I was at a chemistry teacher workshop recently and we participated in a common Hess’s Law laboratory. Part of the procedure required us to measure about 2 grams of solid NaOH and add it to 100 mL of 0.5 M HCl. We also added 50 mL of 1 M NaOH solution to 50 mL of 0.5 M HCl solution. We then compared the energy change of both containers.
After the lab, we had a group discussion about the usefulness of the laboratory to meeting our objectives in our individual classrooms. Two of the teachers present voiced concern over using solid NaOH in a high school setting. A couple other teachers cited 20 years of using it with no issues as evidence that it is not too dangerous to be used in this setting. I don’t accept a lack of accidents as evidence for the safety of a chemical, but I have considered solid NaOH a staple of the chemistry classroom. I believe that I have used a couple of labs in the past that required the use of solid NaOH, but it has been many years.
I would like to have my students engage in this particular laboratory. Are normal safety precautions that I use every day in my high school lab adequate when using small amounts of solid NaOH or should I look for an alternative laboratory? Also, I have several gallon size containers of solid NaOH left over from 20+ years ago (original containers from chemical supplier). I have been using them to mix standard solutions for years and my student lab results are adequate for my level of expectation. Is it acceptable to use these old containers? Can I rely on the chemical MSDS to warn me if I cannot use a chemical past a certain date?
July 14, 2014
Dear Ms. Cullen,
Thank you for contacting the American Chemical Society’s Committee on Chemical Safety with your experiment questions.
Regarding the Hess’s Law experiment:
The Hess’s Law experiment involving dilute sodium hydroxide/solid sodium hydroxide and dilute hydrochloric acid described in your note is a low-risk experiment. Solid sodium hydroxide, in the quantities you mention in your note can be handled quite safely by properly supervised students with ordinary laboratory tools such as scoops or tweezers. The reaction you describe in your note will proceed much slower with the solid reactant due to reduced surface area effects of the solid and any spill of the solid reactant is easily and safely mitigated with little risk of overexposure.
Students may be tempted to pick up the solid sodium hydroxide with their bare hands which may cause minor skin effects. This can be avoided by requiring the use of tools such as scoops or tweezers to handle the solid.
Regarding expiration dates on chemicals and MSDSs:
Generally speaking, the efficacy of a chemical in storage is dependent on many factors: temperature, humidity (especially so for hygroscopic material such as solid sodium hydroxide), light, presence of contaminants, exposure to air (oxygen) etc. Expiration dates on chemicals are often provided by manufacturers and distributors as a “value added” service.
Sodium hydroxide is a hydroscopic, inorganic salt and does not easily undergo chemical change such as oxidation or reduction. Its quality, in terms of the number of hydration waters, is affected by its exposure to atmospheric moisture meaning that over time, the mass of the product will change. However, it remains sodium hydroxide – it does not change chemically (i.e. oxidize or reduce). If your experiment will tolerate the change in mass due to waters of hydration, there should be no problems in using it, provided it has remained free of other contaminants. We have attached an article that appeared in the Chemical Health & Safety, the peer-reviewed journal of the Division of Chemical Health and Safety regarding chemical expiration dates for your review. (Russel Phifer, Reagent Expiration Dates: Fact or Fiction? Chemical Health & Safety, September/October, 1999, p 17 - 20. Posted with permission Division of Chemical Health and Safety of the American Chemical Society.)
The Material Safety Data Sheet supplied with your chemical provides first-level information for the user, but not necessarily information for laboratory use. We recommend that you understand the information in the MSDS, but consult other references such as a Laboratory Chemical Safety Summary found in Prudent Practices or other peer-reviewed sources for more complete information.
Again, thank you for contacting the Committee on Chemical Safety.
Robert H. Hill, Ph.D. Harry J. Elston, Ph.D., CIH
Chair, CCS Chair, Safe Practices Subcommittee
Above information has been provided by recognized authorities from sources believed to be reliable and to represent the best opinions on the subject. This information is intended to serve only as a starting point for good chemical safety practices and does not purport to specify minimal legal standards or to represent the policy of the American Chemical Society. No warranty, guarantee, or representation is made by the American Chemical Society or ChemEd X as to the accuracy or sufficiency of the information contained herein, and the Society assumes no responsibility in connection therewith. This information is intended to provide basic chemical safety guidelines and therefore it cannot be assumed that all necessary warning and precautionary measures have been included and that other or additional information or measures may not be required. Readers should consult pertinent local, state, and federal laws and legal counsel prior to initiating any accident-prevention program.