Ph
The pH of a solution is a measure of the hydronium ion (H3O+) concentration in that solution. The pH is measured on a scale from 0-14. The hydronium ion in a solution results from the selfionization of water.
A drop of pure water is not composed entirely of H2O molecules. Water always contains small quantities of two ions, hydronium ions and hydroxyl ions (OH-). The presence of both of these ions makes water an amphoteric substance. An amphoteric substance can act either as an acid or as a base. An acid is a substance which can donate H+ ions. When a water molecule acts as an acid, it becomes an OH- ion. A base accepts H+ ions. When a water molecule acts as a base, it becomes an H3O+ ion. The self-ionization of water is a reaction that occurs between two water molecules. One water molecule donates an H+ ion to the second water molecule, forming the hydronium and hydroxyl ions. During this reaction, water acts both as an acid and as a base. In a sample of pure water at 25°Celsius, the concentrations of both ions are 1.0 X 10-7 moles per liter (M).
For most solutions, the concentration of hydronium ion is quite small. Even in the strongest acid solutions, such as the hydrochloric acid (HCl) in the human stomach, the concentration of hydronium ions is only about 1 X 10-2 M. In a strong basic solution, the hydronium ion concentration is even less, as low as 1 X 10-12 or 1 X 1013 M.
Expressing the hydronium ion concentration of solutions in terms of molarity (moles per liter) can be cumbersome. In 1909, Soren Sorensen, a Danish biochemist, proposed what is now known as the pH scale. Sorensen developed a simple equation to express the hydronium ion concentrations logarithmically. The pH of a solution is -1 times the logarithm (of the base 10) of the hydronium ion concentration (expressed in moles per liter). The equation for the pH of a solution is: pH = - log[H3O+]. This equation is made even simpler when one realizes that a number's logarithm, to the base 10, is equal to the exponent of the number when 10 is the base. For example, for the number 10-3, the log (103) equals 3. Likewise, the log (10-5) equals 5 and the log (10-8) equals 8. Therefore, the pH for a solution with a hydronium ion concentration of 10- 1 M is 1. The pH of a solution with a hydronium ion concentration of 109 M is 9, and so on.
The pH scale ranges from 0-14. The smaller the pH of a solution, the more acidic the solution. A neutral solution has a pH of 7. Any pH value below 7 indicates an acidic solution, and any pH value above 7 indicates a basic solution. The pH for several common substances can give an idea of what the somewhat arbitrary pH values represent. The pH of lemon juice, which is quite acidic, is 2.5. The pH of a banana is 4.7. Coffee has a pH near 5.2. Saliva, which is somewhat neutral, has a pH of approximately 6.5. Pure water has a pH of 7, it is completely neutral. Human blood has a pH of 7.3, also quite neutral. Borax, a base, has a pH of approximately 9.2. Limewater has a pH of 10.8, and the pH of bleach, a relatively strong base, is about 12.5. Since the pH scale is based on a logarithmic scale, for every one-unit change in pH there is actually a tenfold change in the hydronium ion concentration. For example, if the pH of a solution drops from 9 to 8, the hydronium ion concentration has increased by a factor of 10, from 10-9 M to 10- 8 M.
The pH of a solution can be measured in one of two ways. The simplest method is to use a product called an acid-base indicator. An indicator is a weak acid or base that undergoes a change in color whenever it gains or loses a hydronium ion. Litmus is an example of an indicator. In the presence of an acidic solution, litmus will gain hydronium ions and turn red. In the presence of a basic solution, litmus will lose hydronium ions and turn blue.
Each indicator has a different equilibrium constant, meaning that each has a different pH range over which it changes color. There are many indicators commercially available that turn specific colors in specific pH ranges. For example, thymol blue changes from a red color at pH 0 to yellow above pH 2. Methyl red changes from red at pH 4 to yellow around pH 6. Bromthymol blue changes from yellow at pH 4 to blue around pH 7, and phenolphthalein changes from colorless at pH 7 to pink at pH 9. Common household substances can also be used as crude pH indicators. Red cabbage juice covers the entire pH range. Grape juice is bright pink when added to an acidic solution and yellow when added to a basic solution.
A good way to pinpoint the exact pH of a solution is to test it with several indicators which operate over a broad range of pH values. Strips of paper are available to chemists which are coated with a combination of indicators. Placing a drop of solution on the paper will result in the paper changing color. The color of the paper can then be compared with a color chart to pinpoint the solution's exact pH. The color change in acid-base indicators is due to a chemical reaction between the indicator and the hydronium ions in the solution. An acid-base indicator is much like water in that it can have both an acid and a base form. Adding an acid to an indicator adds hydronium ions to the solution, increasing the concentration of the acid form of the indicator. Adding a basic solution to an indicator removes hydronium ions from the indicator, increasing the concentration of the basic form of the indicator.
The second method of measuring the pH of a solution is much more accurate than using acid-base indicators. This method involves using a device called a pH meter. A special electrode which measures the concentrations of hydronium ions is simply placed in a solution, and the pH of the solution is displayed on the pH meter. Although this method is much more accurate than reading color changes on indicators, it is also much more expensive. The acid-base indicators serve as a good measurement of the pH of solutions whenever exact precision is not necessary.
Sometimes it is necessary to maintain a solution at a constant pH. This is especially true in bodily fluids such as blood, which needs to be kept neutral (between a pH of 7.35 and 7.45). If the pH of blood is allowed to vary outside of this range, serious illness or death may occur. Buffers are substances which control the pH of a solution. A buffer is usually a mixture of acids and bases. This mix of acids and bases allows the buffer to release or absorb hydronium ions, which keeps the pH of a solution constant. The most common buffers are mixtures of weak acids and their conjugate bases. A buffer cannot keep the pH of a solution under absolute control under all conditions. There is a limit to the ability of a buffer to maintain a constant pH, called the buffer capacity. In general, the greater the concentration of buffer in a solution, the greater the buffer capacity.
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