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chemicals. There are many reasons to do this - we might have obtained some slightly im- pure chemicals at a bargain rate, we may wish to remove some suspected impurities, or we might wish to improve the reproducibility of our results. Professional chemists go to great lengths to purify their starting materi- als, and utilize many complicated methods and pieces of equipment. However, there is one method which is easy to perform, gives good results, and isn't expensive. This is called "recrystallization". It doesn't work for all chemicals, but when it does work it is of- ten the method of choice. Let's discuss how it can be used on pyro chemicals.

First of all, let's discuss the theoretical basis behind crystallization. A crystal is a regular ordered arrangement of atoms, ions, or mole- cules (to save time, I will just use the term "ion" from now on since we will be focusing on ionic crystals). The crystal is held together by several types of crystal packing forces. These forces vary from molecule to molecule. Thus, it shouldn't be surprising to learn that different molecules have different preferred crystal packings and crystal shapes. Sodium chloride forms cubes while potassium per- chlorate forms rhombehedrons.

These forces play an important role during the process of crystal formation. Imagine the surface of a crystal which is exchanging ions with a solution. For each ion on the surface of the crystal there are two opposing forces: crystal packing forces which hold the crystal together, and the opposite forces of solvation energy and entropic freedom. If the crystal packing forces are slightly greater than the solvation and entropic forces, then the crys- tal grows. Conversely the crystal will dissolve if the packing forces are less than the dissolv- ing forces. While packing forces are roughly temperature independent, the solvation forces are favored by increases in tempera- ture. This is why things dissolve in hot sol- vents and crystallize from cooling solvents.

During this cooling there is a purification when ions try to fit onto the growing face of

the crystal. There is a better fit between similar ions, and so a growing crystal of po- tassium perchlorate will not include many ions of either sodium or chloride. This prefer- ence is more pronounced when crystal growth is very slow. Thus one can take a mixture of 10% sodium chloride and 90% potassium perchlorate, and recrystallize it to obtain crystals of potassium perchlorate which will be at least 99.9% pure. Now, to a few practical details.

First of all, as can be realized from the previ- ous discussion, the purification will be im- proved if the rate of crystallization is slow. Thus, the hot solution should be allowed to cool as slowly as is reasonable. Secondly, it must be accepted that the process of recrys- tallization loses material. We'll discuss this below. Third, one h a s to do some calculations based on the relative hot and cold solubilities to determine how to perform the procedure. All of the desired material must go into solu- tion at the upper temperature, and none of the impurity should come out of solution at the colder temperature.

Consider 100 grams of a mixture of 10% so- dium chloride in potassium perchlorate. First, we have to look up the solubilities at 0°C and 100°C. Then these values are used to make sure sufficient water will be present to dissolve all of the material at the hot tem- perature, and to determine how much mate- rial we expect to obtain. Purification only happens when the crystals grow from solu- tion. Thus, sufficient water must be present to dissolve all of the desired material. Since 100 mls of boiling water will dissolve 21.8 gms of potassium perchlorate, 412 mls of boiling water will be required to dissolve 90 gms. The impurity doesn't have to be dis- solved, and can be filtered off (more below). When cooled to 0°C, since 0.75 gms dissolves in every 100 mls of water, only 0.75 * 4.12 = 3.09 grams of the potassium perchlorate will remain in solution. Nearly 87 gms will be found as large crystals at the bottom of the vessel. However, we also have to check to make sure the impurity isn't near it's satu- ration point (where it begins to crystallize).

Since 412 mls of cold water will dissolve al- most 150 gms, the 10 grams of impurity in the original sample will still be safely dis- solved. In general, one doesn't want to ap- proach the saturation point more closely than a factor of two.

Thus, the purification can be performed as follows. About 5% more than 412 mls (440 mls) of water is used for every 100 grams of impure perchlorate (the excess helps make up for evaporation during heating, lower boiling points due to reduced atmospheric pressures, etc.). The purest water available should be used. The water is heated in a ves- sel which won't react with the hot chemical solution (a Pyrex beaker is best, but one can use stainless steel or enamelware). The ma- terial is added with constant stirring once the water is boiling. The solution must be filtered if there is undissolved material or cloudiness. This is a difficult process since the solution will tend to cool and deposit crystals in the filter, t h u s clogging it up. It is often a good idea to add excess water to the bulk solution to allow some degree of cooling before crys- tallization occurs and to boil the excess water off after filtration. The filter should be heated by placing it over a receiver containing boil- ing water and pouring some boiling water through the filter apparatus. Coffee filters are convenient for volumes up to a liter or two, but the plastic filter holder isn't designed to allow for easy heating. An old-fashioned metal or glass filter should be used because the steam from the boiling water in the re- ceiver can heat the outside of the filter.

Once the solution has been filtered (if needed) and any water added to assist in the filtration h a s been boiled off, the solution can be allowed to cool. Remember, the slower to cooling the purer the final material. Thus, the container is covered to exclude dust, and the container can be placed in a Styrofoam cooler or wrapped in insulating cloths. Once the solution h a s cooled to room temperature,

it can be further cooled to as close to 0°C as is convenient. The crystals can be collected after a half day at 0°C. This can be done by carefully pouring off the solution, or by using the same filter apparatus described above. The crystals are still damp with traces of the solution containing the impurities, and so a small portion (about l / 1 0 t h as much as the original solution) of pure ice-cold water is used to wash the crystals. The crystals are then dried, ground, and used. Drying is gen- erally accomplished by air drying for a day or more, followed by several hours in a small oven at around 150°C (250°F). Don't use the kitchen oven - a cheap toaster oven at Goodwill will suffice.

This is a very efficient example of this proce- dure. We recover 87 gms of potassium per- chlorate for every 90 gms we start with. How- ever, this is not always the case. Consider barium nitrate; the solubilities are 8.7 gm/100 ml at 20°C, and 34.2 gm/100 ml at 100°C. Thus, 300 mls of boiling water will dissolve 100 grams of barium nitrate, but only 74 gms will crystallize out at 20°C. Even cooling it to ice temperatures will only re- cover a few grams more. This loss of material is typical of purification procedures, and is one of the reasons that most high-purity chemicals are so expensive. However, this loss can be reduced by performing another recrystallization cycle. For example, consider the experiment j u s t mentioned. If the crystals were collected and then two-thirds of the water was removed by boiling, another 17 gms of the barium nitrate would be collected upon cooling. However, there is a risk that these later crops may be contaminated be- cause the ratio of the concentrations of the impurities to the desired material is rising and the driving forces to include impurities in the crystal are becoming proportionately greater. Thus, the pyrotechnist who is per- forming such purifications must walk a fine line between quantity and purity. TIP

MINING THE PYRO TAILINGS HEAP