Understanding Acids Bases and the Ph Scale

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Every aqueous (water-based) solution will have a pH that is acidic, basic, or perhaps neutral. pH is a measurement that is used to quantify aqueous solutions into a range of values known as the pH scale. If the solution is basic it will be higher on the scale, if it is acidic it will be lower on the scale, and if it is neutral it will be in the middle of the scale.

This measurement is very important as people use aqueous solutions everyday and they are present in nature. They are important in chemistry, forestry, engineering, biology, agriculture, oceanography, environmental science and many other disciplines or applications. However, this doesn’t really explain what acids, bases, and the pH scale are.


As defined by Svante Arrhenius in 1884, an acid is a substance or material that releases a proton or hydrogen ion [H+] when placed into a solution with water. For example, when a substance like hydrogen chloride (HCl) is added to water, it dissociates into hydrogen ions [H+] and chloride ions [Cl-]. The strength of the acid will increase with increasing hydrogen ion concentration. The more [H+] that are present in the solution, the stronger the acid. Acids will corrode metals and taste sour (though it is not advised to taste most acids).

Litmus, which is an extract/dye that comes from lichen, is the oldest known pH indicator. It is often impregnated into paper in order to make litmus paper which is an easy test to determine if an aqueous solution is an acid or base. It comes in two types, blue litmus and red litmus. Acids will turn blue litmus paper red, but will not affect the red litmus paper. An example of an acid that you may drink regularly is orange juice, which contains citric acid and ascorbic acid.

Some common acids are: nitric acid (HNO3), sulfuric acid (H2SO4), and ascorbic acid (Vitamin C)


A base is a substance that releases electrons or hydroxide ions [OH-] when placed in a solution with water.  For example when sodium hydroxide (NaOH) is added to water, it dissociates into sodium ions [Na+] and hydroxide ions [OH-]. A stronger base will have more hydroxide ions [OH-] in the solution. Bases tend to feel slippery when touched and have a bitter taste (though it is definitely not advised to ever taste or touch a strong base). Strong bases that dissociate well in water can be quite dangerous because they denature (break down) protein, and a good portion of the human body is made up of proteins. Opposite to the acids, bases will turn red litmus paper blue, but will not affect the blue litmus paper.

Some common bases are: magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2), and potassium hydroxide (KOH)


The measurement of acids and bases, as well as their strength, is called pH and it is approximated through the concentration of hydrogen ions [H+] present in the solution. The equation to approximate pH is as follows:

pH = - log( [H+] )

Using this equation and a calculator, the pH can be approximated. Should the solution be a base, the concentration of hydroxide ions may be substituted and used to determine the pH in the below equations.

pOH = - log( [OH-] )

pH = 14 – pOH

The resultant of these equations will typically be a number between 0 and 14 and this will indicate whether the solution is basic, acidic, or neutral and even give an idea of how strong the solution is. There are some exceptions though as very strong acids and bases could reach numbers outside of this scale.

The pH scale

The pH scale is normally between 0 to 14 and is used to classify and measure acids and bases to show their strength. From 0 to 6 is an acid with stronger acids being a lower number. Neutral is a pH of 7 and everything above 7, 8 to 14, is a base and stronger bases have a higher number on the scale.

The pH scale can also include numbers below 0 and above 14. Sometimes an extremely strong acid or based will give numbers outside the normal range of the scale. For instance, some acids can actually give negative numbers because they are a stronger acid than the lowest number on the normal scale which is zero. Strong bases can also have a number that goes above the number 14 on the scale.

The difference between numbers on the scale is a factor of ten times higher or lower in terms of the amount of hydrogen ion concentration. For example, the number 4 on the scale has a concentration of hydrogen ions ten times higher, and is ten times stronger an acid than the number 5 on the scale. Also, for bases, the number 9 has a concentration of hydrogen ions ten times less than the number 8 and is ten times more basic as the situation is reversed in the basic section of the scale. A good table with this information (hydrogen ion concentrations and the pH scale) can be found at the Science Buddies website. Shown below is the pH scale and corresponding substances that are examples of each number on the scale.

pH scale

Base-14: some liquid drain cleaners and strong sodium hydroxide (lye)

Base-13: oven cleaner and bleach

Base-12: soapy water

Base-11: ammonia solution used for cleaning

Base-10: antacids and the Great Salt Lake in Utah

Base-9: baking soda

Base-8: Salt water (ocean) and egg whites

Neutral-7: Distilled water and blood

Acid-6: Saliva, urine, milk

Acid-5: Black coffee, soft water

Acid-4: Tomato Juice, rainwater (unpolluted)

Acid-3: Orange juice, soda, grapefruit

Acid-2: lemon juice, vinegar, gastric acid

Acid-1: stomach acid (hydrochloric acid) and sulfuric acid

Acid-0: strong hydrofluoric acid and battery acid

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