Recently, Anne Schmidt and I published an article together in the Journal of Chemical Education.1 The article outlines a titration activity that students can carry out at home using only store-bought items. The objective of the activity is to determine the amount of Mg(OH)2 in milk of magnesia.
Vinegar, which is 5.0% acetic acid by mass, is used as the titrant. Therefore, the pertinent chemical reaction at play during the titration is:
Mg(OH)2(s) + 2 HC2H3O2(aq) → Mg(C2H3O2)2 (aq) + 2 H2O(l)
Because this is an acid-base reaction, the pH drops as vinegar is added to the milk of magnesia. Food dyes from natural sources, now available online and at many grocery stores, are used as the indicator. These indicators contain anthocyanins (red), phycocyanin (blue and green), and curcumin (yellow and green). The chemistry of such natural food dyes in powder form has been covered in ChemEd X.2-4 Liquid-based natural food dyes are now available, and these are much more convenient to work with than the powder-based natural dyes.
Careful measurements of mass must be taken when conducting any titration, and this one is no different. Fortunately, kitchen balances capable of measuring mass to the nearest 0.01 g are now available online or in retail stores for around $20.5 That such balances can now be conveniently purchased opens up a host of mass-based experiments that can performed at home – or in the elementary science classroom. Prior to these balances being available, mass-based experiments were difficult if not impossible to be conducted at home.
Video 1 describes the process of completing the titration, and also goes through the calculations required to convert the experimentally collected data into the amount of Mg(OH)2 present in milk of magnesia.
Video 1: Titration of magnesium hydroxide in milk of magnesia, Tommy Technetium YouTube Channel, May 26, 2021
Both Anne and I have had many of our students conduct this experiment remotely. The ability of this lab to be conducted in a remote fashion has been useful to me and my students, many of whom have needed to be off-campus for a time due to issues related to the COVID-19 pandemic.
I have found the liquid-based natural food dyes used in this experiment to be an interesting, convenient source of acid-base indicators. Thus, these food dyes have potential for a variety of applications in science and chemistry classrooms. For example, I have used these food dyes during in-class demonstrations involving acid-base reactions. I have also had students in a science course for non-majors investigate the color changes that occur in these natural food dyes at various pH levels. Video 2 displays just how simply one can generate interesting color changes with these natural food dyes.
Video 2: Color Changing Chemistry Experiment with Food Dyes, Tommy Technetium YouTube Channel, Nov 22, 2021
If you try the titration out, let me know how it works for you. Also let me know if you experiment with the natural food dyes. What variations on the titration experiment might you try? What kinds of experiments and demonstrations can you create with the natural food dyes? I look forward to hearing about your own explorations in the comments.
Happy Experimenting!
References:
- Kuntzleman, Corts, and Schmidt, At-Home Titration: Magnesium Hydroxide in Milk of Magnesia Using an Inexpensive Digital Balance and Natural Food Dye as Indicators, Journal of Chemical Education, July 2021.
- Kuntzleman, Chemical Investigations of McCormick's Color From Nature Food Colors. Part 1: Sky Blue, ChemEd X, Jan 2017.
- Kuntzleman, Investigations of Chemicals in Natural Food Coloring. Part 2: Berry, ChemEd X, Feb 2017.
- Kuntzleman, Investigations of Chemicals in Natural Food Coloring. Part 3: Sunflower, ChemEd X, March 2017.
- Example digital kitchen scale: https://www.walmart.com/ip/KUBEI-Digital-Kitchen-Scale-500g-x-0-01g-Food-Weighing-Gram-Scale-High-Accuracy-Cooking-Mini-Pocket-Portable-Electronic-Small-Jewelry-Automatic/407000660 (accessed November, 2021).
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Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.
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Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.
Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
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Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.
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Comments 2
Titration by weighing
Analysing Milk of Magnesia is one of the few occasions when a titration by weighing (gravimetric) is the only way. It would be impossible to make a dilute solution of magnesium hydroxide in a volumetric flask as the solid would be settling out.
The video https://youtu.be/hFffRZlzqvY shows my microscale version. When I took this method to the States, I found you did not have the claw clamps. The idea is to turn the screw on the clamp to simulate the tap on a burette. I had to improvise by putting a Hofmann clip around the pipette bulb. Interestingly this and the Mohr clip, around rubber tubing, were the original methods of delivering a drop of solution from a burette.
Volumetric titration is deeply engrained in the UK Chemistry teachers’ psyche. It is a required practical in our National Exams, so any deviation is regarded very suspiciously. However, on our CLEAPSS helpline , I was told many times that students were breaking burettes by, clamping too tightly, snapping the taps off chipping of the top of the burette by inserting too wide funnels, greasing taps so much that the grease clogs the tip of the burette, snapping pipettes by inserting pipette fillers and getting pipette fillers full of liquid. 50-ml burettes are very tall for 15-year olds to use so why not use 25-ml versions. No I was told, any deviation may cost marks in an exam. (I was one told that using a 10 ml pipette was making the calculation too easy!) But titration is the most complex operation that students do. It overloads the working memory of the brain. There is just too much to do with manipulation that the whole point of the chemistry is lost.
Titration by weighing can be used as an introduction to the chemistry without the students breaking expensive equipment. Once the principles of titrating are understood, the students can then master the difficult techniques of using the pipette and the burette. And so (we hope) the novice moves to being an expert, just like the teacher (cough, cough). How much titrating have new teachers really done at university for their degrees? I did ask some undergraduates and the number of times quoted to me were low, even zero. Then they go back into schools as “experts”!
The surprise is, that because the densities of the dilute aqueous solutions are similar, the titration by weighing method is quite accurate. (My narrow-tipped pipettes add 0.02g of aqueous liquid per drop)
The balances cost than £10 (and dollars) and we call them digital jewellery balances. Our schools are beginning to buy class sets as it reduces the distracting movement of students around the lab. Just compare them with standard masses, and you will see they are really good. I have carried them around in cars and planes and they still work really well.
There are other reagents in Milk of Magnesia including glycerol, sodium saccharin and a little sodium bicarbonate (affecting the titration?)
Hi Bob:
Hi Bob:
Thank you for sharing your microscale version of this experiment and also the video - great stuff! I wholeheartedly agree that mass-based titrations offer several advantages to volume-based titrations. So much so, that I'd argue that at this point, we chemical educators should increasinglly be moving toward mass-based titrations. Along these lines, I believe that you will find the article linked below and the references included therein to be interesting.
It's always good to hear from you, Bob. I very much enjoy your experiments.
J. Chem. Educ. 2016, 93, 6, 988–989