Acids and Bases

Continuing my discussion of pH, with acids and bases. Some important background information:

  • Bases are also known as alkalis. So a basic solution is also an alkaline solution.
  • A proton is the equivalent of a hydrogen ion (H+).
  • In food, both the acidity and the sugar content are important. Our taste responses comes from a balance of the two. Some foods have a low pH/high acidity but do not taste sour due to their high sweetener content. A good example of this is cola, which has pH ~3.50 but does not taste sour because of the high sweetener (or non-calorific sweetener if you drink diet) content.

    An acid solution occurs when the pH is less that 7 and an alkaline solution is when the pH is above 7. You may have heard of something called litmus paper. This is a quick and dirty method to determine pH. Typically litmus paper turns red in acid and blue in alkali. Additionally, acids typically taste sour and bases taste bitter. If it helps, compare the taste of lemon juice with that of sodium bicarbonate. Lemon juice is an acid solution; it actually contains several acids; with pH 2.2. A solution containing just sodium bicarbonate (5% solution) should have a pH 8.6.

    There are several different definitions of acids and bases that are still useful and practical. Arrhenius defined an acid as a proton donor and a base as a compound that donates hydroxyl ions (OH).

    The reactions would look like this:

    Acid (AH) —->; A + H+

    Base (BOH) —->; OH + B+

    This was alter by Brønsted-Lowry theory of acids and bases, which kept the same definition for acids, but the definition for bases changed to:

    Bases are proton acceptors

    Finally, as far as I am concerned there is the Lewis Theory which is based on electrons rather than protons. A Lewis acid is an electon pair acceptor and a Lewis base is an electron pair donor.

    Any previously defined acid and base will count as a Lewis acid or base, but there are some compounds that are only Lewis acids or bases.

    During food processing it is important to maintain the pH and/acidity. Partly because bacteria cannot grow in a high acid environment. If the pH changed during processing, this would change the way the final product would have to be stored. One way pH is maintained in food and in our bodies, is through buffers. The success of buffers is dependent on the fact that certain acids are weak acids and do not fully dissociate to protons and relevant ions. This will be the topic of my next post on basic concepts.

    References

    Acids and Bases Tutorial

    Introductory Chemistry: Acids and Bases

    Background on acid-base reaction theories

    Lewis Acid and Bases

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    10 thoughts on “Acids and Bases

    1. Pingback: Strong and Weak Acids « Lab Cat

    2. Excellent post!

      Can I add something about alkalis?

      In chemistry, an alkali (from Arabic: Al-Qaly القلي, القالي ) is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases (compounds with pH greater than 7) that dissolve in water. The adjective alkaline is commonly used in English as a synonym for base, especially for soluble bases. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base and are still among the more common bases. Since Brønsted-Lowry acid-base theory, the term alkali in chemistry is normally restricted to those salts containing alkali and alkaline earth metal elements.

      Confusion between base and alkali

      The terms “base” and “alkali” are often used interchangeably, since most common bases are alkalis. It is common to speak of “measuring the alkalinity of soil” when what is actually meant is the measurement of the pH (base property). In a similar manner, bases that are not alkalis, such as ammonia, are sometimes erroneously referred to as alkaline.

      Note that not all or even most salts formed by alkali metals are alkaline; this designation applies only to those salts that are basic.

      While most electropositive metal oxides are basic, only the soluble alkali metal and alkaline earth metal oxides can be correctly called alkalis.

      This definition of an alkali as a basic salt of an alkali metal or alkaline earth metal does appear to be the most common, based on dictionary definitions [1][2], however conflicting definitions of the term alkali do exist. These include:

      * Any base that is water-soluble and. This is more accurately called an Arrhenius base.
      * The solution of a base in water

    3. Help! I’m searching for magazine (5th grade level) for my daughters science fair project. It needs to cover acids and basis. Any direction would be appreciated.
      Thanks in advance for your help!
      Debbie Baugh
      baughdeb@att.net

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