Lowry (1874–1936), who defined acid–base reactions in terms of the transfer of a proton (H + ion) from one substance to another.Īccording to Brønsted and Lowry, an acid A substance with at least one hydrogen atom that can dissociate to form an anion and an H + ion (a proton) in aqueous solution, thereby foming an acidic solution. Brønsted (1879–1947) and the British chemist T. One was proposed independently in 1923 by the Danish chemist J. The Brønsted–Lowry Definition of Acids and Basesīecause of the limitations of the Arrhenius definition, a more general definition of acids and bases was needed. Although these definitions were useful, they were entirely descriptive. In contrast, a base was any substance that had a bitter taste, felt slippery to the touch, and caused color changes in plant dyes that differed diametrically from the changes caused by acids (e.g., turning red litmus paper blue). In ancient times, an acid was any substance that had a sour taste (e.g., vinegar or lemon juice), caused consistent color changes in dyes derived from plants (e.g., turning blue litmus paper red), reacted with certain metals to produce hydrogen gas and a solution of a salt containing a metal cation, and dissolved carbonate salts such as limestone (CaCO 3) with the evolution of carbon dioxide. Although the general properties of acids and bases have been known for more than a thousand years, the definitions of acid and base have changed dramatically as scientists have learned more about them. In fact, this is only one possible set of definitions. The meter is then adjusted so that it reads exactly 4.0 or 10.0.In Chapter 2 "Molecules, Ions, and Chemical Formulas", we defined acids as substances that dissolve in water to produce H + ions, whereas bases were defined as substances that dissolve in water to produce OH − ions. Ideally, the pH meter is adjusted before use by putting the probe into a solution of accurately known pH - for example, pH 4.0 or pH 10.0. To measure the pH accurately, you can use a pH meter which has a probe which you simply put into your solution and get a digital read-out of the pH. It is also possible to get narrow-range indicators which cover a much smaller range of pH more accurately. The results aren't particularly accurate. You don't remember these colours - you just compare the colour you get with the colour on the chart which accompanies the indicator paper or solution. The commonest form is known as full-range universal indicator, and changes colour in a regular way from pH 1 to pH 14. This is a mixture of indicators which change colour at all sorts of different pHs. Nevertheless, it picks out most acids and alkalis, and is easy to use. If it turns blue, all you can say is that the pH is 8.3 or greater. So if it is turns red, all you can say is that the pH of what you are testing is 4.5 or less. In between it is various shades of purple. It comes in two forms - red litmus and blue litmus.Ī disadvantage of litmus is that its colour change isn't very sharp - it changes colour over quite a wide range of pHs - from pH 4.5 to pH 8.3. Litmus is nearly always used as litmus paper rather than a solution. They can be in solution or as papers soaked in solution and then dried. Indicators are substances which change colour depending on the pH of a solution. So an alkaline solution is one with a pH greater than 7. If a solution has a pH higher than 7, it is said to be alkaline. So an acidic solution is one with a pH less than 7. If a solution has a pH of less than 7, it is said to be acidic. So a solution with a pH of 5 is ten times more acidic than one with a pH of 6. The higher the number, the more alkaline a solution is.īecause of the way the scale is defined, each shift of 1 pH unit means a ten times shift in the acidity or alkalinity of a solution. The lower the number, the more acidic a solution is.Ī pH above 7 shows the solution to be alkaline. The key points are:Ī pH of 7 shows the solution to be neutral.Ī pH below 7 shows the solution to be acidic. The pH scale is used to measure the acidity or alkalinity of a solution. It also introduces some common ways to test whether a solution is acidic or alkaline. This page introduces the pH scale to measure the degree of acidity or alkalinity of a solution.
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