this video mini lecture continues our coverage of chapter 2 chemistry comes alive in this video we will finish section 6 looking into ph and buffers we'll continue the discussions of acids and bases by examining ph and buffers in order to talk about buffers we'll need to first differentiate between strong and weak acids and bases though now at the conclusion of this video you should be able to define ph and differentiate between acidity and alkalinity and describe the carbonic acid bicarbonate buffer system you will also be able to successfully explain the concept of ph for the semester student learning objective but describing the structure and function of each class of organic compounds is going to spread out over four more videos but let's get started with the concept of ph let's suppose you make two solutions of salt water using sodium chloride in the first you dissolve five grams of salt in one liter of water and in the second you dissolve a hundred grams of salt in one liter of water which solution is saltier and how do you know well the second one of course we could measure the salinity and we could tell that it has 20 times as much salt in it we can also taste it and be an option some of our taste buds detect sodium ions and we could definitely tell the difference between that much concentration but suppose we have two different acidic solutions that were made by someone else how could we determine which one has more acid in it we'd have to measure something but what well let's recall what acids do when added to solution they donate protons so the more acid in a solution the more protons it has and therefore if we can measure the concentration of protons in the two solutions we could tell which one is more acidic well bust out the champagne because we can measure that we determine the acidity of a solution using a scale called the ph scale it is a measure of the free hydrogen ion concentration of a solution notice the brackets here that's the symbol for concentration the greater the hydrogen ion or proton concentration the more acidic it is now the ph scale is an inverse logarithmic scale of the hydrogen ion concentration which means two things first the lower the ph the higher the hydrogen ion concentration therefore the lower the ph the more acidic it is and for each whole number change in ph the hydrogen ion concentration changes by 10 times so in a solution with a ph of 6 we have 100 times as many protons in the solution as we do a solution with a ph of 8 and therefore a ph of 6 is 100 times more acidic than a ph of 8. now as you can see here the scale runs from 0 to 14. in pure water some of the water molecules will ionize into hydrogen ions and hydroxyl ions and pure water has equal concentrations of the two ions so on the ph scale this is measured as a ph of 7.0 and is considered neutral neither acidic nor basic or alkaline if an acid a proton donor is added to a solution the free proton concentration increases giving it a lower ph and the hydroxyl ion concentration actually decreases meanwhile if a base a proton acceptor is added to a solution the free hydrogen ion concentration decreases giving it a higher ph a solution with a ph under 7 is considered acidic while a solution with a ph over 7 is considered alkaline or basic some common acids shown here include hydrochloric acid and lemon juice we also have some physiological examples of acids like gastric juice stomach secretion stomach acid not listed here we also have vaginal secretions sweat and some intracellular compartments for instance some common bases shown here include bleach ammonia lye and sodium hydroxide we also have examples of physiological bases including pancreatic secretions bile seminal secretions and blood notice here we show a blood ph of 7.4 which is slightly basic that set point is actually a range though and it ranges from 7.35 to 7.45 now as we'll see in more detail next semester it is essential for our blood to stay in that range a blood ph below 6.8 or above 8.0 is fatal now how we maintain our blood ph in this narrow range is through the use of buffers so what is a buffer well now that we've got a grasp on ph and how it measures acidity and alkalinity we need to see what buffers are but first we need to get one more concept in the difference between strong and weak acids and bases here are a list of acids and bases from before some of these are terms strong and some are termed weak the difference between them is based on how they disassociate in solution when a strong acid or base is added to a solution it disassociates completely for example hydrochloric acid ionizes completely into protons and chloride ions and there are no hydrochloric acid molecules left in solution likewise sodium hydroxide ionizes completely into the hydroxide ions and sodium ions and there are no sodium hydroxide ions or molecules left in the solution everything is disassociated into the ions but weak acids and bases on the other hand only partially disassociate into their constituent ions for instance carbonic acid here ionizes into protons and a bicarbonate ion but not completely and so some intact carbonic acid molecules remain in the solution now because acids donate protons and bases except protons they act somewhat to cancel each other out in what's termed a neutralization reaction and there's an interesting consequence of this along with the complete ionization for strong acids and bases if we add equal amounts of a strong acid and a strong base they neutralize into water and assault so for instance here we have hydrochloric acid that's going to disassociate into a hydrogen ion and a chloride ion and sodium hydroxide that disassociates into hydroxide ion and sodium ions and if these are present in equal concentrations then the hydroxide ions will accept the proton from the hydrogen ions here forming water molecules and the salt and the chloride ions form salt now of course the sodium chloride is broken up in solution as sodium ions and chloride ions but the end result is still salt water all right finally we're ready to talk about buffers so to the heart of the matter let's revisit carbonic acid which was one of the weak acids because it's a weak acid it does not disassociate completely which is why we wrote it like this we have our carbonic acid molecule it ionizes into protons and carbonic carbonate bicarbonate ions but some of the carbonic acid molecules remain because strong acids and bases disassociate completely the ionization in solution is not reversible but this partial disassociation of weak acids and bases makes these reactions reversible so let's write the reaction like this now if nothing else happens the rates of these two reactions will reach an equilibrium point remember that when we're at equilibrium the concentrations of the chemicals do not change for every carbonic acid molecule that disassociates into a proton and a bicarbonate ion another proton and bicarbonate ion combine to form a new carbonic acid molecule keeping the concentrations of all three of these the same the other thing to remember is that since one of those chemicals is protons we have a ph value for the solution but as long as the concentration of proton stays the same which it will at equilibrium the ph also doesn't change but what happens if we add an acid like hydrochloric acid well if we add hydrochloric acid to a solution it disassociates into a proton and a chloride ion and as the free proton concentration increases the solution becomes more acidic and so the ph drops quite a bit but what happens if we add hydrochloric acid to a buffer like our carbonic acid solution here first remember that this is two reactions a forward reaction and a reverse reaction which is which the forward reaction is the decomposition reaction breaking down carbonic acid into the proton and bicarbonate ions the reverse reaction is a synthesis reaction which forms carbonic acid from protons and bicarbonate ions if we add hydrochloric acid to this the hydrochloric acid is going to disassociate into protons and chloride ions just like it would any other time however in this case the carbonic acid reaction is occurring as well and the protons are reactants for the reverse reaction to understand why this is significant remember that the concentration of reactants is a major factor determining the rate of chemical reactions so if we increase the concentration of one of the reactants in this case the protons the rate of the reverse reaction increases but the rate of the forward reaction stays the same so many of the protons from the added hydrochloric acid end up reacting with available bicarbonate ions to form new carbonic acid molecules and because only free protons contribute to the ph these protons that get bound up into these new bicarb carbonic acid molecules can't affect the ph so by absorbing many but not all of the hydrogen ions that were added when we added the hydrochloric acid the ph of this solution decreases but only a little bit if we were to add sodium hydroxide to a carbonic acid solution instead of hydrochloric acid we'd have a similar situation but in reverse remember that pro bases are proton acceptors and so it's going to pull free hydrogen ions out of solution that will increase the ph but again because hydrogen ions are reactants for the reverse reaction removing some of them is going to reduce the rate of the reverse reaction decreasing the concentration slows down the reverse reaction but leaves the forward reaction rate unchanged so carbonic acid molecules are going to disassociate into hydrogen ions and bicarbonate ions at their normal rate replacing most of the hydrogen ions that were taken out of the solution by the addition of the sodium hydroxide and its hydroxide ions because of this the ph is going to increase but it's not going to increase by nearly as much as if we didn't have this reaction occurring and this is the nature of buffers they are solutions made from weak acids and bases and are reversible reactions in equilibrium they resist large changes in ph from the addition of acids or bases by either absorbing added protons from an acid or releasing protons to replace ones lost by the addition of a base this particular buffer system is called the carbonic acid bicarbonate buffer system it won't come up as part of our physiological mechanisms this semester but it is very important next semester and so it will be on every exam it's a major player in maintaining the ph of our blood in its narrow range of 7.35 to 7.45 and it is also an essential part of our blood's ability to carry carbon dioxide from body cells to the lungs for excretion in this video we learned about the ph scale and how it measures acidity and alkalinity in a solution we learned about strong and weak acids and bases and how weak acids and bases can be used as buffers we also got our first taste of the carbonic acid bicarbonate buffer system which will be featured heavily next semester in discussing the respiratory and urinary systems if you have any questions over the content in this video please do not hesitate to post them in the q a discussion thread or to ask during class