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pH Scale

by Ron Kurtus (30 May 2006)

The pH of a material is a measure of how acidic or alkaline a substance is. Because it is the measure of activities of Hydrogen ions, the initials pH stand for Potential of Hydrogen.

Acids have pH values under 7 and alkalis have pH values over 7. If a substance has a pH value of 7, then it is neither acidic or alkaline and considered neutral.

The pH scale is logarithmic, meaning that greater numbers are multiples in strength. pH devices include litmus paper to determine the pH values.

Questions you may have include:

This lesson will answer those questions.



pH of various materials

The following table shows the pH values of various materials, from acids to neutral to bases.

 

Material

pH

Acid

Battery acid -0.5
  Stomach acid 2.0
  Lemon juice 2.4
  Cola drink 2.5
  Vinegar 2.9
  Orange juice 3.5
  Acid rain 4.5
  Coffee 5.0
  Milk 6.5
     

Neutral

Pure water 7.0
     

Base

Healthy human saliva 6.5-7.4
  Blood 7.4
  Sea water 8.0
  Hand soap 9.0-10.0
  Ammonia 11.5
  Bleach 12.5
  Lye 13.5

pH Table for Common Substances

Logarithmic scale

The pH scale is logarithmic. That means a difference of one pH unit represents a tenfold or ten times change. For example, the acidity of a sample with a pH of 5 is ten times greater than that of a sample with a pH of 6. A difference of 2 units, from 6 to 4, would mean that the acidity is one hundred times greater, and so on.

The logarithmic scale is handy in being able to list items in a form that is easy to chart. Otherwise going from a pH of 1 to 7 would be like going from 1 to 10,000,000.

pH measurement

A common way to measure the pH of a liquid is by the use of litmus paper. This is a special type of paper containing a chemical that will tell you whether a substance is acidic or alkaline by its color. Acids turn the paper red and bases turn it blue.

There are also more sensitive pH papers available that can give more accurate readings.

There are natural pH indicators such red cabbage juice, which will change its color when subjected to an acid or a base.

Gardeners also use a battery-powered pH meter to measure the pH of the soil.

Summary

The pH scale measures how acidic or alkaline a substance is. pH stands for Potential of Hydrogen. Acids have pH values under 7 and alkalis have pH values over 7. The pH scale is logarithmic, meaning that greater numbers are multiples in strength. pH is often measured with litmus paper.


Measure to determine strength


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