Acids have been know since ancient times. Anything that is sour is acidic. The first intentionally produced acid was vinegar which is produced by the action of bacteria on wine, beer, or mead. Acids are a waste product of bacterial metabolism. That is why bacterial grown contributes to tooth decay. When bacteria are grow in an alcoholic beverage in the presence of oxygen they produce acetic acid, which is the acidic component of vinegar. In fact, when any food or beverage "goes sour" it is from bacteria producing acetic and other organic acids.
For most of antiquity, vinegar was the only commonly available acid. But as metals came into wider use it became apparent that different metals had different properties. But ores seldom contain only one kind of metal-- they are usually mixtures of different metal compounds. For example, the lead ore we are using in class is mostly lead sulfide, or galena. But it also contains minor amounds of zinc sulfide and silver sulfide. It is very desirable to be able to separate these different metals, particularly the precious metals like silver and gold from the "base" metals like lead and zinc. All metals are more or less soluble in acid, but different metals are soluble in different acids. So technologically it is very desirable to be able to produce a host of different acids for separating metals from each other.
Sulfuric acid was originally produced in alchemical laboratories by heating the mineral, green vitriol, FeSO4(H2O)7. Biringuccio tells us how to find this mineral:
Vitriol ore is found many places in Italy and abroad. Some say that wherever it is found it is an indication of gold ore, but I cannot approve this a certain. It is mined everywhere from open mines because of evil and unbearable odors like those of sulfur or worse. It's location is recognized by various signs, particularly by its many odors, which show where it is so that it is not necessary to seek it by means other than these and by the sight.
He then goes on to describe the extraction of vitriol from its ore by solution
and recrystalization in a manner similar to what we have used to isolate potash
and lime. The refined vitriol is then heated in a furnace and the vapor
given off is condensed:
FeSO4(H2O)7(s) -----> H2SO4(l) + FeO(s) + 6 H2O(g)
This sulfuric acid, or "oil of vitriol" was for centuries produced in small quantities in alchemical laboratories for metalurgical use. Sulfuric acid might have remained a laboratory curiosity but for a development in the textile industry.
One of the most valuable dyes of the Middle Ages was indigo (blue). Blue dye was available in Europe from woad, a plant which grows in cool climates. But indigo, a tropical plant, produces much more intense blues. Like black walnut, this dye is soluble in its reduced (yellow) form and insoluble in its oxidized (blue) form. If cloth is to be dyed in tropical climates, it is easy enough to use. Steep the plant in water, let it ferment anaerobically (like mead), dip your cloth in the yellow dyebath, and when it is exposed to air, the dye becomes blue and insoluble. But if you want to import indigo to Europe, you have a problem. Once the dye is oxidized, it is insoluble in water. How can you use it to make dye? This fundamental chemical problem caused indigo to be an expensive color, and consequently one which everyone wanted.
In 1744, it was discovered that indigo would dissolve in sulfuric acid, forming a water-soluble dye called indigo carmine. If sulfuric acid could be produced economically, blue cloth would be made available at incredibly low prices. This discovery started a drive to find more efficient and cheaper ways to produce sulfuric acid. As the price dropped, people found more and more uses for this acid and it has become one of the most important chemicals in industry.
The manufacture of sulfuric acid presents us with an interesting lesson in
industrial economics. We have seen that the roasting of sulfide ores produces
sulfur dioxide as a waste product. For example:
2 PbS(s) + 3 O2(g) -----> 2 PbO(s) + 2 SO2(g)
From the beginning of metal smelting to the mid 18th Century, sulfur dioxide was simply sent up a chimney into the atmosphere. Over long periods of time, the sulfur dioxide slowly reacts with oxygen and water in the atmosphere producing sulfuric acid:
2 SO2(g) + O2(g) -----> 2 SO3(g)
SO3(g) + H2O(l) -----> H2SO4(l)
This is one important source of acid rain. Consequently, a smelter was not the ideal place to build your dream home. But the world was big in those days, the wealthy simply didn't live near a smelter, and the environmental lobby was nonexistent.
The discovery that indigo could be used to dye wool changed the situation dramatically. Now there was a demand for sulfuric acid but no way to produce it cheaply in the quantities demanded by the textile industry. In 1746 John Roebuck developed the lead chamber process for the manufacture of sulfuric acid. Prior to this time, sulfuric acid had been produced in glass bottles several pounds at a time. But the lead chamber process could produce sulfuric acid by the ton.
In the lead chamber process, sulfur and potassium nitrate are ignited in a
room lined with lead foil. Potassium nitrate, or saltpeter
is an oxidizing agent which we have seen when we discussed explosives.
The saltpeter oxidizes the sulfur to sulfur trioxide according to the
6 KNO3(s) + 7 S(s) -----> 3 K2S + 6 NO(g) + 4 SO3(g)
The floor of the room was covered with water. When the sulfur trioxide reacted with the water, sulfuric acid was produced:
SO3(g) + H2O(l) -----> H2SO4(aq)
Notice that this process depends on cheap supplies of saltpeter and produced yet another air pollutant, nitrogen monoxide. Thus we have to pay for a nitrogen source, saltpeter, and all of the nitrogen winds up going up the smokestack. Saltpeter could be replaced with its less expensive cousin, sodium nitrate ("Chile saltpeter") but nevetheless you wind up paying for nitrogen which winds up as a waste product. Saltpeter was a significant expense. In his textbook of 1806, Chaptal brags:
Numerous experiments which I have made in my manufactory, to economize the saltpeter employed in the fabrication of oil of vitriol, have several times exhibited the results here mentioned.
A significant advance came in 1835 when Joseph Gay-Lussac invented a process
for recovering the nitrogen in nitrogen monoxide and recycling it to replace
the saltpeter as a source of nitrogen.
4 NO(g) + O2(g) + 2 H2O(l) -----> 4 HNO2(l)
4 HNO2(l) + 2 SO2(g) -----> 2 H2SO4(aq) + 4 NO(g)
This accomplished two things simultaneously: it reduced the dependence on expensive saltpeter and at the same time sharply reduced nitrogen monoxide emissions. You can guess which of the two was the more important at the time. The only expenses now were sulfur and enough saltpeter to make up for lost nitrogen monoxide.
The major source of sulfur at this time was Sicily, where the mines were controlled by a cartel which fixed the price. But as sulfide ores were increasingly being processed in smelters, the waste gas, sulfur dioxide, from the smelters became an important resource for sulfuric acid production. If a smelter could build a sulfuric acid plant on site, it could turn its waste gas into valuable sulfuric acid. One man's trash is another man's treasure indeed!
In the 1890's Herman Frash invented a method to extract molten sulfur from the substantial deposits of Texas, Louisiana, and Mexico and the resultant drop in the price of sulfur made it competitive with sulfide ores as a sulfur feedstock. Today sulfuric acid is produced from both sulfur and sulfur dioxide by the contact process which replaces the lead chamber with a vanadium oxide catalyst.
Sulfuric acid, H2SO4, is today the chemical produced in the largest quantity in the world. U.S. production alone is approximately 40 billion kilograms annually. Its chief use is in the manufacture of fertilizers. It is also used for the manufacture of other chemicals and for the processing of wood pulp in the manufacture of paper. Sulfuric acid can be bought at many hardware stores as an industrial strength drain opener (not to be confused with the more common lye based drain openers). It is also the "battery acid" of the lead storage battery.
As we have said, sulfur is found in deposits of elemental sulfur and can be mined using traditional techniques. The Frash process uses steam to melt sulfur underground so that it can be extracted in wells, similar to oil wells. Alternately, sulfur dioxide produced as a biproduct of metal smelting can be used as a source of sulfur, both in the lead chamber and the modern contact process for the manufacture of sulfuric acid.
The following sulfide ores are included in the mineral collection. You will be called upon to identify them as part of the acid quiz. In most cases the materials is far from pure. The chemicals listed are those which are present in largest concentration. These materials are classified as minerals (M, homogeneous) or rocks (R, heterogeneous). Rocks are further classified as igneous (IR), sedimentary (SR), or metamorphic (MR).
|Number||Mineral or Rock||Chemical||Formula||Picture|
|2||Chalcopyrite(M)||Copper Iron Sulfide||CuFeS2|
Nitric acid, HNO3, is the second most important acid, ranking 12th in U.S. chemical
production (7 billion kilograms). It is a cornerstone of both the fertilizer
and explosives industries. Nitric acid was historically produced from sulfuric acid
by a metathesis reaction:
KNO3(s) + H2SO4(l) -----> HNO3(g) + KHSO4(s)
Nitric acid was produced at high temperatures in the gas phase and then condensed into liquid nitric acid. This manufacturing method suffers from dependence on cheap supplies of potassium nitrate or saltpeter. Today this dependence has been broken by producing nitric acid from ammonia, which in turn is manufactured from the nitrogen in the air.
The third most important acid is hydrochloric acid, HCl. It is an important industrial
chemical and is sold in hardware stores under its older name, muriatic acid, for
cleaning swimming pools. Hydrochloric acid can also be produced from sulfuric
acid by a metathesis reaction:
NaCl(s) + H2SO4(l) -----> HCl(g) + NaHSO4(s)
The HCl has is then dissolved in water to produce aqueous hydrochloric acid.
We have seen that acids can be produced from salts, such as vitriol, saltpeter, and table salt. It is also possible to go the other way. When an acid reacts with an alkali (in a metathesis reaction) a salt is produced. For hydroxides, the driving force for the reaction is the production of water:
For carbonates, the driving force for the reaction is the evolution of carbon dioxide:
Similar reactions can be written for nitric and hydrochloric acids.
The acid quiz will consist of three questions on the following topics:
Sulfur is a slightly flammable yellow solid. It is not particularly toxic or reactive. Of course, your shouldn't eat it, but you should become familiar with its characteristic aroma.
Saltpeter is a strong oxidizer, hence its use in gunpowder. It should not be mixed with fuels unless it is your intention to make pyrotechnic mixtures. While not particularly toxic, you should not it eat by the spoonful. If you do, call a doctor and induce vomiting.
Sulfur dioxide and sulfur trioxide are noxious gases and should be handled in a fume hood. They have pungent aromas which are unmistakable. A little whiff won't hurt you, but if you get a lungfull, get fresh air immediately. If a cough develops, see a doctor.
Dilute acids are relatively harmless. Soft drinks, pickles, and tomato sauce are all acid foods. Acids taste sour.
Concentrated acids, on the other hand, are quite hazardous. They will cause acid burns on the skin, can blind you if splashed in the eyes, and can kill you if ingested. Some industrial drain openers contain concentrated sulfuric acid. If it can open a drain, it is not good to drink. If you happen to drink some, do not induce vomiting and head immediately for the emergency room. I hope you make it.
If acid is splashed in the eyes, flush them with water and head for the emergency room.
If you get some on the skin, just wash it off immediately. You only need medical attention of a blister or burn forms.
Information on chemical hazards is summarized in a Material Safety Data Sheet for each compound. These sheets often tell you more than you want to know, but they are worth looking at.
You will replicate the most primitive production of sulfuric acid from sulfur
and saltpeter. The reaction will take place in a test tube with very little
air, so we must count on the saltpeter to supply all of the oxygen.
Begin by balancing the reaction:
KNO3(s) + S(s) -----> K2S + N2(g) + SO3(g)
H2O + KNO3(s) + S(s) -----> K2S + N2(g) + H2SO4(l)
Then use stoichiometry to calculate the number of grams of saltpeter needed to react with 1.0 g of sulfur. You will bring these calculations with you to the lab when you are ready to make sulfuric acid. You will also need a 2 L soda bottle to take the place of the original lead chamber.
Begin by weighing 1.0 g of sulfur and your calculated weight (from the quiz) of saltpeter and place this mixture into a clean, dry, test tube. Close the test tube with a rubber stopper fitted with a piece of glass tubing. Rinse your 2 L bottle with water and drain it, leaving the walls of the bottle wet. Put the glass tubing into the bottle and hold the test tube over a Bunsen burner. The sulfur and saltpeter will react producing a mixture of nitrogen monoxide (brown gas) and sulfur dioxide (clear gas). These react further in the presence of water to produce nitrogen and sulfur trioxide, which dissolves in the water to produce sulfuric acid.
What you will see initially is a violent reaction as the sulfur and saltpeter are heated. Billowing clouds of gas will be produced and the material may even catch fire. This is nothing to worry about. The gas that is produced will be brown in color and will go out through the glass tubing and fill the 2 L bottle. The bottle will seem to be filled with brown, hazy mist: a mixture of nitrogen oxides and sulfur dioxide. The gas will turn clear and trasparent over the course of several minutes as the nitrogen oxides oxidize the sulfur dioxide to sulfur trioxide. Sulfur trioxide dissoves in water to produce sulfuric acid.
I will test your sulfuric acid, first by smelling it (it should be almost odorless) and then with test paper (it should turn the paper red). If it passes these tests, you pass the project. Otherwise, you fail but may try again another day.