What Is pH of Sodium Carbonate in Water?

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Sodium carbonate, also known as washing soda, is a common ingredient in laundry detergents. When dissolved in water, it tends to form solutions with pH values between 11 and 12.

Water

Close up of water.
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Water, H?O, undergoes a process known as autodissociation, in which it separates into a hydrogen ion (H?) and a hydroxide ion (OH?):

H?O ? H? + OH?

pH

Litmus paper in solution testing the pH.
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pH is actually a measure of the amount of H? in a solution and ranges from 0 to 14.

In general terms, a pH greater than 7 indicates an alkaline (or basic) solution (more OH? than H?). A pH less than 7 indicates an acidic solution (more H? than OH?). A pH of 7 indicates a neutral solution, such as water (H? and OH? are equal).

Sodium Carbonate

Baking soda on black background.
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Sodium carbonate (Na?CO?), also known as washing soda, produces sodium ions (Na?) and carbonate ions (CO?²?) when dissolved in water:

Na?CO? ? 2 Na? + CO?²?

The sodium ions have no influence on the pH of the resulting solution. The carbonate ions, however, are basic and make the solution alkaline by increasing the amount of OH?:

CO?²? + H?O ? HCO?? + OH?

Concentration and pH

Adding sodium carbonate to water in a beaker is easy.
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The important part of pH is that it depends on concentration. That is, dissolving two tablespoons of sodium carbonate in a glass of water will result in a higher pH than dissolving one tablespoon.

Mixing Instructions

Blue jar with water mixture.
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One gram (0.035 ounces) of sodium carbonate dissolved in water and diluted to 1.0 liter (about 1 quart) will produce a solution of pH 11.37.

Five grams (0.18 ounces) of sodium carbonate dissolved in water and diluted to 1.0 liter (about 1 quart) will produce a solution of pH 11.58.

Ten grams (0.35 ounces) of sodium carbonate dissolved in water and diluted to 1.0 liter (about 1 quart) will produce a solution of pH 11.70.

References

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