The theme for National Chemistry Week (NCW) for 2016 is “Solving Mysteries through Chemistry: Exploring the Chemistry of Fibers and Forensics”. To prepare for NCW, I’ve been trying to learn as much as I can about forensic science and chemistry. One of the more interesting topics I’ve learned about is how cocaine (C17H21NO4) adheres to and can be detected on paper currency. I thought I would take on the challenge of trying to figure out a way – using materials commonly found in high school chemistry laboratories – to detect this illegal drug on $5 bills. Turns out this challenge was a bit tougher than I initially imagined it might be!
Cocaine is an illicit drug that is generally bought and sold using cash. Because both sellers and users handle the drug and money, cocaine can become adhered to fibers in paper currency during purchasing transactions.1-4 Furthermore, people sometimes use banknotes to inhale the drug. The cocaine adhered to bills is transferred during note to note contact, such as in cash registers and bank counting machines.1 Several analytical techniques (gas and liquid chromatography, mass spectrometry, capillary electrophoresis and more)1-3 have been used to investigate the amount of cocaine present on paper currency. The amount of cocaine on any single bill varies widely based on location, age, and other factors. Of note, 1.3 milligrams of cocaine was found on a single $1 bill in Ohio!1,2 In general, 90% or more of banknotes across the globe contain traces of cocaine.1-3 While amounts vary widely, U.S. bills of varying currency can be expected to have roughly 3 to 30 micrograms of cocaine.2-3
Cobalt (II) thiocyanate, Co(SCN)2, can be used to test for the presence of cocaine in protonated form (C17H22NO4+), due to its reaction with cobalt thiocyanate to form a blue product:5-6
[Co(SCN)(H2O)5]+(red, aq) + 3 SCN-(aq) + 2 C17H22NO4+(aq) –> [(C17H22NO4)2Co(SCN)4](blue, aq) + 5 H2O(l)
In the Illustrated Guide to Home Forensic Science Experiments4 by Robert and Barbara Thompson (a FANTASTIC resource for NCW 2016, by the way), it is stated that a 0.11M solution of Co(SCN)2 can be used to detect cocaine in the amount of 60 micrograms or more. (if you don’t have Co(SCN)2 on hand, a solution of a mixture of 0.11 M CoCl2 and 0.11M KSCN is reported to work equally well).4 Because most bills contain much less than this amount, the probability of using this test to detect cocaine on a single banknote is low. According to Thompson and Thompson, your chances of achieving a positive test increase if you live in an urban area where cocaine use is prevalent.4 However, not all studies agree with this claim.1
I set out to see if I could collect enough cocaine from U.S. $5 bills for a positive test using Co(SCN)2. To do so, I used methanol as the solvent to transfer cocaine from banknotes to a Q-tip. I used methanol rather than water as the solvent because methanol dissolves both the acidic and basic forms of cocaine, whereas water only dissolves the acidic form.7
To conduct tests, I placed 2 – 3 drops of methanol on a Q-tip, and then ran the moistened portion of the Q-tip along the entire front surface of a $5 bill. I then placed 2 – 3 fresh drops of methanol on the Q-tip and repeated the process on the opposite side of the $5 bill. Using the same Q-tip, I repeated this process again for a total of five different $5 bills. Each time, I tried to rub the very same portion of the Q-tip over the surface of each side of all five $5 bills. This was done to so as to concentrate material on to the same area of the Q-tip to increase the probability of a positive test. After concentrating material on the Q-tip, I added 2 – 3 drops of 0.11M Co(SCN)2 to the appropriate area of the Q-tip. Finally, I added 2 – 3 drops of 12M HCl to convert any basic form of cocaine on the Q-tip to the acidic form (recall it is the acidic form that reacts with cobalt (II) thiocyanate to form the blue complex). You can see this procedure carried out in the video below:
You will note that the positive results achieved were subtle. In fact, some might argue that I didn’t actually achieve a positive test. Nevertheless, I could see shifts to blue color several (but not all) trials. In a total of 19 trials, 7 resulted in a positive test, 11 were negative, and one test was deemed to be inconclusive (an extremely faint color change to blue was observed).
If you think this experiment looks a bit time consuming, you’re right. If you’d rather not go through the hassle of trying to extract cocaine from currency, diphenhydramine (C17H22NO+ in acid form) tablets can be used to spike banknotes to easily illustrate how this test works. Similar to cocaine, diphenhydramine (the active ingredient in Benadryl) reacts with cobalt thiocyanate to form a blue product. A likely reaction for this process is:
[Co(SCN)(H2O)5]+(red, aq) + 3 SCN-(aq) + 2 C17H22NO+(aq) –> [(C17H22NO)2Co(SCN)4](blue, aq) + 5 H2O(l)
Spiking bills with diphenhydramine adds the benefit that you can pretty much guarantee strikingly positive test results. In the video below you can see how to carry out the cobalt (II) thiocyanate test with diphenhydramine. Because it is much easier to achieve and identify a positive test with diphenhydramine, it’s a good idea to try this experiment to see how a strongly positive test might look when testing for cocaine on bills.
If you try out this experiment on your own, I’d love to hear from you. I’d also like to hear any comments about this experiment. Perhaps you think the results I achieved are too subtle to indicate the presence of cocaine on $5 bills. Let me know if you think this is the case. Also, let me know if you have any suggestions on how to improve chances of detecting cocaine using this (or any other) method. Remember, however, I’m looking for tests that can be conducted using materials found in high school labs! Given that I live in (and collected all $5 bills tested from) a rural town of about 7500 people, I’m interested to see if stronger and positive results occur more often using banknotes obtained from large cities. I’m also interested to see if $1, $10, $20, or bills from other countries provide a greater chance of achieving positive results. Finally, let me know if you might you use this experiment during NCW, if you successfully used this method to detect cocaine on any currency, or if you were able to achieve a higher probability of positive results than I. Please share your thoughts, suggestions for improvements, and results of experiments in the comments below. I look forward to hearing from you and your students!