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2015 April 29

More model fitting in lecture

Filed under: Circuits course — gasstationwithoutpumps @ 22:05
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Today’s lecture was all about fitting models for the electrode data. I started by showing them how one could hand-sketch Bode plots, at least for RC and RL circuits.  We did a hand plot and a gnuplot plot for the $R_{s} + (R_{p} || Z_{c}(C))$ model with arbitrary values, showing the initial horizontal $R_{s} + R_{p}$, the final horizontal $R_{s}$, and the diagonal at $\frac{1}{2\pi f C}$.

In class I went through trying to do fits to data collected for stainless-steel electrodes, and showing how to debug various problems (it was all live-action plotting—I did not script my actions).  The biggest problems were getting very bad fits (in one case from taking the log of the function but not the log of the data, in another case from having bad initial values) and singular matrices (mainly from having variables in the function that didn’t affect the fit, though in some cases from trying to fit complex models to real data without taking absolute value of the complex model).

It turns out that the standard R+(R||Z_C) model is very hard to fit to the data we collected for the stainless steel electrodes.  The oxide coatings don’t leak much current, so we had no low-frequency plateau for estimating the parallel resistance from.  I suggested making the parallel resistance infinite and using a simple R+Z_C serial connection.  That can model the data well at high frequencies, where the change in |Z| is fairly small, but at low frequencies the model is poor.

I came up with a different model on the spur of the moment (not one I had ever tried before on electrode data): $R + \frac{1}{j \omega^\alpha D}$ with a capacitor-like element having a smaller slope that the normal 1/f slope of a capacitor (about 0.6).  This turned out to fit the data quite well.  I don’t have a convincing physical explanation for the exponent α, but I suspect it has to do with diffusion times for ions near the surface of the electrode and depletion regions in the electrolyte.

In the new model, the R term probably corresponds to the bulk properties of the electrolyte solution and the $\frac{1}{j \omega^\alpha D}$ term to the surface chemistry at the electrode, so 1/R should be proportional to the concentration of the NaCl, I think.  I wonder whether students will get that result in their fits.  I’m thinking that I should rewrite some of the book to incorporate this model.

I ended by trying to model some of the data collected by students that did not work well—they had a huge inductance uptick at high frequency (fitting nicely to something like a 3mH inductance).  I’ve no idea how they got that data, as I saw their setup and they couldn’t have had more than a few µH of stray inductance.  Other students had small upticks at the high frequencies that were almost certainly stray inductance, since moving the voltmeter leads to connect directly to the electrodes eliminated the uptick, which did not happen with the students whose data I tried modeling.  I showed students how to model the uptick with an additional inductor, but I really don’t know what went wrong with the student data—I didn’t see any problems with their setup or recording, so I can only assume we all missed something.

Some of the students at least are getting the idea that modeling is not forcing your data to fit the theory in the book, but looking for regularities in the data.

2015 April 28

First half of electrode lab a bit long

Filed under: Circuits course — gasstationwithoutpumps @ 20:47
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Monday’s lecture went fairly well—I used my post  Comments for class after grading as lecture notes, and pretty much covered everything, though not necessarily in the order presented there.

Today I spent a long time in the lab, from about 9 a.m. to after 6 p.m., because it takes a fair amount of time to set up and clean up when we are dealing with liquids (in this case, salt water) in the electronics lab.  I have to make sure that everything is in secondary containment tubs, so that nothing gets spilled.  (It irks me that the EE faculty don’t bother enforcing the clearly posted “no food or drink” rule on their students, and I’ve had to chide several EE students coming into the lab with cups of coffee and open bowls of food—I often see drink containers in the room trash.  I spend hours making sure that my students don’t spill anything, but the EE students routinely spill their drinks (judging from the mess on the never-cleaned floor.)

I did two demos today: one planned, one unplanned.  The planned demo was of vernier calipers, which the students used to measure their stainless steel electrodes.  The unplanned demo was of what happens if you pass a large current through a salt solution.  I considered that grade-school chemistry (I’m sure I was in grade school when I took two carbon rods from inside batteries and passed a current through them, measuring the amount of H2 and O2 that bubbled off—I even looked at the difference between AC and DC (initially by using Al foil on a turntable to do the switching, but 33rpm (0.55 Hz) was still too high a frequency to get any electrolysis, and I had to switch to a DPDT switch and a watch to manually get something like 0.1Hz to get small amounts of electrolysis. OK, I admit that was a science-fair project (6th grade? 7th?) to measure the amount of electrolysis as a function of frequency, but electrolysis was not a strange subject for middle school students.

No one in the morning class had any idea what would happen if you passed a current through a salt solution, and I couldn’t even get guesses.  In the afternoon class, I badgered the students a bit more and finally got someone to realize that H2 would bubble out, and (after a bit more badgering) got them to predict which electrode this would happen on. With 6V and a 1A limit, I got vigorous bubbling (about 0.6A current drawn), and the other electrode produced a yellowish color in the solution (probably an iron oxide).

Note: all but one or two of these students have taken at least a year of college chemistry that included a full quarter on electrochemistry, but they had never seen a demo of the H2 reaction, despite it’s being the standard reaction of defining half-cell potentials. (Most of them had seen the reaction before, since it happens in gel electrophoresis boxes all the time and nearly all of them have done gel electrophoresis in biochem labs, but not one of them put 2 and 2 together and realized what was going on.)

What prompted the electrolysis demo was students asking why the readings kept changing on their ohmmeters when they tried to measure DC resistance. I tried through socratic questioning to get them to realize that the ohmmeters work by measuring the voltage across the device under test while passing a known current through, and that passing a DC current through an electrode will result in chemistry on the surface of the electrode, changing the electrode properties. I got some groups as far as realizing that there was a current, but no one seemed to realize that having a current meant there must be redox reactions taking place on the surfaces of the electrodes, changing the surface properties.

Did I already mention that almost all of these students have had a quarter of electrochemistry? I wonder what (if anything) is taught in that class! I’ve never taken it, so I’m relying entirely on what I learned in grade school and high-school—I would have thought that a college-level class would have more than what I recall from a high-school sophomore class in the 60s.

The rest of the lab went fairly smoothly, but a number of students saw a change in the behavior of their electrodes at high frequencies. This was unexpected (I’d not see the effect in my versions of the experiments), so I spent some time debugging the problem. I’m pretty sure that the problem was long wires—the students were getting a series inductance added to their electrodes. About 1.5µH would be enough in some cases to cause the observed phenomenon, though in other cases a much larger inductance would seem indicated. For one student, I suggested hooking up a voltmeter right at the electrodes rather than at the other ends of the wires to the electrodes—the saw a 2-fold reduction in voltage at 1MHz, which pretty much cancelled their apparent increase in impedance. We’ll discuss the problem in class tomorrow, and I’ll suggest modeling the electrodes not with just the standard R+(R||C) model but with L+R+(R||C), with the extra L corresponding to the long wires in their test setup.

We used up about half the salt solutions a colleague had made for me, and we’ll use up the other half on Thursday. It seems we need at least 110ml/student, so next year I’ll probably want to get 150ml/student or even 200ml/student, so that we don’t run out. Today we characterized stainless steel electrodes (which are highly polarizable) and on Thursday we’ll characterize Ag/AgCl electrodes (which are non-polarizable). So I’ll have another long day in the lab on Thursday.

2013 August 26

I won’t teach chemistry

Filed under: home school — gasstationwithoutpumps @ 09:32
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As I mentioned in Homeschooling chemistry this year? “My son needs to take chemistry this year.  He doesn’t have much interest in the subject, …”

Since we couldn’t get him into the community college courses (the waitlist is still full), and the AP course at the local high school might let him in, if there was room, in the spring, it looked like I was going to have to learn it with him, like we did for physics.  While it would be good for me to learn general chemistry, I’m not sure I’m going to have time this year to give it the attention it would need.  I’m very afraid that we would slip on the schedule repeatedly, to my son’s detriment.

My son and I looked at online courses, and found one somewhat pricey one that seems to have gotten good reviews (not only in the customer testimonials, but also in one of the homeschool mailing lists, which tends to have less selection bias).  Although this is just AP chem (slightly less rigorous than the book I’d planned to use), we decided that the external schedule would probably be enough of a benefit to be worth the price to us.

The course is ChemAdvantage, taught by Peter Moskaluk.  It is costing me

$738.95 for tuition (with handling charges)$239.45 for the chemistry kit (with shipping)

$16.98 for the textbook for a total of$995.40.  If I decide to do the labs with him, I’ll need to get another pair of safety goggles, pushing the course over $1000. That’s more than the community college would have been for 2 semesters, but a lot less than UCSC Concurrent Enrollment. I would still have preferred the community college course, where he would have learned to use real chem lab equipment, but the California legislature has decided that education needs to be strictly rationed, so that they can continue to pay for imprisoning ever larger fractions of the population without charging corporations and wealthy individuals any taxes. (Actually, I’ve already spent more than$1000 on the course, because I bought the new AP chem student lab manual, which we probably won’t have any occasion new to use, since the ChemAdvantage course comes with its own lab manual.)

I might still make a colorimeter, just for fun.

2013 August 13

Lab kits for homeschooling chemistry

Filed under: home school — gasstationwithoutpumps @ 10:10
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In my previous post, I mentioned the Advanced Microchemistry kit, from Quality Science Labs.

Another kit that has been mentioned in e-mail is one that I actually looked at earlier and forgot to include in yesterday’s post: the CK01 kit from the Home Scientist.  Instead of 16 labs, it is set up for 39 labs, and it costs only $174 instead of$220.  They also provide the lab manual on-line for free, so it is easy to see what the labs are and whether they are worthwhile.  They also do one other very clever marketing ploy—they sell the same kit for $5 more including a second set of splash goggles for the parent. Since any sensible home school teacher will be nearby while the labs are being done, including a second set of goggles is just good sense. Let me compare the 2 kits side-by-side: Quality Science Labs, Advanced Microchem$220
The Home Scientist,
CK01
$174 acetic acid 0.1M acetic acid 6M ammonia 6M ascorbic acid ascorbic acid (500mg tablets) barium nitrate 0.1M Bromothymol blue indicator Bromothymol blue 0.1% n-butanol calcium nitrate 0.1M calcium nitrate 0.1M charcoal, activated cobalt chloride moisture test paper (reusable) copper copper copper nitrate 0.1M copper (II) sulfate 1M distilled water food coloring 0.1M hydrochloric acid 0.1M hydrochloric acid 6M iodine/iodide solution, 0.1M iron iron (II) sulfate, 0.1M iron (III) chloride, 0.1M lead (II) acetate, 0.1M magnesium magnesium sulfate magnesium sulfate methyl orange, 0.1% methyl red, 0.02% nickel nickel nitrate, 0.1M oxalic acid, 0.5M palmitic acid pH paper wide-range pH paper phenolphthalein paper phenolphthalein 0.5% phosphoric acid, 1M potassium bromide, 0.1M potassium dichromate, 0.1M potassium ferricyanide, 0.1M potassium hydroxide, 0.1M potassium iodide, 0.1M potassium iodide, 0.1M potassium permanganate, 0.01M potassium permanganate, 0.1M salicylic acid sodium acetate, 0.1M sodium bicarbonate (650mg tablet) sodium bisulfite, 1M sodium borate, 0.1% wrt boron sodium carbonate, 1M sodium ferrocyanide, 0.1M sodium hydroxide, 0.1M sodium hydroxide, 6M sodium salicylate, 220 ppm wrt salicylate sodium sulfate, 0.1M sodium sulfide, 0.1M sodium oxalate, 0.1M sodium thiosulfate 1.0M sodium thiosulfate 1.0M starch indicator solution sulfuric acid, 1M thymol blue, 0.04% turmeric reagent vegetable oil zinc zinc nitrate, 0.1M alligator clips 2 alligator clip leads 9v battery 9v battery battery adaptor (9v) 15mL plastic beaker 30mL plastic beaker (2) 50mL plastic beaker (2) 100mL plastic beaker 150mL plastic beaker 50mL glass beaker 250mL borosilicate glass beaker (6) 15mL centrifuge tubes (6) 50mL centrifuge tubes chromatography paper chromatography paper coffee filters conductivity tester cotton swabs cotton balls and swabs 3 oz cup 8 oz foam cup, with cover 9.5oz plastic cup digital scale funnel glass rod 15cm stirring rod safety goggles safety goggles 10mL graduated cylinder 10mL graduated cylinder (plastic) 100mL graduated cylinder (plastic) index card 1cc measuring spoon multimeter paraffin wax felt tip pen purple Sharpie felt-tip pen graduated pipette graduated pipettes thin stem pipette mini pipette 24-well plate 24-well plate 96-well plate 96-well plate rubber bands 6″/150mm ruler sandpaper steel wool stop watch spoon/spatulas stainless steel spoon/spatula stoppers for test tubes syringe 10mL oral, with cap test tube 12×75mm (6) test tubes 16×100mm> test tube brush test tube clamp test tube rack digital thermometer partial immersion thermometer tongs toothpicks wood splints washing bottle wire gauze wire gauze The Quality Science Labs kit has some more expensive items (the digital scale, the multimeter, and the digital thermometer), but also includes more common household stuff and fewer chemicals. I already have a multimeter (better than what they provide), and a digital scale that measures to 0.01g is only$10. I’m not sure a digital thermometer is any better than the alcohol-based thermometers I already have (or that come in the other kit). If we need higher precision or to record over a time period, I already know how to set up a thermistor with the Arduino data logger.  The Home Scientist kit has somewhat more dangerous chemicals (higher concentrations of the strong acids and bases, for example).

Right now, it looks like the CK01 kit is the better buy for us, and I can read the manual to see if the experiments align with the new AP chem curriculum.

Neither kit provides a heat source, so we’ll either use the gas stove or buy a small alcohol heater to work outside.  We also need to get gloves, and various household supplies (like distilled water, distilled vinegar, …).  Both kits have a list of needed supplies—the list for the CK01 kit is much longer, probably because it is designed for twice as many labs.

Based on this side-by-side comparison, I think I’m more likely to get the CK01 kit.  It seems to have more of the stuff I’d need and less of the stuff that I already have.  I also like the marketing better (full details of the kit and the lab manual free on the web site).

2013 August 12

Homeschooling chemistry this year?

Filed under: home school — gasstationwithoutpumps @ 21:31
Tags: , , ,

My son needs to take chemistry this year.  He doesn’t have much interest in the subject, but one of his top choices for college (Harvey Mudd) requires entering freshmen to have had chemistry, and it would be easier to take it during the school year than to scramble to try to fit it in next summer.

Originally, we had planned for him to take a community college chemistry course, so that they could deal with the lab safety instruction, lab equipment, and proper disposal of chemical waste.  The community college course is perhaps a little lower level than what would be optimal for him pedagogically, but it is low cost and reasonably convenient.  Unfortunately, as a high school student he is among the last to be allowed to register, and by the time he was allowed to register not only were all sections of the chem class he needed filled up, but all the wait lists were also.  He could have gotten onto the wait-list for a lower-level chem class, but that would have gone painfully slowly for him, even if a slot opened up to let him register. So community college was out for us.

We heard that the local high school will sometimes let home-school students into AP courses (if there is room), but they have AP chem scheduled only for the second half of the year (the busier half for him), and their compressed schedule would require him to put about 1/3 of his attention on chemistry—something he’s reluctant to do.  Given the high probability that they would decide that their AP class is full, trying to use the local high school course seems too risky.

I considered having my son take Chemistry at UCSC, but that would be rather pricey at 5331 (not counting books or lab fees) as they stretch general chem out to 3 7-unit courses. My son is also reluctant to committing to a 3–4-day a week commute up the hill for the entire school year. I’ve also heard from some of the brighter students that the class goes very, very slowly—possibly because they have to pass more than 1000 students a year, many of whom have even less interest in chemistry than my son. I looked a little at online courses, but none of them looked very promising. Most were barely high-school level chemistry and quite a few seemed to be associated with young-Earth Creationist religious groups, which I will not knowingly support. Even if there are some good ones out there, my son prefers books to videos, and most online courses are aimed at those who would rather watch than read. So, it looks like I will be homeschooling chemistry this year, like I home-schooled physics for the past 2 years. Unfortunately, I feel even less prepared for chemistry than I did for physics, so I’ll have to be learning general chemistry alongside my son. I have had only 4 chemistry classes in my life: a high-school chem class in 1968–69, a totally useless chemical thermodynamics without calculus class around 1975, a biochem class on replication, transcription, and translation before 2000, and a graduate protein structure class in 1998. None of these prepare me for teaching a general chem course, and only the high-school chem class 45 years ago had any wet-lab component. Increasing the challenge for me is that I’ll have very little time this year, with a higher-than-usual teaching load again this year. (I can’t really complain about it, since I volunteered to create a freshman seminar for the bioengineers WInter quarter—my chair even tried to talk me out of doing it as overload, though he recognized the need for the course.) Over the summer, I picked up a textbook for free: Zumdahl’s Principles of Chemistry. The chem department was discarding their TA copies of the old edition, since they are switching to a newer edition. (The book is one of many that suffers from publisher churn—getting out a new edition every three years just so that students have to pay full price, rather than getting used copies of the book.) I did a little checking on the web, and Principles of Chemistry seems to be a step up from Zumdahl’s Chemistry, which is commonly used for AP courses, which is in turn a step up from Zumdahl’s Introductory Chemistry, which is sometimes used for regular high-school chemistry. I’m not sure where World of Chemistry fits in the collection of books. I’m not sure I would have picked Principles of Chemistry as my first-choice text—it is almost 1100 pages not counting the Appendices and seems to written in a rather wordy style. The book is packed full of gee-whiz sidebars and pictures and bios of famous (and not so famous) chemists. I suspect that the actual content could be conveniently presented in a book a third the size. But it is hard to argue with “free”, and it is a commonly chosen text for 1st-year college courses. I still have to pick an order for covering the material, since the preface indicates that there are several chapters that can be presented in different orders: “the chapters on atomic theory and bonding (12–14), thermodynamics (9,10), and equilibrium (6–8) can be moved around quite easily. In addition, the kinetics chapter (15) can be covered at any time after bonding.” How I’m going to choose the best ordering for the material, when I don’t really know any of it, remains a mystery to me. Of more concern to me is setting up appropriate labs. I don’t think we really did enough labs in physics, and it was much easier for me to design physics labs and jury rig equipment than it will be for me to do the same in chemistry—so I worry about how we will do sufficient labs. I joined the AP Chemistry teachers’ mailing list and asked for help there. One teacher pointed me to Quality Science Labs, who make most of the chemistry kits that online courses use for their chem labs (they support Johns Hopkins CTY, Apex Learning, and ChemAdvantage, among others). Their web pages did not enumerate what was in their kits, so I sent them a query, and they suggested their Advanced Microchemistry kit, which they say has been updated to align with the new AP Chemistry curriculum. They also sent me a list of the contents of the kit and promised to put that list up on their web site. The kit does not seem to include the experiments that are suggested for the new curriculum [http://apcentral.collegeboard.com/apc/members/courses/teachers_corner/221821.html]: Investigation 1: What Is the Relationship Between the Concentration of a Solution and the Amount of Transmitted Light Through the Solution? Investigation 2: How Can Color Be Used to Determine the Mass Percent of Copper in Brass? Investigation 3: What Makes Hard Water Hard? Investigation 4: How Much Acid Is in Fruit Juices and Soft Drinks? Investigation 5: Sticky Question: How Do You Separate Molecules That Are Attracted to One Another? Investigation 6: What’s in That Bottle? Investigation 7: Using the Principle That Each Substance Has Unique Properties to Purify a Mixture: An Experiment Applying Green Chemistry to Purification Investigation 8: How Can We Determine the Actual Percentage of H2O2 in a Drugstore Bottle of Hydrogen Peroxide? Investigation 9: Can the Individual Components of Quick Ache Relief Be Used to Resolve Consumer Complaints? Investigation 10: How Long Will That Marble Statue Last? Investigation 11: What Is the Rate Law of the Fading of Crystal Violet Using Beer’s Law? Investigation 12: The Hand Warmer Design Challenge: Where Does the Heat Come From? Investigation 13: Can We Make the Colors of the Rainbow? An Application of Le Châtelier’s Principle Investigation 14: How Do the Structure and the Initial Concentration of an Acid and a Base Influence the pH of the Resultant Solution During a Titration? Investigation 15: To What Extent Do Common Household Products Have Buffering Activity? Investigation 16: The Preparation and Testing of an Effective Buffer: How Do Components Influence a Buffer’s pH and Capacity? I can’t get the PDF for the AP chem lab manual (you need a password for that, which in turn requires passing an AP audit—too much paperwork for one student), but I will order the student lab manual in hardcopy, from the College Board store, like anyone else can. College Board charges18 for the manual, $5 for shipping, and$2.01 for tax.  The tax amount is clearly wrong, since even here in California the sales tax is not (yet) 11.17%.

I think we may be able to do some of these labs: certainly I could hack together a phototransistor and an LED or laser diode to measure “amount of transmitted light”—that’s essential the same circuitry as the optical pulse monitor. Even the mechanical setup should be within my capabilities: drilling some holes through a block of wood to hold a test tube, the LED, and the phototransistor, like for my failed attempt at a pulse oximeter.

Instead  of the new AP labs, the microchem kit has 16 other labs [http://www.qualitysciencelabs.com/lab-manuals/advanced-microchem-manual/, available separately from the kit for $30]: • Lab 1 Gravimetric Analysis • Lab 2 Mole Ratios • Lab 3 Redox Titration • Lab 4 Electrochemistry: Galvanic Cells • Lab 5 Enthalpy of Fusion of Ice • Lab 6 Enthalpy of Reaction • Lab 7 Investigation Colorimetry: Light Path and Concentration • Lab 8 Types of Compounds • Lab 9 Paper Chromatography • Lab 10 Types of Chemical Reactions: Evidence for Chemical Changes • Lab 11 The Effects of Temperature and Particle Size • Lab 12 Analyzing Concentration vs. Time Data • Lab 13 Reversible Reactions • Lab 14 Solubility Equilibrium • Lab 15 Acid-Base Titration • Lab 16 A Buffer Solution Some of these look like they overlap with the suggested AP labs (Investigation 1 and Lab 7, or Investigation 14 and Lab 15, or Investigation 16 and Lab 16). I don’t know how “cookbook” the Advanced Microchem labs are—I need something that is straight-forward enough to be doable without a real chem teacher around, but not so routine as to be a pointless exercise. Designing labs like that is tough, and from what I’ve heard, most first-year chem labs in college don’t succeed at it (being very cookbook). Unfortunately, it does not look like Quality Science Labs sells the kit and the manual unbundled (you can get the manual without the kit, but not the kit without the manual), so I can’t really study the manual before buying the kit—at least not without wasting$30.

I’ll also need to buy some more safety equipment (another pair of splash goggles, probably some gloves).  I’ve been told that Flinn Scientific is a good source for high-school chem supplies (also Materials Safety Data Sheets and various teaching resources).

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