Strong and Weak Acids

In earlier posts, I discussed the concept of pH and acids & bases. If you remember, H+ ion concentration is often measured as pH as pH = -log [H+]. When dissolved in water, all acids undergo a dissociation reaction: Continue reading

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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

    The Hodge Scheme

    Hodge Mechanism

    This is such a cool reaction mechanism. It was designed by John E Hodge in a what is now a citation classic. It sums up the Maillard Reaction, which is as complicated as the reaction scheme above suggests. This is such a classical scheme that it is known as the “Hodge Scheme”. I find that pretty impressive; it would be great if there was ever a “Lab Cat” reaction scheme.

    Hodge was an African American who gained an MA from the University of Kansas in 1940 and worked for the USDA for more than 40 years. Sadly, there was no Wikipedia page for this amazing man, so I started one. If any one knows anymore information about him, please, please add to the Wikipedia page.

    Reference:
    Hodge, J. E. (1953). “Chemistry of browning reactions in models systems.” Journal of Agricultural and Food Chemistry 1(15): 928-943.

    Citation Classic (pdf)

    John E Hodge Bio

    pH

    In my post on magical properties of water I tried to explain why Essentia water could not have a high concentration of “active” hydrogen while also having a high pH. I thought it might be useful to write a post explaining pH. This could be considered to be part of the on-going basic concepts series.

    So pH is defined as the negative log of hydrogen ion activity:

    pH = -log aH+

    where aH+ is the hydrogen ion activity. Where activity is the effective concentration of hydrogen ions rather than the actual concentration. This is because in solutions that contain other ions, activity and concentration are not the same. Generally the activity is equal to concentration so the equation above is typically written as:

    pH = -log[H+]

    Where the square brackets are used to signify concentration; one of those chemistry jargon things. Like in math you have + for adding etc.

    The fact that hydrogen activity = hydrogen concentration (aH+ = [H+]) is only true in dilute solutions. It is not true if there is a lot of ions present as these alter the ionic strength, which in turn alters the aH+. Additionally, the measurement of pH is not so accurate in a concentrated solution as the additional ions interfere with the workings of pH meters.

    So assuming that our solution has low ionic strength and therefore a valid pH value, what does pH mean?

    pH is an indication of how many hydrogen ions are present. The lower the pH the higher the hydrogen ion concentration. Hydrogen ion concentration is an indication of acidity. So a low pH is associated with acidity. Conversely a high pH is associated with alkalinity and is an indication of a low concentration of hydrogen ions.

    Conveniently, water has a pH 7 and is considered to neutral – that is water is neither acid or base as it dissociates into equal concentrations of H+* and OH ions which just happens to be 1×10−7 mol/L giving a -log10[H+] of 7!**

    pH can be used to compare the acidity of foods. So orange juice (pH 3.3-4.2) is more acidic than milk (pH 6.4-6.8). pH is also important because bacteria and other micro-organisms cannot grow or survive in an acidic environment. So if food is treated with acid will be preserved. For example, converting lactose into lactic acid reduces the pH, so yoghurt is a way of preserving milk. Sauerkraut (pH 3.3-3.6), chutneys (3.5-4.5), pickles (pH 2.6-3.8), etc. are other foods that are preserved in this way.

    Foods with a low pH have a separate legal definition because of their high resistance to micro-organisms, Clostridium botulinum in particular. Acid foods are defined as foods that have a natural pH of 4.6 or less and acidified foods are defined as low-acid foods to which acids or acid foods have been added.


    Footnotes
    * It is more accurately to put H3O+ but H+ is easier to understand.
    **It is interesting writing these posts as I realize that I know need to do a post on the self-ionization of water and on acids and bases. I also want to explain acidity and buffers as these concepts are important in food science.

    References:

    What is pH?

    pH at Wikipedia

    The FDA has a great list of foods and their pH.