Introduction
The ashes left over from a fire are not dirt. In fact, they contain a number of compounds that are very useful to us - certainly in primitive cultures. Early people discovered that wood ashes could be used for cleaning. Some writeres have suggested that soapy suds were first noticed when fatty tissues and ashes were washed away with water from human sacrifice sites! The major components of wood ashes are potassium carbonate (potash) and sodium carbonate (soda ash). From a chemical standpoint these two compounds are very similar. So similar that while ashes have been used for millennia the difference between sodium and potassium carbonate was only recognized in the 19th century.
Potash was man's first base. A base is a material which produces OH- ions in solution. Bases are the opposites of acids which donate H+ in solution.We also refer to a base as an alkali. Alkali's have a bitter taste and a slightly soapy feel when rubbed in the fingers. The isolation of alkali is necessary for making true soap.
Even today potassium and sodium carbonate remain extremely important
chemicals. U.S. Production of sodium
carbonate alone was 9 billion kg making it the 11th most-produced chemical
in the U.S.
Background
pH
pH is a scale that measures the acid-base levels of a solutions. Neutral (neither acid nor base) is 7 on the pH scale. A strong acid would be a "1" and a strong base would be a "14". As you can see, the lower the number, the more acid a solution is, and the higher the number the more basic, or alkali, the solution is. The scale goes from 1 to 14. Because it is based on logarithims, a one-unit change (example from 4.5 to 3.5) in pH is a 10X increase in acidity or concentration of H+ ions.
One of the most convenient ways to measure pH is with an indicator.
An indicator is a substance which changes color
when the pH changes. There are many indicators. You may have noticed,
for example, that tea turns from dark brown to tan when lemon juice (an
acid) is added. We will use wide-ranges test paper o determine pH. When
moist the paper turn a different color depending on the pH of the test
solution. By comparing the papers color to a standard scale the solution
pH can be determined.
Solubility
One of the most fundamental problems in chemistry is that of purifying
a substance. We have seen that most things in
nature are actually mixtures, either homogeneous or heterogeneous.
But to have any control over chemical reactions, a
chemist must first be assured that his starting materials are pure.
Later in the course we will look at distillation as a
technique for purifying substances that are gases or liquids under
ordinary conditions. For solids, however, we turn to
recrystalization as our primary purification technique.
We have seen that a mineral, particularly a crystalline mineral, is
essentially a pure substance, either element or
compound. How are such minerals formed in nature? One possibility is
that the mineral crystals cooled from molten rock. Another is that the
crystals formed from substances dissolved in water. It is this mechanism
which we will try to exploit in this project.
You know from everyday experience that sugar is soluble in water.
If you add a single grain of sugar to 2
liters of water, the sugar will all dissolve--no solid sugar will remain.
And if you add a second grain, it too will dissolve. In fact, if you add
a teaspoon of sugar, the entire amount will dissolve. You may add a second
and a third teaspoon, but anyone who has ever added sugar to tea or coffee
knows that there comes a point where the water is saturated with sugar,
that is, all the sugar that can dissolve, has dissolved. Any sugar in excess
of this amount will just settle to the
bottom as a solid. The amount of a material which will dissolve in
a given amount of water is called its solubility. Here
are solubilities of some common substances that we are interested in:
Substance Solubility (gr/100 ml water)
| Cold | Hot | |
| potassium carbonate | 147 | 331 |
| calcium chloride | 75 | 159 |
| sodium chloride | 36 | 39 |
| potassium chloride | 34 | 57 |
| sodium carbonate | 22 | 421 |
| calcium carbonate | 0.001 | 0.002 |
"Suppose, then, that I start with an ancient sea. Dissolved in the seawater are all kinds of things dissolved out of the soils and rocks in the area which drains into the sea. Some of these substances, like sodium chloride, have high solubility while others, like calcium carbonate, have low solubility. Eventually, the geological conditions change, the sea is cut off from the ocean, the drainage patterns change, and the sea begins to dry out, perhaps over the course of hundreds of thousands of years. The Dead Sea and the Great Salt Lake are two modern examples of such a situation. Now, as the sea evaporates it becomes more concentrated until it becomes saturated in the least soluble material it contains. Like the excess sugar added to tea, this least soluble material falls to the bottom and is deposited as a layer, perhaps calcium carbonate. As the evaporation continues, the substances present are deposited in reverse order to their solubilibies. Finally, the most soluble substances present are deposited as the uppermost stratum as the sea gives up the last of its moisture. The substances deposited depend on what was present in the original sea, and the order in which they are deposited depends on their relative solubilities."
"One more hitch in the story is that solubility depends on temperature. Notice that while the solubility of sodium chloride is about the same in hot and cold water, the solubility of sodium and potassium carbonate is much greater in hot water than in cold water. We will exploit this property in separating the soluble carbonates from the other components of wood ash."
Density & Specific Gravity
The density of an object is expressd as the
mass of the object divided by its volume. For example, we know that a ml
of water has a mass of one gram. Therefore, the density of pure water
is: 1gr/1 ml = 1.0 gr/ml
If an object has a density has a density of less than 1.0 gr/ml,
charcoal for example, it will float. Material that sinks in water must
have a density of greater than 1.0 gr/ml.
Because a water solution has dissolved substances, which also
have mass, the density of aqueous solutions is greater than 1.0 gr/ml.
For example, ocean salt water has a higher density than lake water, so
you float better. If you divide the density of a solution by the density
of water, 1.0 gr/ml, you get a ratio without units called specific gravity.
Hopefully, you see intuitively that a high specific gravity means more
solutes are in solutions. The doctor, for example, can measure the specific
gravity of your urine to tell if your kidneys are functioning properly.
Specific gravity can be measured with a device known as a hygrometer.
Wood Ashes
"Whatever we extract from wood ashes must be there to begin with. Wood ashes are a complex heterogeneous mixture of all the non-flammable, non-volatile minerals which remain after the wood and charcoal have burned away. Because of the presence of carbon dioxide in the fire gases, many of these minerals will have been converted to carbonates. Burned soil may also be present. So the ashes probably contain predominately sodium and potassium carbonate, sodium and potassium chloride, silica, and calcium carbonate."
If we add the ashes to water, the soluble potassium and sodium salts
will dissolve while the insoluble silica and calcium
carbonate will settle to the bottom. We can then drain off the water
(containing the "good stuff") and throw the insoluble material away. To
separate the chlorides from the soluble carbonates, we will exploit the
greater solubility of the carbonates in hot water. We will bring the liquid
to a boil and continue boiling until enough water boils away for an
insoluble precipitate to form. This is very likely a mixture of sodium
and potassium chloride. From this point, we will
continue boiling until half of the remaining water is removed. At this
point we can be reasonably certain that only the
soluble carbonates remain in solution. We will carefully pour off the
hot liquid into another container, leaving the solid
material behind. As the liquid cools to room temperature, the less
soluble sodium carbonate will precipitate leaving the
more soluble potassium carbonate in solution. Finally, the remaining
solution can be drained off and boiled to dryness,
producing solid potassium carbonate.
One of the observations you make should be that it takes a lot of wood
to make a little ash and a lot of ash to make a
little potash. Thus, while it is not particularly difficult to extract
potash from wood, you will go through an enormous
amount of wood to produce commercial amounts (pounds and tons) of potash.
This will have implications for us later in the semester."
Safety and Common Sense
Just because potash and soda ash are "natural" and you are extracting
them from wood ashes, doesn't mean they are
safe. Just because they are chemicals with chemical names and formulas
doesn't make them dangerous. When thinking
about chemical hazards, you should always consider the amount and concentration
of the substance in question. Potash
makes up only a small percentage of wood ashes, which are not particularly
hazardous. But as the potash is extracted,
concentrated, and purified, it becomes more deserving of care.
Potash and soda ash are relatively strong alkali's. The are moderately
caustic, which means they will damage skin.
Consequently, you should not rub them all over your body or get them
in your eyes and you should not eat them.
Nevertheless, they don't warrant paranoia. If you get some on your
skin, wash it off. You should wear glasses to protect your eyes, but if
some gets in, you should splash cold water into your eyes. You should not
eat it.
Instructions
Our goal is to extract as much of the soluble carbonates from wood ashes as possible while leaving behind the insoluble components. You will need a couple of handful of wood ashes, some water, our old friend, the 2 L soft drink bottle, and a plastic cup.
Place your wood ashes into the 2 L bottle until it is about 1/3 full
of ashes. Fill the rest of the bottle with hot water, place the cap on
the bottle and shake it up. The soluble carbonates (as well as any other
soluble materials) will dissolve while
the insoluble silicates, carbonates, aluminosilicates, and any other
insoluble materials will settle to the bottom. Any
charcoal present will float to the top. Place the bottle where it will
not be disturbed and let it sit overnight.
If you were interested in further purifying your product, you could
recrystalize it again. This time you would start with
your crude product (instead of ashes), dissolve it in water, boil it
until it was almost dry, and filter it while hot to remove any materials
less soluble than the carbonates. You would then allow it to cool and the
carbonates would precipitate out leaving anything more soluble still in
solution. By repeated application of this procedure, you could even separate
sodium carbonate from potassium carbonate. But for our purposes, you crude
potash should be alkaline enough. To test it for yourself, just taste it.
It should taste bitter, like soap.
Criteria
Quiz
Know how soluble compounds are deposited geologically.
Know how recrystallization can be used to
purify compounds.
Know how the pH scale is used to quantify
acidity and alkalinity.
Know the difference between baking soda and
washing soda.
Know what specific gravity is
Project
To pass: Your potash must
be grey or white, with no obvious contamination. A wet pH test paper should
indicate "base" when touched to your potash. You must also calculate
the specific gravity of your solution. I will check the actual S.G. with
a hydrometer.