Pop Rocks: Candies, or Keys to Understanding Titration?




There are many myths surrounding the ever-popular Pop Rocks™ candy. The fizzy, gas-producing candy seems to be the stuff of urban legend and tall tale! When Pop Rocks™ candy first became commercially available in the mid-nineteen seventies, the stories of torn-open tummies and Coca-Cola™ induced explosions had parents literally panicked. Several Poison Control centers actually had to create emergency phone lines to assist hysterical parents and assure them that their children would not be harmed by the fizzing candy.

How did such a myth begin? Because Pop Rocks™ fizzes and pops when it is put into the mouth, parents believed that their children might be ingesting the gas created and that their stomachs could be seriously harmed - they even thought their children might explode! And although this myth is totally unfounded (eating Pop Rocks™ might not be nutritious, but it certainly won't cause you to detonate), there is some truth to the way these parents were thinking about Pop Rocks™.

Pop Rocks™ are formed by the addition of sugar and other sweet ingredients to a pressurized chamber filled with carbon dioxide, CO2. The carbon dioxide-candy mixture is heated to excesses of 320 degrees Fahrenheit and then super-cooled under large pressures (50 atm or more!). Upon release of this huge pressure, the mass of candy fractures into tiny "rocks". The "rocks" contain pockets of the CO2 gas originally introduced into the mixture. Therefore, when Pop Rocks™ dissolve in your mouth, the gas is released from the pockets with a popping sound.

Your job in lab today will be to determine the amount of carbon dioxide per gram of the candy. You will do this by dissolving the Pop Rocks™ slowly and collecting the CO2 gas that escapes. There exists a problem in determining how much CO2 gas evolves, however, as the lab is not equipped to measure small volumes of gas with much accuracy. Instead, you will convert the CO2 gas into a measurable quantity. By using 0.1 M NaOH, you will convert the gas to sodium carbonate, Na2CO3 (aq). If this chemical could be isolated, one could then calculate the moles produced via a balanced chemical equation like the one below:

However, since you are collecting an unknown amount of CO2 (g), it is not clear how much NaOH should be used to covert CO2 (g) into CO32- (aq). Therefore, after the above reaction takes place, you will have some NaOH left over. The major species in the collection vessel will be Na+, CO32- , and OH-.

You will titrate this mixture twice with HCl. In the first titration, you will use the indicator phenolphthalein, which is pink in basic solution and clear in acidic solution. In the second titration, you will use methyl orange. Methyl orange is a red liquid which will be yellow at the beginning of the second titration and will turn red-orange at the endpoint of the titration.

Two reactions will take place with the addition of acid in the first titration. The excess OH- will be neutralized, and the carbonate ion will be converted to bicarbonate, HCO3- (aq). These two reactions are shown below:

After the first titration, the major species in solution is the bicarbonate ion, HCO32- (aq). You will titrate this ion using HCl to form the species H2CO3 (aq). The amount of H2CO3 (aq) formed is exactly equal to the amount of CO2 (g) evolved from your sample of Pop Rocks™! The reaction is as follows:

When you have found the amount of CO2 (g) evolved by the Pop Rocks™, you have completed the main objective for this lab.