Indicators

Added: 2026-05-12

[00:00:07]Hello everybody. Welcome to my class again. This is CHM 2090 chemistry for biology. And today we're going to talk about our first lab experiment here.

[00:00:19]Okay. In the kitchen for chemist is about acid base buffers and indicators.

[00:00:26]While buffers is not a solution that makes you go buffer, but it's a bit more intricate. So, inside the lab here, you wear your safety goggles, you wear your gloves and personal protective equipment. Okay? Make sure you're safe, right? And then I'll be interacting with you. You see other demonstrators there.

[00:00:42]Hopefully, you're excited, engaged, ask questions.

[00:00:48]So, the first part of this experiment one is to measure pH. We talk about it in class. PH is the potent of H+ concentration with an indicator. Now why indicators? Like I mentioned in class here, these are weak acids that get protonated no or deprotonated and they absorb differently, right? When there are different forms here with with the H+ or without H+ there and because they absorb light differently, there's a change in the color.

[00:01:17]We don't go deep again because this is the formula that you work with for the past few sessions.

[00:01:24]Now the form here you want to see is in acidic part and the non acidic part here. The key difference is that when you have the non acidic part here, this right becomes a n single double bond here. Which means that now there's something called a conjugations in a pi system in chemistry which might be a bit too much for you to comprehend at this time. But what it's saying is when you are taking this form here the whole molecule is flat is planer and therefore it has different absorption of light here right this is a sp3 hybridized atom nitrogen that the whole part is no longer flat but it's like you don't see my two palms here it's like kind of like oblique okay whereas on the right hand side is flat so as usual we denote it using a right ah we acid partial show the associations with double hit arrow to form the conjugate base a minus and H+. Now here in this table I'm going to show you the list of the major and the common indicators right you find in the literature you've got mal violet you know bromephenol blue metal orange metal red limus bromal blue phopalin elizarin yellow right and these are the colors on the acidic side and the basic side and take note here when we denote the term the color on the acidic side and the basic side it does not mean below or above seven because each of them change right not at page seven but they have their own scale So you need to know right where would they change color it is like this point where they will switch color. So with reference to their own midpoint right you will know that what is more acidic to them what is a bit more basic to them. So what it's saying is right for meta violet okay if I go below 0.0 is going to be yellow.

[00:03:20]If I add a bit of base there above 1.6 six it'll be violet and then you ask me what about if I have 08 in the middle you mix yellow and violet what color do you get think about it okay so similarly for metal rate right if I have um pH is 500 right inside the zone the color you see will be a mix of red and yellow which is orange get it so phopalin if I have a pH of nine you'll pale pink. You mix pink with colors, it's a bit pale pink. Still a tinge of pink, but much less. This is like very solid pink. Okay, not hot pink, but just pink. Pink. Pink. So, you get the idea.

[00:04:07]Now, this is how it looks like in the lab here. Middle orange is red. This kind of red, not crimson red. And then for the yellow is that kind of yellow, not the bright gold yellow.

[00:04:18]And you see that we have the different color across the pH level. Okay, we took a photo for you for orange and this one of the uh indicators you'll see in the lab here. So I just want you to connect the formula structure with this color here. Okay.

[00:04:37]All right.

[00:04:40]Now you know that an indicator has different color at different pH. What it means is you can measure what's the pH.

[00:04:48]In the past when we didn't have the pH meter, this is what we observe here.

[00:04:53]Okay. So in this case the pKa of this acid is 3.5.

[00:04:59]Okay. Which means that at this pH 3.5 you have equal amount of these two the weak acid and conjugate base. We call this the maximum buffering capacity.

[00:05:12]Again new term later on.

[00:05:15]try to correspond the structure with the color. Again, please don't memorize, but you'll be given a sheet there. But I want you to concept to understand that you know when he has some form of deeproinations, the color change because now the molecule is no longer planer.

[00:05:31]Okay.

[00:05:34]So if I plot it using the y- axis of fraction and x axis is the pH there then you notice that how much of this acidic part that's protonated get dissociated.

[00:05:47]So therefore you see when you shift right the tinge of color changes right I talk about between red and yellow you have got orange but there's also like dark orange light orange because as you shift it to the basic side of this indicator it's going to be more yellow and less red.

[00:06:04]Okay so here's the UV absorption spectra to show you right that your lambda max is the absorption wavelength right of the different colors. So for yellow it absorb maximum at the 435 nanometers.

[00:06:20]For red color it is 520 nanometers. But what I'm saying is right for this this plot came from the individual plot there. So if I have a very acidic solution of this we acid there and I scan it through the UV spectra phototer.

[00:06:34]This is the graph I get. Okay. The one in in lime. Okay. And then it tells you that oh it is red color. You know if I cover my eyes and say what color is it?

[00:06:41]You tell me the measure frequency wrong.

[00:06:44]The measure wavelength is 520 mm. Okay, straight away I know it's red. Okay, I might not be able to see but you tell me I can translate from the wavelength to the color. So indeed, oh I see it's red color but the instrument tells me this number this red. So similarly when we scan this you know cuette right UV spectra phototer for the basic part of this weak acid we have a yellow solution. You get me?

[00:07:09]Okay, here are just some uh rearrangement of the formulas that you have learned from me in the past. Here, remember the Henderson Hazabach equation, the HH equation there, rearranging it. You cross multiply, you know, and then move it around. You have a fraction, you get that. Okay, it looks complex, but I would suggest you to just slowly work it out.

[00:07:30]Okay, remember we talk about P something is equal to negative log something.

[00:07:34]Okay, this is a constant, right? is only dependent on temperature. Okay.

[00:07:43]So because I mentioned you have a list of these um very common indicators there, right? Mal orange congo red photin and tamop blue. I want to give you a challenge. So there's no free lunch here. There's no giveaway. So for the first time in this course here last year it was the old one. Okay. When I come in I want to make sure you learn very well. We'll only give you one that has been done before. Okay. by your seniors right of blue and these are the color but for the rest of indicator we make it very new for you that means there's no record here whatsoever okay you can look it up maybe other places in the world they do it but not UCD chemistry okay so we want you to try for the first time in the lab we tested it is very safe but we want you to be safe so when you have no reference I think it's very brand new like a scientist right how do people get Nobel prize after years of you know going research to a to the topic they're so passionate about because there's no no um history just because they feel so strongly about it. So t blue has been done before. This is the color. You can check against your color but again just put it aside first if you want to. But the others all crystal talin burmo phenol blue nitrousin yellow burl crystal green. Now try it.

[00:09:03]Pick the color. Take a photo right.

[00:09:05]Record it like a scientist, like a pro.

[00:09:09]Part two of this experiment is to measure using the pH meter. Remember just now we used the color to measure indicator. Now we have some scientific instrument that can give you exact number. Okay, which is a sensor to measure H+ ions there. Cool. So this has a electro this is more of electrochemistry is about um the electro conductivity there. Okay. This is sensitive only to H+. Why you say why not hydroxide? Can't we use measure hydroxide and subtract it? But remember hydroxide is soap. soap you know when you don't um dry your hair you know when you still have some shampoo on your on your hair what happen stick together right that's why we need to have a conditioner so what it means is right for hydroxide you tend to lump things together but as it's a bit better they don't really stick so much okay you don't want to clock the whole sensor the electro it becomes ineffective there okay so this how it looks like you know in a schematic diagram here we also try to measure the acidity of coat here you know slightly acidic Not so good for a tea if you don't drink too much. Okay.

[00:10:12]And I have a little trick for you how to use a pH meter using like Google Glass.

[00:10:17]Have a look there. I think it's very interesting. So they have a concept of oh um you have different u models, different uh brands of the pH media but why we to do calibration using different uh buffer solutions different pH you know three seven and nine you know. Yeah support me. Okay support.

[00:10:37]So you look at this equation there you're not um surprised because you've seen it in blood member the natural buffer in our body in blood carbonic acid okay hydrogen carbonate PK 6.4 for there right so our body pH is tightly controlled because if not our regulatory system uh will be affected we affect our metabolism we get sick we go to hospital okay so just like this as mentioned in class when we saw each other we have another kinds of uh buffer right the phosphate you know you have phosphoric acid and then dissociate you know bit by bit triproic acid you have got hydrogen phosphate and this is how it looks like here. Okay.

[00:11:21]So you see that here you have a deeprotonation right one H+ drop off and then you have this uh we call resonance okay so become flat just like here your dihydrogen phosphate right losing one H+ here then this whole part resonate okay three part and then you get more flat which means you you kind of modify the structure and then modify the property okay now here why this important because right you to do some comparison and simulating the effect of muscular exercise. Here we introduce something called lactic acid.

[00:11:57]Latic acid is where when you store in your muscle for too long you get cramp.

[00:12:01]So when you see a oh I'm cramm I didn't stretch right or you working too much here without oxygen there you get cramp.

[00:12:08]So cramp could be because you have lack of oxygen and then you know you can't compress here. What you do is stretch right? We have current stretch because we want to allow blood with oxygen there you know to have this reaction to relax the lattic acid.

[00:12:22]So we want to simulate this okay not giving exercise but in a chemical way.

[00:12:29]Lactic acid structure is this right?

[00:12:31]When it's deepated you get that too much acid you get cramp.

[00:12:36]Okay remember this from just now.

[00:12:42]Okay.

[00:12:45]So this is more of the biochemistry of biology which I do not go too much but I'm just saying that when you are you don't get much cramp if you stretch your muscle when you are breathing very normally when you are having loss of oxygen you get cramp when you are lacking oxygen you are over using your muscles per unit time and then the blood is not going to the piece of muscle and it's a mechanism in your body to say stop because you know you cannot function right stop. So remember our body will react in a way to help us recover not to overexert it. Okay. So the ATP but I'm not going through is not related to at least the chemistry part of the class but you should know it about the glucose you know under go the causes the ATP exchange the energy unit in our body there. Okay here I want to just show it as a y axis versus x axis there when you have oxygen. Okay, it's a ar aerobic oxidation exercise. How much right of the lactic acid versus the latate which is the deprotonated form of it is there in the body. Cool. Okay. So and we have this by having a human right a sportsman you know doing a cycle exercising we measure with a different amount of oxygen in the mouse there and then we track the whole whole body system there and I do that because you know I serve in the army and you know when you're over the certain age you need to check make sure your heart is good you don't overexert your body yeah so I do have some muscles too okay right so now here's a test for yourself there what molecules are called acids. How do you define them? What molecules are called bases? The definitions. What is the pKa of a molecule? Can you tell me and explain a solution of pH indicators change the color? Why? You know, with different pH, how do buffers work? Okay. What property you want to look at?

[00:14:49]Will the temperature affect the pH of the buffer? How much HCl you need to add to this concentration of the acid so that the pH will be raised.

[00:15:01]Clue Henderson has a back equation.

[00:15:04]Okay. And for now stay safe and all the best and see you in that P. Cheers.

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