Which pH has the highest concentration?

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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.

The pH scale

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 Ions

Since 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:

At room temperature, the balance between the H+ and OH– concentrations in water are related by this expression ([H+] means "concentration of H+"):

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 pH

To calculate pH, you should know the concentration of hydrogen ions (H+) in your solution. From there, the pH can be found by:

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 More

The 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

• Acid
• Base
• Chemical
• Water
• Acid rain
• Water cycle
• Or explore a random page

To explore pH further please see the Chemistry LibreTexts page.

References

1. ↑ Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/2/23/216_pH_Scale-01.jpg
2. ↑ US EPA. (July 8, 2015). What is pH? [Online], Available: http://www.epa.gov/acidrain/measure/ph.html
3. ↑ Nordstrom and Alpers, Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California Proc Natl Acad Sci U S A, vol. 96, no. 7, pp 3455-3462, 1999 [Online], Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC34288/
4. ↑ Licht, "pH Measurement in Concentrated Alkaline Solutions" Anal. Chem., vol. 57, pp. 514-519, 1985 [Online], Available, http://pubs.acs.org.ezproxy.lib.ucalgary.ca/doi/pdf/10.1021/ac50001a045

	pH Scale Introduction and Definitions: Acidic and basic are two extremes that describe a chemical property chemicals. Mixing acids and bases can cancel out or neutralize their extreme effects. A substance that is neither acidic nor basic is neutral. The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8. Pure water is neutral. But when chemicals are mixed with water, the mixture can become either acidic or basic. Examples of acidic substances are vinegar and lemon juice. Lye, milk of magnesia, and ammonia are examples of basic substances. Practice: HNO3 + KOH --> +   H3PO4 + 3 NaOH -->   +
	Ionization of Water: Water molecules exist in equilibrium with hydrogen ions and hydroxide ions. H2O <--> H+ + OH- The water equilibrium constant is written as: Kw = [H+] [OH-] Experimentally, it has been found that the concentration of: H+ = OH- = 10-7 Therefore: Kw = [10-7][ 10-7] = [10-14] (To multiply exponential numbers - simply add the exponents.) The values for Kw, H+, OH- concentration all indicate that the equilibrium favors the reactant (water molecules). In other words, only very small amounts of H+ and OH- ions are present. Effect of Acids and Bases on Water Equilibrium: If an acid (H+) is added to the water, the equilibrium shifts to the left and the OH- ion concentration decreases. Water Equilibrium: H2O <--> H+ + OH- If base ( OH-) is added to water, the equilibrium shifts to left and the H+ concentration decreases. Water Equilibrium Principle: The multiplication product (addition of exponents) of H+ and OH- ion concentration must always be equal to 10-14. BOTH H+ and OH- ions are ALWAYS PRESENT in any solution. A solution is acidic if the H+ are in excess. A solution is basic, if the OH- ions are in excess.
Number Representation and Logarithms  Number  Exponent Notation  Log of the Number  1000  103  3  100  102  2  10  101  1  1  100  0  0.1  10-1  -1  0.01  10-2  -2  0.001  10-3  -3  0.0001  10-4  -4 Solve for any of the others given one  H+  OH-  pH  pOH  0.001        0.00001          6        4	Definition of pH, pOH, and pKw: The concentrations of hydrogen ions and indirectly hydroxide ions are given by a pH number. pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is: pH = - log [H+] similarly, pOH = - log [OH-] and p Kw = - log [Kw] . Logarithms of numbers that are multiples of ten are merely the exponents of the number including the sign. See the table on the left for a review. The method to find logs of numbers that are not multiples of ten are found by using a calculator. The method is not discussed here. Example: If an acid has an H+ concentration of 0.0001 M, find the pH. Solution: First convert the number to exponential notation, find the log, then solve the pH equation. H+ = 0.0001M = 10-4; log of 10-4 = -4; pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH The purpose of the negative sign in the log definition is to give a positive pH value.  Example: If the base has an OH- concentration of 0.001M, find the pH.  Solution: First find the pOH, (similar to finding the pH,) then subtract the pOH from 14. OH- = 0.001M = 10-3; pOH = -log [OH-] = -log (10-3) = +3 = pOH pH = 14 - pOH; pH = 14 - 3 = 11 = pH pH Principle: pH and pOH must always equal pKw (14).
Click for larger image	pH Scale: The pH scale, (0 - 14), is the full set of pH numbers which indicate the concentration of H+ and OH-ions in water. The diagram on the left gives some relationships which summarizes much of the previous discussion. pH Scale Principle: H+ ion concentration and pH relate inversely. OH- ion concentration and pH relate directly. The following statements may be made about the pH scale numbers. Complete some of them. a. Increasing pH means the H+ ions are decreasing. b. Decreasing pH means H+ ions are increasing. c. Increasing pH means OH- ions are d. Decreasing pH means OH- ions are  Most H+ ions: pH = 4; or pH = 5.    Most OH- ions: pH = 10; or pH = 13.    Least H+ ions: pH = 12; or pH = 13.    Least OH- ions: pH = 8; or pH = 9.    If acid was added to a solution of pH 4, the pH would increase or decrease?    If acid was removed from a solution of pH 3, the pH would inc. or dec.?    If base were added to a solution of pH 9, the pH would inc. or dec.?    If base were added to a solution of pH 2, the pH would inc. or dec.?     If acid were added to a solution of pH 13, the pH would inc. or dec.?    If base were removed from a solution of pH 12, the pH would inc. or dec.?

What is the highest concentration of pH?

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Which acid has higher concentration?

Which pH solution is the most concentrated base?