The pH scale measures acidity or alkalinity of a solution from 0 to 14, with 7 indicating neutrality. SPL Sørensen introduced the concept in 1909. Soil pH affects gardening success, and pH is important in determining wine quality and paper acidity. Natural pH indicators include red cabbage, and indicators like methyl violet are used for precise results.
The pH scale gets its name from the words potential of hydrogen. It is a scale used to measure the acidity or alkalinity of a solution. The pH scale uses a range from 0 to 14, with 7.0 indicating neutrality. Numbers starting at 7.0 and moving towards 0 indicate acidity, while numbers starting at 7.0 and moving towards 14 indicate alkalinity, so the scale divides acids from bases. We owe the concept of pH to the Danish chemist SPL Sørensen, who introduced it in 1909.
There are several categorizations for solutions measured by the pH scale. A common one is this:
pH 0 – 2strongly acidicpH 3 – 5weakly acidicpH 6 – 8neutralH 9 – 11weakly basicopH 12 – 14strongly basic
Interestingly, while some people associate corrosion with acid, a number of organizations call pHs of 0-2 and 11.5-14 on the pH scale corrosive.
While litmus paper is used to indicate whether a solution is acidic or basic with a simple red/blue color indication, more precise results are sometimes needed. To find the equivalence point in acid-base titration, for example, the choice of the correct indicator is critical to obtaining adequate results. The indicators used, such as methyl violet, bromothymol blue and alizarin yellow, have very specific color reaction bands that can be matched to the equivalence point.
A number of plants are natural pH indicators, including red cabbage, a fact you may have seen revealed in your kitchen. You may also see the soil pH revealed by the plants in your garden, because soil pH is one of the factors that determine the success or failure of gardening initiatives. Generally, plants experience their best growth in soils with a pH close to neutral, although some plants, particularly blueberries and azaleas, need acidic soils to grow well. One of the interesting gardening tricks accomplished by altering the pH is to encourage the flowering of blue – if the soil has a pH of 5.5 or lower – or pink – if the soil has a pH of 6.5 or higher – hydrangea flowers from identical plants. Soil with an intermediate pH can produce purple flowers, a mix of pink and blue.
The pH of different substances is also of interest in other fields. The correct pH is important in determining the quality of the wine, best determined using a pH meter. The pH of wine ranges from 2.9 to 4.2 and is considered best between about 3.0 and 3.5 – different winemakers give slightly different figures. The pH scale is also the measurement used to determine if paper qualifies for the description as acid-free.
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