Lab Equipment: pH measurement with paper and meters [Part 1]

Greetings fellow nerds. When doing chemical reactions, sometimes the acidity or the basicity is important. This is usually defined as pH and measuring it can be very useful for getting a reaction right. Now there are a few ways to measure pH. The cheapest, simplest and most reliable is pH paper. On the left here I got blue litmus paper, which if dipped into an acidic solution will turn pink or red. Now I don’t have it here, but there is also a red litmus paper that turns blue if dipped into basic solutions. Litmus paper is good for simple acidic or basic measurements. But for a more precise determination we have this more advanced pH paper on the right. It consists of multiple strips of pH indicators that have different transition points. Just apply a small drop of the solution you want to measure to the strip. Dipping a strip is actually not the best way to use pH paper because you might contaminate your sample with the indicators used in the paper. Anyway, wait for the color change, and then compare the color to a reference chart to find the pH. For things like fieldwork and quick measurements, this is great. For even greater precision, an expensive but high-tech solution is to use a pH meter. I got this expensive laboratory model, but you can get cheaper ones off eBay for about forty dollars. It’s not as good as lab grade stuff but it’s still very useable. Another company that sells a broad range of pH meters at various price points is omega engineering. I put the link in the video description. Now let me show you its components. This blue cylinder is the pH meter which houses all the electronics to measure pH. And this plastic tube mounts the glass electrode. It’s currently in a potassium chloride storage solution. Electrodes must be stored in a 4 molar solution of potassium chloride to keep the probe surface hydrated and active. Do not let them try out, and do not store them in distilled water, as this leaches out the ions in the glass that make it work. Now let me show you the actual glass electrode. And here it is, this glass bulb is what measures pH. If you look inside there is a metal wire, this wire is the inner reference electrode that measures the potential across the glass. Now to measure a voltage you need two electrodes so if I can find it… here… there it is. Can you see that little nub there? That is the outer reference electrode. This electrode works with the inner reference electrode to measure the potential across the glass and determine the pH. For the glass electrode to work you must immerse the electrode all the way up to this reference electrode. Otherwise you’ll get erratic and irreproducible readings if you don’t. Now I’m going to dip the electrode into some distilled water to wash off the storage solution. So let’s start taking measurements. But before we can do that we have to calibrate the electrode. To do a calibration you need to purchase these calibration buffer solutions. Buffer solutions are carefully prepared samples that have an exactly known pH. They are very cheap and a must have for any chemist serious about the quality of their pH measurements. You can buy them from omega engineering as well. So here is sample of the calibration solution. This particular one has a pH of 7.01 at 25 Celsius, which is the current temperature of my lab. I’m putting it in this beaker ’cause the vial can’t hold the electrode by itself. Now I’m dabbing off the excess water. Be careful not to touch the glass bulb itself, you don’t want to contaminate or scratch the bulb. Now simply insert the electrode into the calibration solution and give it a few minutes to equilibrate. As you can see the electrode has drifted quite a bit since it’s last use so we need to reset it. On an expensive meter we just press the right commands. On these smaller ones there is usually an adjustment screw or similar that we can use. Let me adjust this one here… There we go we’ve now calibrated this pH point to its proper reading, but what about other points? Let me get a different pH calibration buffer solution and see if the measurement is correct. This solution has a pH of 4.01. As you can see here the pH meter says it’s off. The slope of the pH response is not correct. On a more advanced instrument we could calibrate this new point and the onboard computer would automatically readjust the slope. But for this cheaper instrument we don’t have that kind of ability. If we readjusted this point we would misadjust the previous point. Now this error is acceptable for many experiments as the change in pH is more important, but in part two of my video on pH measuring tools, I’ll show you how to use a calibration curve to compensate for this error. So please subscribe, rate and comment.

90 thoughts on “Lab Equipment: pH measurement with paper and meters [Part 1]

  1. Thank you! I couldent rember for the longest time what color was what on my red and blue litmus paper. Keep making great videos! Alot of us are learning from them. Thanks again -SuperRichard37

  2. dip the Blue paper NOT into the base
    Thats how my chemistry I teacher taught us. you think its B – B but its not

  3. Knowing the pH of foods we eat is good. Some say having an acidic body is bad for health unless you're trying to eliminate germs.

  4. @wowggscrub yeah…. but there are a lot of factors you have to consider like ionic strength, dissolved gases, purity of chemicals, stability, ion activity and a host of other issues in order to maintain a predictable pH. It can be done, but not worth it.

    Also, how do you tell if there's a mistake? use the mis-adjusted pH meter? 🙂

    Most companies that sell pH meters also sell the solutions, so if you're willing to go that far to get a meter, it's not much harder to get the solution.

  5. @brainanator we have to slog through this in order to get to some more interesting stuff.

    (psst… we're going to make ammonium nitrate)

  6. the glass electrodes meters are pretty accurate once adjusted to the 7.0 buffer solution. the reason they get off in between test is the distilled water. no matter how much you try you cant get all the water off and this throws the pH up for acid solutions because it dilutes them and the down for base solution. I have found if you test with a second vial after the first one washes the water off it is more accurate.

  7. Hmm, I guess my litmus paper is weird. It's halfway between the red and blue litmus paper you mentioned.
    Your talk on calibrating the meters reminded me of one meter that was WAY of in its calculations.

  8. @C6R1S Adjusting to a particular buffer like 7 should be done with some knowledge of whatever chemistry you're doing, if you're working near 7 in your experiment then go for it. But if you're working near 4 then get a pH 4 buffer and calibrate to that. Likewise for 10. This is to minimize any slope or proportional errors in the electrode or the meter.

    Picking a calibration point arbitrarily is not a good idea.

  9. this is the best video because just today we started learning about this units
    pH measurement and watching this video made this unit just a whole lot easier 😀
    thank you nerd rage <3

  10. @alexdenton02 thanks!

    i do agree, this is one of my more epically boring videos, but we have to slog through this to get to some better stuff. Driving fast cars might be fun, be we gotta build the car first.

    Thanks for understanding 🙂

  11. Im surprised you didnt go into what buffer solutions are, the whol acid and conjugate base or base with conjugate acid. Is the reason you dont get buffers at ph 7 due to this?or can you get them and i havent seen them.

  12. @dudomaniac I guess it was some cheap buffer solution i was using then, plus I mainly use it to test the pH of water for my plants ;-P

  13. You can't say you don't have red litmus paper, You just made some by dipping the blue paper in the acid,
    I think it would be good if you where to show that effect in upcoming vids

  14. @iwan0t0smith technically you're right, but practically it's not the best way of going about it. The acid used was quite a bit and a lot of it sticks to the paper. If then i dip the paper into something only weakly basic it wouldn't change color because the acid sticking the paper would overpower it. Red litmus paper is best made by using *just enough* acid to convert it and nothing more. So it would be sensitive to weakly basic solutions.

    I didn't have such paper. thus the line in the video.

  15. That porous plug is called the reference junction, that creates an electrical connection, but only a slow diffusion connection; the reference electrode is on the inside; usually Ag wire/ satd. AgCl, or Ag/AgCl in satd. KCl ([Cl] affects [Ag] through Ksp common ion). Hg/Hg2Cl2 used to be the best reference electrode, but Hg sucks for safety/health. Hence Ag, but Ag is is affected by light. I would prefer Cu complexer precips to Ag and Hg, don't know why they are not used. Cu is still noble enough

  16. @C6R1S if it's for plants then go for it. Plants are more forgiving of pH differences (to a point though…) than some chemical reactions.

    I'm just being anal because I used to do analytical chemistry, maximizing accuracy and precision was an epic quest in itself.

    But yeah for plants, go for it, around pH 7 is alright. But still use good buffer solutions. 🙂

  17. @Sillybillydilly You still need to immerse the electrode up to that part in order to obtain an accurate and stable pH reading.

    Anyway, you are right that exposed part is the plug but it is still part of whole reference electrode assembly, and you need the whole thing behind it in order for it to work.

    When you point to a car you usually only see the car body and not the engine, but the whole thing is needed and it isn't wrong to call what you're pointing the car, rather than the car body.

  18. @Sillybillydilly As for the construction of the reference electrode, you're absolutely right but i felt it would clutter the video too much. I left it out since the design didn't really change how to use it.
    As for mercury, i think that's no longer used, i could be wrong through, i think silver is still a major component of most references although i'm unsure.

  19. @NurdRage True but I still think id would be an interesting demonstration of how litmus paper works.

    I really like your vids btw

  20. @jbohbot1



    Besides, it wouldn't work anyway, the excessive voltage will likely cause the plates to disintegrate and fill your acid with particles of lead and lead dioxide.

  21. @martha2ful technically yes….. but safety and liability would be an issue, any substance that powerful would also dissolve human flesh very effectively. Even a small splash out when you drop something would be a horrific.

    I recommend just a mulcher or similar mechanical approach to grind the matter into a paste and then flush it or compost it.

  22. @ZyGaming While the approach of a chemistry teacher depends on the teacher, the subject itself is 99% boring with only 1% cool stuff. I might look cool on youtube because i show mostly the 1% cool stuff. But to get to that 1%, you have to first slog through the 99%

    Is your teacher *actually* boring? or is the stuff they're teaching boring? 😉

  23. It is a coincidence that I had completed an experiment which involved an pH meter on Friday in the same week you uploaded this video about pH measurements. This is also my first time to use a pH meter.
    The objective of the experiment was to measure the dissociation constant of acetic acid and formic acid by preparing buffer solutions of the acids and their corresponding salt of same concentration.
    Thank you for providing background information of pH meters.

  24. How much of each material do you need? In class My group is doing this project and we would like to know how much of alcohol, K-Permaganate, and S-acid is needed for the reaction to occur. Thanks!

  25. lol i am doind this cauase i have end of year exam tomorrow and i am revising cause the exam decides my GCSE groups i wanna get top group again i have been top twise. wish me luck

  26. @NurdRage
    I came here from your fluorescence thermochromism video (which was totally awesome btw) cause you said it would probably bore us out of our minds…in fact I find this most informative…i'm not sure what I would actually put this into practice with but still…none the less…very informative…I have measured PH before but only for watering specific plants and stuff that prefer certain PH levels…haha…

  27. hey a joke :
    how to defferentiate acid pH from alkalic ones
    drink it.
    if tastes sour, that's its acid, if tastes bitter : alakalic
    put your finger in it
    burns : low pH
    sticky : high pH

    liabilities : death, scar, and a trip to hospital

  28. Hello.
    How is yellow pH paper used? It comes with a colour scale in which yellow is about 5.5 Of course if I dip it in distilled water it does not change, but according to the colour scale it is indicating 5.5 and not 7 which is green.
    Thanks for any help you can provide.

  29. The way they teach is boring. Full of theoretical stuff we'll never use.
    I kinda want to learn something practical and useful in real life – like making plastics or distillation and syuff 🙂

  30. in our chemistry classes the pH meter probes (glass electrode) were just stored in air and rinsed with tap water after use

  31. I like the pH liquid better. It is easier to use. It's been a long time that I have studied chemistry.

  32. Will adding super glue or hairspray to shampoo change it's PH, if I check the shampoo when I first get it, can I tell of someone has tampered with it. I will have to use PH strips.

  33. Are you sure you were rinsing your probe in DI? It showed a pH of ~8.3. Even after calibrating it in the pH 7 buffer, you put it back in the DI and it hops back up to ~7.7. I would expect those numbers for tap water with all its ions, which is a bad idea for maintaining the accuracy of your probe. If it was DI, the pH should be much lower than 7 on account of unbuffered CO2 gas exchange, as well as mixing with the residual storage solution from the bulb. If it was definitely DI, you might want to check your DI system.

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