Acidity and basicity, proton concentration, the pH scale, and buffers.
Even if you’ve never set foot in a chemistry lab, chances are you know a thing or two about acids and bases. For instance, have you drunk orange juice or cola? If so, you know some common acidic solutions. And if you’ve ever used baking soda, or even egg whites, in your cooking, then you’re familiar with some bases as well.
You may have noticed that acidic things tend to taste sour, or that some basic things, like soap or bleach, tend to be slippery. But what does it actually mean for something to be acidic or basic? To give you the short answer:
- An acidic solution has a high concentration of hydrogen ions (H), greater than that of pure water.
- A basic solution has a low H concentration, less than that of pure water.
To see where this definition comes from, let’s look at the acid-base properties of water itself.
Autoionization of water
Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules. This process is called the autoionization of water:
The letters in parentheses just mean that the water is liquid (l), and that the ions are in aqueous (water-based) solution (aq).
As shown in the equation, dissociation makes equal numbers of hydrogen (H) ions and hydroxide (OH) ions. While the hydroxide ions can float around in solution as hydroxide ions, the hydrogen ions are transferred directly to a neighboring water molecule to form hydronium ions (HO). So, there aren't really H ions floating around freely in water. However, scientists still refer to hydrogen ions and their concentration as if they were free-floating, not in hydronium form – this is just a shorthand we use by convention.
So, how many water molecules in a pitcher of water will actually dissociate? The concentration of hydrogen ions produced by dissociation in pure water is 1 × 10 M (moles per liter of water).
Is that a lot or a little? Although the number of hydrogen ions in a liter of pure water is large on the scale of what we usually think about (in the quadrillions), the number of total water molecules in a liter – dissociated and undissociated – is about 33,460,000,000,000,000,000,000,000. (Now there's something to think about with your next glass of water!) So, autoionized water molecules are a very tiny fraction of the total molecules in any volume of pure water.
Acids and bases
Solutions are classified as acidic or basic based on their hydrogen ion concentration relative to pure water. Acidic solutions have a higher H concentration than water (greater than 1 × 10 M), while basic (alkaline) solutions have a lower H concentration (less than 1 × 10 M). Typically, the hydrogen ion concentration of a solution is expressed in terms of pH. pH is calculated as the negative log of a solution’s hydrogen ion concentration:
The square brackets around the H just mean that we are referring to its concentration. If you plug the hydrogen ion concentration of water (1 × 10 M) into this equation, you’ll get a value of 7.0, also known as neutral pH. In the human body, both blood and the cytosol (watery goo) inside of cells have pH values close to neutral.
H concentration shifts away from neutral when an acid or base is added to an aqueous (water-based) solution. For our purposes, an acid is a substance that increases the concentration of hydrogen ions (H) in a solution, usually by donating one of its hydrogen atoms through dissociation. A base, in contrast, raises pH by providing hydroxide (OH) or another ion or molecule that scoops up hydrogen ions and removes them from solution. (This is a simplified definition of acids and bases that works well for thinking about biology. You may want to visit the chemistry section to see other acid-base definitions.)
The stronger the acid, the more readily it dissociates to generate H. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions when it is placed in water, so it is considered a strong acid. The acids in tomato juice or vinegar, on the other hand, do not completely dissociate in water and are considered weak acids. Similarly, strong bases like sodium hydroxide (NaOH) completely dissociate in water, releasing hydroxide ions (or other types of basic ions) that can absorb H.
The pH scale
The pH scale is used to rank solutions in terms of acidity or basicity (alkalinity). Since the scale is based on pH values, it is logarithmic, meaning that a change of 1 pH unit corresponds to a ten-fold change in H ion concentration. The pH scale is often said to range from 0 to 14, and most solutions do fall within this range, although it’s possible to get a pH below 0 or above 14. Anything below 7.0 is acidic, and anything above 7.0 is alkaline, or basic.
pH scale, ranging from 0 (very acidic) to 14 (very basic/alkaline) and listing the pH values of common substances.
The pH inside human cells (6.8) and the pH of blood (7.4) are both very close to neutral. Extreme pH values, either above or below 7.0, are usually considered unfavorable for life. However, the environment inside your stomach is highly acidic, with a pH of 1 to 2. How does the stomach get around this problem? The answer: disposable cells! Stomach cells, particularly those that come in direct contact with stomach acid and food, are constantly dying and being replaced by new ones. In fact, the lining of the human stomach is completely replaced about every seven to ten days.
Most organisms, including humans, need to maintain pH within a fairly narrow range in order to survive. For instance, human blood needs to keep its pH right around 7.4, and avoid shifting significantly higher or lower – even if acidic or basic substances enter or leave the bloodstream.
Buffers, solutions that can resist changes in pH, are key to maintaining stable H ion concentrations in biological systems. When there are too many H ions, a buffer will absorb some of them, bringing pH back up; and when there are too few, a buffer will donate some of its own H ions to reduce the pH. Buffers typically consist of an acid-base pair, with the acid and base differing by the presence or absence of a proton (a conjugate acid-base pair).
For instance, one of the buffers that maintain the pH of human blood involves carbonic acid (HCO) and its conjugate base, the bicarbonate ion (HCO). Carbonic acid is formed when carbon dioxide enters the bloodstream and combines with water, and it is the main form in which carbon dioxide travels in the blood between the muscles (where it’s generated) and the lungs (where it’s converted back into water and CO, which is released as a waste product).
H+ + HCO3- <--> H2CO3
If too many H ions build up, the equation above will be pushed to the right, and bicarbonate ions will absorb the H to form carbonic acid. Similarly, if H concentrations drop too low, the equation will be pulled the left and carbonic acid will turn into bicarbonate, donating H ions to the solution. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.
Want to join the conversation?
- so how does having more H+ per molar make a liquid able to burn a hole in a wall?(26 votes)
- The H+ are super reactive. The more of these you have in solution, the more reactive your solution will be. If it is highly acidic, then the H+ ions may react with the paint on the wall and otherwise disintegrate materials that make up the wall too.(37 votes)
- why does the human blood need to be at a certain Ph(2 votes)
- Enzymes in the body are very specific, so they can only operate at certain pH and temperature. If the pH levels go up or down too much in the body, the enzymes will start to denature (unfold), and will no longer work properly, which would have disastrous effects on the human body.(73 votes)
- In our class we "made yogurt", the milk's pH was 6.The final pH with the yogurt was 4. What happened to the pH of the milk as it turned into yogurt, and why did this change occur?(13 votes)
- Bacteria digest the sugars in the milk, producing lactic acid as a waste product. This is one reason yogurt tastes tangy.(25 votes)
- Why does cytoplasm have a lower pH compared to blood?(6 votes)
- Because the cells maintain an ionic gradient between the inside and the outside of the cell as part of how it uses energy from different molecules to do work. Organic molecules such as proteins help to maintain this gradient. Some are too large to go diffuse through the membrane and hold a charge (like an ion), and others actively pump ions one or both ways to maintain the gradient.(14 votes)
- Can you explain this another way, I have not been able to fully understand how it works out. Thanks.
"This number may seem a little arbitrary, but it has the handy property of causing the mass of one mole of an element in grams to equal the mass of one molecule of that element in atomic mass units."(7 votes)
- In the video "Definition of pH," (https://www.khanacademy.org/science/biology/water-acids-and-bases/acids-bases-and-ph/v/introduction-to-definition-of-ph) Sal draws a diagram of the logarithmic scale number line with bleach at 12. The image of the same type of number line (https://ka-perseus-images.s3.amazonaws.com/7ca9aecccf7e9d5caaf1ea10d2835c81f4036708.png) has bleach at 13. So does bleach have a pH of 12 or 13?(4 votes)
- I know that, say, 7.0 is 10 times more acidic than 8.0 (although it isn't acidic). If that's the case, then how much more acidic is, say, 8.0 than 8.2? Thanks!(2 votes)
- Since pH is based on base-10 logs (log₁₀) you just need to do the inverse function.
That means 10⁰⋅² or approximately 1.58 times more acidic.
If this seems mysterious, you might want to review the material on exponentials and logs — for example:
Does that help?(5 votes)
- how can a buffer "know" when there are too many of too few H+?(1 vote)
- It is all probabilistic. If there are many H+ ions, there is a greater chance that they will bump into HCO3- ions, forming greater quantities of H2CO3.
If there is a "shortage" of H+ ions, then there will be a smaller quantity of H2CO3 being formed, and since the reaction works both ways, H2CO3 will decompose into H+ and HCO3- more often(4 votes)