evenly distributing the stresses throughout the cells in
manner similar to concrete reinforced with steel rods.
Wood consists of approximately 99.5% combustible material, which can be burned (oxidized)
to give yield the original starting materials, water and carbon dioxide:
(C6H10O5) n + 6n O2 →
5n H2O + 6n CO2
The remaining 0.5% consists of minerals that do not burn and are instead converted to a grey,
powdery ash. Most of the ash (80%) is not soluble in water. The remaining water–soluble
components of ash, potash (K2CO3), soda ash (Na2CO3), and arcanite K2SO4, are of interest to us in
this experiment.
The major component of ash is potash. When potassium carbonate dissolves in water it
+
2
splits into two potassium ions (K ) and one carbonate ion (CO3 ¯). The carbonate anion in solution
can react in two different ways. First, it could reaction with the water to form bicarbonate and
hydroxide:
2
CO3 ¯ + H2O
→
HCO3¯ + HO¯
Clearly, this will raise the pH by increasing the number of hydroxide ions. Additionally, carbonate
can reaction with any hydrogen ions in solution:
2
+
CO3 ¯ + H
→
HCO3¯
This decreases the amount of H in solution, which also raises the pH. Therefore the carbonate makes
the water more basic. In fact, potash and soda ash were the only bases know in the ancient
civilizations. They used these chemicals for making glass, making soap, and embalming mummies.
Time Management
The isolation of crude water–soluble ash requires about an hour.
recrystallization to get pure potassium carbonate requires about ½ an hour.
The subsequent
Overview
•
•
•
•
Weigh ashes
Extract water–soluble salts
Filter solution
Evaporate water
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•
Recrystallize product to purify
A. Extracting water soluble salts
Materials
ashes
filter paper
1000 mL Erlenmeyer flasks
Büchner funnel
500 mL filtration flask
1. The ashes and the potash you will be isolating are mild
caustic agents. You must were disposable gloves for this
experiment.
Safety: As always you should be
wearing your safety glasses.
2. Weigh out about 200 g of ashes, but be sure to note the
exact amount.
Note: The balances can not weigh
anything over 150 g and your weigh
boats will not hold that much either.
You will have to weigh out several
portions, noting the exact amount of
each portion and the total weight.
3. Describe the ashes in your notebook: color, particle size,
homogeneity, etc.
4. Transfer the ashes to a 1-L Erlenmeyer flask.
Hint: A powder funnel may help.
5. Add 500 mL of de-ionize (D.I.) water.
6. Stopper the Erlenmeyer flask with an rubber stopper.
7. Shake or swirl the flask for several minutes.
Question: What is the purpose of
shaking/swirling?
8. While the contents of the flask are settling, set up a
filtration apparatus: clamp a filtration flask to stand, add
the porcelain Büchner funnel, then add a piece of filter
paper.
9. Slowly, carefully decant the aqueous layer into the
Büchner funnel and collect the liquid in the filtration
flask.
Hint: decant means to pour so that
the solids stay behind in the flask.
Hint: you will not be able to pour
all of the liquid into Büchner funnel.
You will have to do it in several
portions, waiting in between for the
water to filter out the funnel.
B. Isolation of crude water-soluble ash
Materials
beaker
boiling chip
hotplate
1. Transfer your aqueous solution from part A to a 500 ml
beaker.
2. Test the pH of the aqueous layer using pH paper.
Question: Is the solution basic or
acidic? Why?
3. Add a boiling chip to the solution.
Question: What is the purpose of
the boiling chip?
4. Place the beaker on a hotplate and turn the hotplate onto
about ½ of its maximum setting.
Warning: never turn the hotplate to
its highest setting. That could
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shorten its lifetime and/or burn out
its heating element.
5. Let the water evaporate slowly. This will take at least an
hour.
C. Purification of potash
Materials
2x 100 mL Erlenemeyer flask
boiling chip
hotplate
pipet & bulb
1. After all the water has evaporated, scrape together the
solid material in the beaker using a spatula.
2. Transfer the solid into a 100 mL Erlenmeyer.
Warning: never turn the hotplate to
its highest setting. That could
shorten its lifetime and/or burn out
its heating element.
3. In the second 100 mL Erlenemeyer add about 25 mL D.I.
water, a boiling chip, and bring water to a boil on the
hotplate.
4. Slowly add about one pipet of hot water to the flask with
the crude potash.
5. Continue to add the minimum amount of hot water until
the crude material is all dissolved.
Hint: don’t add too much solvent
(water) or else the potash won’t
precipitate out.
6. Take the flask off the hotplate and let it cool slowly. As
the solution cools to room temperature, crystals should
form.
Hint: the quality of crystals formed
depends on the rate of cooling.
Slower cooling gives better crystals.
7. After the solution has cooled to room temperature, then
place the flask in an ice bath.
C. Isolation of potash
Materials
small filter flask
Büchner funnel
1. Set-up a vacuum filtration apparatus: clamp a filter flask
to a stand, attach thick-walled rubber tubing from the
vacuum port to the filter flask, insert Büchner funnel
with adaptor, and add filter paper to the funnel.
filter paper
A filtration flask looks like an
Erlenmeyer with a side arm.
A Büchner funnel is a white
porcelain funnel with holes.
2. Turn on the vacuum.
3. Pour the contents of the small Erlenmeyer into the
Büchner funnel. The water should pass through and the
solid (the potash) should collect on the filter paper.
4. Let the vacuum continue to run for a few minutes while
the crystals air dry.
5. When the crystals are dry weigh them and record the
results in your lab notebook.
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Questions
1. What is the chemical formula for potash? Soda ash?
2. How grams of potash did you collect? How many moles is that?
3. What is the weight percent potash in the original ashes you started with?
4. Explain how a recrystallization works.
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