20.3.

You probably know enough by now that you could reproduce the Leblanc process without further instruction. The four-step process would involve heating sulfuric acid and salt. You would heat the product from this first reaction with charcoal and limestone in a crucible similar to the one used to smelt metals in Chapter 9. After heating this crucible to red heat in a kiln, you would crack it open and extract the soda in the same manner that you extracted potash in Chapter 8. But you would simply be repeating, for the most part, processes you have already performed in previous projects. Of the steps in the process, only the first step is substantially different from what you have already done and so this project will focus on the reaction of sulfuric acid and salt.

Begin by using Figure 20-2(a) to answer the following stoichiometric questions:

Q: How many grams of sulfuric acid are needed to react with 2.00 grams of sodium chloride?
Q: How many grams of sodium sulfate can be produced from 2.00 grams of sodium chloride?

The answer to the first question tells you how much concentrated acid to use. The answer to the second stoichiometric question is the theoretical yield, the amount of product that you would expect to get if the reaction went perfectly. You now know how much acid and salt to use and how much sodium sulfate to expect.

You are going to heat sulfuric acid and salt in a flask and you ought to take a few precautions to accomplish this safely. Obviously, since the reaction is going to produce hydrogen chloride gas, you should do this either outdoors or in a fume hood. You are also going to be boiling a concentrated solution which is prone to "bumping," that is, to boiling suddenly and violently. Chemists mitigate against this by adding boiling chips or boiling beads to such a solution. Any chemistry laboratory will have such things on hand, but if you are working at home, a few chips of broken glass or stone from Chapter 2 will do nicely. Finally, you will be handling a hot flask so you ought to have leather work gloves or beaker tongs handy. With these few precautions, you are ready to proceed.

Weigh a medium (100-250 mL) Erlenmeyer flask on a centigram balance and record the weight in your notebook. Tare the balance, add a few boiling chips, and record their weight. Tare the balance again and add the calculated amount of sulfuric acid using a medicine dropper or transfer pipette. If you happen to add too much acid, remove a few drops with the dropper and dispose of it in the sink, washing it down the drain with plenty of water. In a separate cup or weighing boat, weigh out 2.00 grams of sodium chloride and record this weight in your notebook for completeness.

Use a spatula or spoon to add the sodium chloride to the acid a little at a time. The acid will fizz and hydrogen chloride gas will be evolved. If you happen to get a whiff of hydrogen chloride gas, imagine what it must have been like to live next door to an early soda factory. Use a strip of pH test paper, the chemist's virtual nose, to determine the pH of the gas. Continue adding sodium chloride until you have added the entire 2 grams. Normally we would calcine the acid-salt mixture in a furnace at red heat, but we can get by with a kinder, gentler heat by engaging in successive rounds of calcination and dissolution. There are two ways you may calcine your flask. You may mount it on a lab stand and heat it over a spirit lamp or you may heat it on a hot-plate. To prevent the acid from spattering, place a cork loosely into the flask. Light the spirit lamp or turn the hot-plate to its highest setting and heat the flask for 10 minutes, as shown in Figure 20-3. Snuff the lamp or remove the flask from the hot-plate, remove the cork, and let the flask cool for 10 minutes. Human nature being what it is, you will probably be tempted to cut this time short, but if you rush things you are likely to break your flask. When it is cool enough to handle with your bare hands, weigh the flask and record the weight in your notebook. You can now subtract the weight of the empty flask and the boiling chips to get the weight of your product. Compare this weight to the theoretical yield you calculated earlier.

Figure 20-3. Calcination

Chances are your actual yield is higher than your theoretical yield, meaning that not all of the HCl has departed. Add 5 mL of water to the flask with a medicine dropper or transfer pipette and swirl the flask to bring as much of the solid as possible into solution. Fill the pipette with the resulting solution and use it to wash all of the solid from the sides of the flask down into the bottom, as shown in Figure 20-4. Then test the solution by placing the last drop from your pipette on a strip of pH test paper; it will probably be acidic. You are about to calcine this solution a second time, but the concentrated solution may spatter as it dries out. To prevent this, place a cork loosely into the mouth of the flask and place it back on the heat source, as shown in Figure 20-3(L). Once the solution has stopped spattering the cork may be removed, as shown in Figure 20-3(R). Calcine for a total of 10 minutes and then cool for 10 minutes. Weigh the cool flask once again and calculate the yield of your product. It should be less than before if you have successfully driven off more HCl.

Continue this cycle of calcination and dissolution; it may take four or five rounds to drive off all of the HCl. When the weight stops dropping, that is, when the weight from your final calcination is the same as the one before, you have probably succeeded in driving off all of the HCl. The flask now contains anhydrous sodium sulfate. Chances are, you are completely underwhelmed at this point. All this time spent slaving over a hot flask for nothing more than some white powder. Mirabile indeed! The best is yet to come, my friend, if you will be patient but a while longer.

Figure 20-4. Dissolution

The sal mirabile which brought a glow to Glauber's cheek, was not anhydrous sodium sulfate, but sodium sulfate decahydrate, Na2SO410 H2O. To make it, add only 3 mL of water to your flask, not 5 as before. Warm the flask gently over the spirit lamp or hot-plate, but do not bring it to a boil. The flask should not be so hot that you cannot hold it comfortably in your bare hand. Swirl the liquid around the flask and use your pipette as before to rinse the solid from the sides of the flask. Your goal is to get as much sodium sulfate into as little warm water as is physically possible. Spend a full 10 minutes swirling, rinsing, and warming; this time is well spent. Not all of the solid will dissolve. Use your pipette to transfer the warm, saturated solution to a small Petri dish or watch glass and allow it to cool. If you have used the fire gently and with great skill, you will witness the behavior which gave Glauber such high opinion of this salt:

 

This sal mirabile being rightly prepared, looketh like Water congealed or frozen into Ice; it appeareth like the Crystals of Salt-petre, which shoot into a long Figure; also it is clear and transparent, and being put to the Tongue, melts like Ice. It tasteth neither sharp, nor very salt, but leaveth a little astringency on the Tongue. Being put upon burning Coals, it doth not leap and crackle after the manner of common salt, neither coneiveth flame like Salt-petre, nor being red hot, sends forth any smell; which gifts or endowments no other salt possesseth.

 — Rudolf Glauber, A Treatise on the Nature of Salts[1]

Figure 20-5. Coagulation

With the first crystals of Glauber's salt in hand, add another 2 mL of water to the remaining solid in the flask and use your pipette to bring the solid into solution. Add this solution to the crystals in your Petri dish and allow the excess water to evaporate overnight. If you allow the crystals to dry longer than that, they may revert to anhydrous sodium sulfate, depending on the temperature and relative humidity. Now, I know what you are probably thinking; these crystals are not nearly as mirabile as the hype might have led you to believe. It is difficult to compete with color television and video games. I am amazed that you have read this far in the book with an attitude like that. But consider that without Glauber's salt, there would have been no cheap soda, without soda no cheap glass, and without glass, no television and video games. And who knows? If crystals get under your skin, as they did mine, and Glauber's before me, then you may become interested in growing larger and more beautiful crystals, water-soluble gemstones like the large crystal of Glauber's salt shown in Figure 20-5(R). If so, allow me to recommend Crystals and Crystal Growing,[2] the Bible of amateur crystallography.

ImportantQuality Assurance
 

Compare the theoretical yield to your actual yield of anhydrous sodium sulfate. Your product should be very nearly neutral in pH; tape your final pH test paper into your notebook. Photograph your crystals and tape the photo into your notebook as an everlasting memorial.

Notes

[1]

Reference [18], p 32.

[2]

Reference [86]