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If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.  Figure 1. Various substances arranged by their pH.[1] The term pH is an abbreviation for power of Hydrogen, which is a measure of how acidic or basic a chemical solution is. Most solutions are either acidic or basic (also called "alkaline") - a substance that is neither acidic nor a basic is "neutral".[2] Acids and bases are very important chemically, and are found almost everywhere. One example in the context of energy is acid rain, which is formed when water interacts with various pollutants in the atmosphere. Pollutants dissolved in the rain water cause it to be acidic, which can cause damage to the environment and man-made structures. Read more about acid rain here. A pH value is used to describe a water-based solution. In general, a small pH value describes a solution that is acidic, and a larger pH value describing solutions that are less acidic (more basic). The pH scale is centered on 7 - meaning that a solution with a pH of 7 is perfectly neutral (neither acidic nor basic). Some examples of common substances and their place on the pH scale is shown in Figure 1. The pH value of a solution directly measures the concentration of hydrogen ions (H+) in the solution. Acids have a large H+ concentration, which translates to a small pH value. Bases have very little H+. Basic solutions have large pH values - greater than pH 7. While pH values have been measured as low as -3.6[3] and as high as 17.6 [4], most solutions encountered in nature range between pH 0 to pH 14. pH is a logarithmic scale. This means that for each one-digit change in pH, the acidity (H+ concentration) changes by 10 times. For example, a solution with a pH of 4 has 10 times more H+ than a solution with a pH of 5. A solution of pH 3 will have 100 times more H+ (it is 100 times more acidic) than a solution of pH 5. Hydroxide and Hydrogen IonsSince pH is measured in water-based solutions, there is always a balance between H+ (hydrogen ions) and OH– (hydroxide ions). This is because water can break down into these two ions: [math]H_{2}O \rightarrow H^{+} + OH^{-} [/math]At room temperature, the balance between the H+ and OH– concentrations in water are related by this expression ([H+] means "concentration of H+"): [math] 10^{-14} = [H^+] \times [OH^-] [/math]This equation shows that the more hydrogen ions a solution has, the fewer hydroxide ions it must have. So: an acidic solution that has a large amount of H+ will have very little OH–. And a basic solution that has very little H+ will have more OH–. A solution that is exactly neutral will have the same amount of H+ and OH–. Calculating pHTo calculate pH, you should know the concentration of hydrogen ions (H+) in your solution. From there, the pH can be found by: [math] pH = -\log_{10}([H^{+}]) [/math]Since this is a logarithmic calculation, in order to change the pH by one unit (say, from 2 to 3), the concentration of H+ must be changed by 10 times (say, from 0.01 mol/L to 0.001 mol/L). Discover MoreThe PhET simulation below was graciously provided by the University of Colorado. This simulation can help explore how pH changes as liquids mix. For Further Reading
To explore pH further please see the Chemistry LibreTexts page. References
pH scale ranges from 0 - 14. It measures the acidity or basicity of a solution. A measurement of 7 is neutral, less than 7 is acidic and a basic is higher than 7. pH is measured based on the concentration of hydroxide and hydrogen (or hydronium) ions in solution. A solution containing hydrogen (H+ ions) or Hydronium ions (H3O - which is made from the reaction of hydrogen ions with water) indicate a pH lower than 7 or an acidic solution. A solution containing hydroxide ions (OH-) creates a basic solution with a pH greater than 7. It is possible to measure a pH that is greater than the bounds of the scale since the calculation for the scale is based on the negative log of the concentration of hydrogen (or hydronium) ions in the solution. pH=-log[H3O] therefore [H3O] = 10^-pH Actually, this balance of hydrogen ions and hydroxide ions determines the pH of the water. When the hydrogen ions outnumber the hydroxide ions, the solution is acidic. If the reverse is true, then the solution is alkaline. If the temperature does not vary, the following relationship between the densities of hydrogen ions (H+) and hydroxide ions (OH-) is found with any solution: [H+][OH-]=Kw=10-14(=fixed) at 25℃ (Kw is called the ion product of water or dissociation constant of water.) [H+]=[OH-] , [H+]=[OH-]=√(Kw)=√10-14=10-7 If you know the value of either [H+] or [OH-], you can find the value of the other. Thus, we measure only [H+] and use it as the standard for pH. In this way, pH is determined by hydrogen-ion concentration. So, pH is defined by the following formula: pH=-log10[H+] The concentration of hydrogen ions in any solution we are likely to encounter will range from 1 mol to 0.000001 mol per liter of solution. However, solutions with extremely low hydrogen-ion concentration could conceivably rack up a pretty long parade of zeros after the decimal point. Danish biochemist S.P.L. Sorensen was the first to use the pH system we know today, which defines inverse numbers of hydrogen-ion concentration shown in common logarithm as pH. That is to say, pH=-log10[H+] In the case of a neutral solution, [H+]=10-7 , which we call a pH of 7. This means, for example, that a hydrogen-ion concentration of a solution with a pH of 4 is 10-4mol/l, meaning it contains 0.0001 mol of hydrogen ions in a solution of 1 liter. In the same way, a solution with a pH of 5 contains 10-5mol/l of hydrogen ions, a solution with a pH of 6 contains 10-6mol/l of hydrogen ions, while the solution with a pH of 7 contains 10-7mol/l of hydrogen ions. You will notice that if you compare solutions with pH of 4 and pH of 7, the difference in pH is only 3, but the concentration of hydrogen ions with a pH of 7 is 1000 times as high as with a pH of 4.The following figure also shows the advantage of exponents, which can express large variations in value that range over many orders of magnitude. The number of hydrogen ions in the cube on the left (10-4mol/L) is 1,000 times larger than in the one on the right (10-7mol/L); this is equivalent to a difference of 3 in the pH value. (A single blue ball represents 10-7mol/L of hydrogen ions.) Next page Hydrogen-Ion Activity |
Which solution has a greater concentration of hydrogen ions a solution with a pH of 3 or pH of 7?
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