9 comments by Robert
There is more information that will be relevant to school teachers and technicians. CLEAPSS (www.cleapss.org.uk) obtains many complaints about phenolphthalein solution when used as an indicator. Teachers blame the technicians first and then the technicians call CLEAPSS on the Helpline. When adding a strong alkali to a weak base, phenolphthalein is used but the pale fuchsia-coloured endpoint appears and then disappears. This is caused by carbon dioxide (acidic) in the atmosphere, dissolving into the weakly alkaline solution and reducing the pH. It is recommended that if the colour is present for 10 to 15 seconds, then the endpoint has been reached.
Another call concerns what happens when 1M sodium hydroxide is added to phenolphthalein solution as the mauve colour slowly disappears. Universal Indicator (UI) also contains phenolphthalein. At pH values above 13, the colours of UI deviate from the expected range. These two effects are caused by another alteration in the structure of the molecule at high pH values greater than 12 and the conjugation of double bonds is lost again. The solution turns slowly colourless again.
You can access the structures on the Wikipedia page (https://en.wikipedia.org/wiki/Phenolphthalein). The article implies there is another change in structure at pH values of 0 and lower. I have never tried this. There is a great gif there as well illustrating the changes in structure with pH.
The hazard classification of phenolphthalein in the article also needs to be updated. The classification from the European Chemical Agency is Signal Word: "Danger"
• Germ cell mutagenicity, Category 2; H341(Suspected of causing genetic defects)
• Carcinogenicity, Category 1B; H350 (May cause cancer)
• Reproductive toxicity, Category 2; H361f(Suspected of damaging fertility)
This has caused consternation, especially when the substance has been cited as a Substance of Very High Concern (SVHC). I am reading between the lines here but the labelling of most solutions of substances carrying these Hazard Statements is usually 0.1% (w/v) but for phenolphthalein, it is 1%. This could be a “nod” to its use as an indicator so that chemists can buy a 0.99% solution and not have any hazard markings on the bottle (except “flammable” due to the ethanol present). Unfortunately, some countries and school safety organisations do not allow any SVHC substances to be used in their schools. Fortunately, the HSE in the UK still allow CLEAPSS to issue guidance on the chemical through its Hazcards and Recipe Sheets. That is because we look critically at the level of risk, not solely at the level of hazard.
As you can imagine, this will affect many titration exercises, experiments which have taken place in schools for over 100 years without any reported incidents. Well, except one story, I inherited when joining CLEAPSS. One teacher went on and on about the use in Ex-Lax and one student that a joke could be played on a fellow student by pouring the contents of a dropping bottle of the indicator into the student’s drink. When the student did drink the laced orange juice, the student collapsed. The laxative issue was not the problem. The solution was made up in 60% ethanol (120 proof) and the student suffered alcohol poisoning. The use of a stomach pump solved the issue. Be careful telling anecdotes!
Green chemistry for schools has flitted in out of schemes of work but they are available.
For esterification and preparation of aspirin, strongly acidic ion exchange resin can be used as an example of a heterogrneious catalyst.
I doubt if any synthetic organic chemist would use sodium dichromate as an oxidising agent for oxidising alcohols and carbonyl compounds. Hydrogen peroxide is the greenest of oxidising agents with a high atom economy and can be used with a small amount of iron(II) ions present (Fenton's reagent)
You can read more about it here https://microchemuk.weebly.com/2-blog-is-this-supposed-to-happen/archives/07-2017.
In the 1970s I had a course of the chemistry in Norfolk for Y9 where I am my colleague made contact with local chocolate makers, breweries, fruit cordial suppliers and the we did work on extracting oil and making soap from rape seed, testing local water, etc. We also looked at ancient local crafts such as charcoal burning and dying. In the 6th form the w would take the students to the University to see spectrometers and the Nuffield Chem Eng students would be shown around the local chemical engineering factory. I should think all ex teachers have similar stories.
Then came double science to scupper the first idea!
AS I passed my Hafnium birthday, I really think there is nothing new. My interest in microscale chemistry (www.microchemuk.weebly.com) was being fuelled by a local teacher, Andrew Farmer, who died young but left ideas in School Science Review, (School Science Review, 54, 189, 695-701, Jun 73). Prof Alex Johnstone was doing it as well. The J Chem Ed had a series on projecting reactions onto a screen using a new-fangled slide projector. Then came the overhead projectors. Now wehave visualisers.
In many ways everyone is just reinventing the wheel but improving the design.
ANODin brings Positive relief
“Err Teacher, all this supercooling, depression of freezing point, hand warmer stuff, and variation of boiling point on the top of Mount Everest you are on about; is it in the exam, do we have learn and memorise it.”
“Is it in the test you are going to set us next week?”
“No, but …..”
“So why are you telling us about it?”
“Because it is interesting! You all complain about “what is the point of all this?" And when I show you relevant applications you all go on about “is it in the exam?” Can I ever please you?”
“Not going to climb Everes though”
If you think it confuses them (or you) don’t teach it. Just do the normal freezing point experiment you have there (which assumes the molecules are small solid spheres with no attractions between them. This is done very early on the schemes of work so they have not met ions, molecular architecture and rotating bonds. Nor have they done potential energy diagrams, activation energy and entropy.
And these added extras are not in the AQA GCSE syllabus (and do check the others). So why bother? (I did have a picture here but I cannot add it)
Oh yes, you want to make chemistry interesting and it is the exceptions to general rules that make chemistry interesting (look at the melting points of the hydrides in the p block elements). So this is all tied up with progression teaching isn’t it. Surely, nobody just teaches a topic and then does not revisit it until it is revised for an exam?
Thank you for reminding me of cyclohexane which has a Mpt of 6.5°C and Bpt of about 80°C. This means all 3 states are available within the temperature range of water from the freezer and the electric kettle. You would be allowed to put 0.5ml of cyclohexane in a test tube (with an anti-bumping granule) and use hot water from a kettle (do not use a Bunsen). (I feel a microscale application coming on!.
This is not what caesium chloride solution looks like. I went to the source and found it was cyan dye.
If you want to try this, the CLEAPSS conductivity indicator (GL166 on the CLEAPSS website) is ideal and can be made quite simply. Place a puddle (1 to 1.5cm diameter) of distilled water on a plastic (polypropylene is best) sheet. The LED on the indicator does not light up. Now move with a wooden splint 1 grain (yes 1 grain) of salt into the puddle. The LED lights up after the salt dissolves. This does not happen with sugar (but interestingly does with brown unrefined sugar). Conductivity of solutions is much neglected now with the emphasis on electrolysis. Electrolysis is a process which enables the electric current to seemingly pass through the solvent.
The picture of the knife illustrates perfectly all that is happening here (err, that is irony).
The plastic used in laminated sheets is not a hydrocarbon. It means that the “puddle” or hemispherical drop is not quite as good as when the puddle is on a sheet of polypropylene. It all hinges on the contact angle. The reason for using a plastic folder rather than a well-plate is that one can insert the worksheet with instructions into the PP folder. This reduces cognitive load and the experimenter does not have to keep asking “what do I do next?”
Clearing up is just a wipe with a paper towel so it avoids the washing of numerous test tubes. The sheets can be photographed and inserted into lab books. Photographing is so easy now and close-ups with cropping produces some beautiful effects.
Observe very carefully at the green iron(II) hydroxide precipitate in the article and you will see a faint brownness appearing which will in 15 minutes, the ppt will be very brown. This brings up the usual remark from students “Is this supposed to happen?” Always a great with observation excercises.
One American when trying these methods suddenly said, “Gee in a little you can see a lot”. A great strap line I thought. The word “puddle” was coined by Prof Bruce Mattson at Creighton University. See his method for doing precipitates with cocktail sticks which they call tooth picks! https://www.youtube.com/watch?v=f2FA1p5KHCE
Thank you for pushing forward some of these practical ideas that have been developed at CLEAPSS. They were initially answers to safety and cost problems at first but in your words, the educational advantages become apparent. You did not have enough space for the little Hofmann but the electrolyte is sodium sulfate solution, not 2M sulfuric acid as used in a large Hofmann, which is why bromothymol blue can be used to indicate the acidic and alkaline at each electrode. (Other indicators are affected by the oxygen (at a little ozone) coming off at the anode positive electrode.) With 5 ml of hydrogen and 2.5ml of oxygen and can get a squeaky pop in a test tube and relight a glowing splint. Which may surprise teachers who think this is all too small. “Dynamite” soap bubbles can be made and 7.5 ml the gas mixture will knock your socks off (forget balloons) and I can fire plastic pipette bulbs with the energy from that reaction but there is a trick to that if you want to send the blub over 10 metres into the air.
Teachers should also be aware of the CLEAPSS conductivity indicator, again using carbon fibre rods, in which a LRD lights up in tap water but not in distilled or deionised water. But if 1 grain (yes 1 grain) of salt is introduced into a 1.5cm diameter puddle of pure water on a polypropylene sheet the LED lights up. The sodium chloride has dissolved. The electrostatic attraction of water molecules to ions outweighs the electrostatic attraction of sodium and chloride ions in the crystal. And ye in many text books, the ionic bond talk of as a “strong” bond.
You write “Most UK examination boards allow teachers flexibility in how electrolysis experiments are carried out and in the choice of solutions investigated” but at the Technicians meeting at Hatfield yesterday, the technicians said that teachers will do not do these experiments because they think that being “micro” the students cannot manipulate the parts, cannot see what is happening and it is not the equipment in the text book which will penalise them when students take exams. These procedures are there to teach with, not just for examinations. The availability of visualizers, webcams and digital microscopes makes these techniques come alive on the projection screen.
You might righty remove the words anode and cathode from the article but teachers will not because there will be exam points for that. The idea that you can PROGRESS to these terms is not easy in teaching when books are full of the words. We do tend to overload our students with words and ideas in one fell swoop instead of progressing with ideas from one year to the next up to GCSE.
Unfortunately attempts to provide teachers with CPD on items like this do not recruit due to limited funds being available for this type of CPD which is why RSC outreach is so important. Teachers will see some of these procedures on July 4th at a meeting in Birmingham on July 4th (http://www.rsc.org/events/detail/25676/transitions-chemistry-from-8-to-18-and-beyond) and at the VICE meeting in York, http://www.rsc.org/events/detail/26401/Variety%20in%20Chemistry%20Education%20and%20Physics%20Higher%20Education%20Conference%20(ViCEPHEC)%202017.
Bob Worley (www.microchemuk.weebly.com)