What is pH?

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  • Written By: R. Kayne
  • Edited By: O. Wallace
  • Last Modified Date: 13 May 2019
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A pH (potential of Hydrogen) measurement reveals if a solution is acidic or alkaline (also base or basic). If the solution has an equal amount of acidic and alkaline molecules, the pH is considered neutral. Very soft water is commonly acidic, while very hard water is commonly alkaline, though unusual circumstances can result in exceptions.

The pH scale is logarithmic and runs from 0.0 to 14.0 with 7.0 being neutral. Readings less than 7.0 indicate acidic solutions, while higher readings indicate alkaline or base solutions. Some extreme substances can score lower than 0 or greater than 14, but most fall within the scale.

A logarithmic scale means that there is a ten-fold difference between each successive full number on the scale. An acidic solution reading 4.0 represents a ten-fold increase in acidic molecules over a 5.0 solution. The acidic difference between a 4.0 solution and a 6.0 solution is 100 times greater (10x10).

Hydrochloric acid or muriatic acid is an extremely caustic solution that sits at the extreme end of the acidic scale. This chemical is often used to lower the pH of highly alkaline water, such as in the treatment of swimming pools and aquariums. Only small amounts of hydrochloric acid are required, relative to the amount of water being treated.


Also low on the acidic scale at 1.5 – 2.0 is gastric acid, or stomach acids that help us digest food. Colas come in at 2.5, even more acidic than vinegar at about 3.0! It might also be surprising to learn that beer is slightly more acidic than acid rain, and coffee only slightly less acidic.

Pure water has a neutral pH and human saliva hovers close to neutral, while our blood is slightly alkaline. Seawater hits the scale between 7.7 and 8.3, and products like hand soap, ammonia and bleach score high on the alkaline scale running from 9.0 – 12.5. Highly alkaline baking soda or sodium bicarbonate is often used to increase the base of acidic water.

Aquarists rely heavily on related measurements for proper fish keeping. Large bodies of water such as lakes and oceans have very little pH fluctuation, making fish intolerant of swings. Decaying plants, left over fish food and even fish waste all have a tendency to create acidity in an aquarium, while certain types of rocks and shells can continuously release trace amounts of calcium, boosting alkalinity.

Due to chemical water treatment and other factors, tap water in many large cities throughout the U.S. tends to be alkaline with a pH close to 8.0. Though drinking tap water with alkalinity is not harmful, the declining quality of tap water over the years has resulted in many people opting for faucet or pitcher filters to remove chlorine, chloramines, pesticides and other substances. These filters do not alter the pH of the water.


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Post 40

pH stands for the potential of hydrogen present in liquids. The pH scale ranges from 1 to 14; neutral is 7. A pH higher than 7 is called alkaline, while a pH lower than 7 is called acidic.

The pH can be checked simply by performing a drop test, with the pH reagent that are now commonly available. The color code in the scale will indicate the pH value of water.

Post 35

I did an experiment and sodas can't dissolve foods because the foods make the soda's ph more basic.

Post 30

How about pH in lemons?

Post 29

This article is very helpful.It gives a brief summary of pH. --Maryland

Post 27

this is a nice website. Very helpful and comprehensive.

Post 26

really helpful to know the abbreviation of pH. and the details below are really helpful to me. babu, india.

Post 25

I am doing my science fair project on the chemical analysis of snow, and I am doing some pH testing. This is a great website! Very helpful!

Post 22

Great website! I am using this while I have an issue in A and P 2. This website has very good answers and a very simple way of explanation (simple vocabulary).

Post 20

The website is informative and useful.

Post 19

This is a good website.

Post 17

@loveme2k (comment 10): A good base is likely to give up it's OH- ion. NaOH dissociates in water, giving you Na+ and OH-. But alcohols do not, which explains the low alkalinity of alcohols.

Post 14

It's pH because hydrogen is capital, and potential isn't.

Post 13

its very good site for me. quick answers to my questions.

Post 11

why does it have to be written pH?

Post 10

Why does the OH group of NaOH etc. give a strong basic pH, while the OH group on the alcohols does not?

Post 7

What is pH with acid water?

Post 6

What is the big idea of Particles? How do you use that to describe the difference of acid and alkali?

Post 5

The pH, alkalinity or acidity is also measured in soil. Some plants prefer more of an acidic soil, such as azaleas and marigold, while others will grow better in a more alkaline soil, such as iris and calendula. The higher the number the more alkaline the soil is. Usually a soil will not have less then 3.5 acidity, or greater then 8 alkalinity.

Post 3

Why pH is limitted up to 14.0? Why liquid with pH 16 is not possible? pl.clarify.

Post 2

How can the ph of a buffer solution remain unchanged even after addition of small amts of acid/alkali?

Post 1

how accurately can the pH of an unknown solution generally be measured?

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