by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Today’s pop quiz: What is important for proper vineyard cultivation, relatively high in ocean water compared to rain water, and causes us to add lime to our soil to grow some of the plants we like the best?
Give up?
It’s pH, or the concentration of hydrogen ions in a substance. In my classes there is probably no other scientific concept that can be so interesting yet so misunderstood; pH is the Greta Garbo of chemical traits, mysterious yet aloof. But gardeners on Nantucket and in the Northeast have to contend with acidic soils and have learned, perhaps going back thousands of years, how to work with our acidic, young, peaty soil. And when you take a sip of wine at the Nantucket Wine Festival, you’ll be enjoying a dance of many different acids and the result of centuries of work to understand how the right combination of chemicals can somehow become magical.
I teach kids as young as seven about pH, so it’s not a difficult concept, but most people just learn that orange juice is an acid and milk is a base and go about their daily lives. The pH scale was invented by the Danish chemist Søren Sørensen in 1909. pH (scientists argument on what the “p” stands for, can be potential hydrogen or power of hydrogen ion concentration but effectively is the “negative logarithm thereof” ) is a measure of the acidity or basicity of an aqueous solution. “Pure water” with absolutely no minerals or salts dissolved in it, is neutral, with a pH close to 7.0 at 25 °C (77 °F); this means the proton concentration is 10−7 moles per liter or 0.0000001 mol/L. Something that is 1 pH has a hydrogen ion concentration of 0.1 mol/L, which would hurt (a lot).
Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. This value varies with temperature and slightly by other things dissolved in the water that can help buffer or affect the pH. When an acid is dissolved in water, the pH will be less than 7 (as measured at 25 °C (77 °F)). When a base, or alkali, is dissolved in water, the pH will be greater than 7 (as measured at 25 °C (77 °F)). A solution of a strong acid, such as hydrochloric acid, at concentration 1 mol/L has a pH of 0. A solution of a strong alkali, such as sodium hydroxide (also known as caustic soda or lye), at a concentration of 1 mol/L, has a pH of 14. Thus, measured pH values will lie mostly in the range 0 to 14. Since pH is a logarithmic scale, a difference of one pH unit is equivalent to a tenfold difference in hydrogen ion concentration. Other items measured on a logarithmic scale include earthquakes, measured in Richter numbers, and sound and electromagnetic power measured in decibels (dB) and decibel-milliwatts (dBm) respectively.
But we are missing something here: hopefully you recall that in water hydrogen ions tend to hang out in pairs along with one oxygen molecule; you know, H2O. So what happens to the other OH? Well, it can be a bit sordid and complicated, but you might remember from high school that water molecules carry a dipole, or net charge, across the molecule. This dipole causes each molecule to behave like a little magnet with a positive and a negative end. This dipole causes water molecules to be attracted to each other; the positive hydrogen is attracted to the negative oxygen of a nearby molecule. Because the oxygen atom in water tends to monopolize the electrons in the molecule, the hydrogen protons are only loosely held to the molecule, they really aren’t appreciated by the oxygen. The attraction between adjacent water molecules allows them to swap hydrogen protons. In fact, many molecules that contain hydrogen can swap protons with water molecules (I told you it was sordid).
Well, when they split up, H becomes positive (H+) (good for it), and it grabs another water molecule (or another anion, it’s not picky) and become a H3O+ (called hydronium) ion and the OH it left is called hydroxide and becomes negative (OH-) (not taking it as well as the hydrogen ion even though its possessiveness of electrons started it), and it is measured as pOH or the concentration of OH-. When you add an acid like hydrochloric acid (HCl) to water, it separates into 2 ions: a positively charged hydrogen proton and a negatively charged chlorine ion. The positively charged hydrogen proton (H+) combines with water and increases the concentration of H3O+ ions. The lower the pH the MORE H3O+are in solution (negative logarithm). All of these exchanges of protons do most of the work in solutions, helping our blood to exchange iron and oxygen, causing nutrients to be taken up with plants, and in the right combinations, making wine taste better (or not) and helping plants grow in different soils.
This week as I thought about the Nantucket Wine Festival, I was reminded of how important pH in the soils can be for growing grape varietals and of the layers of acidity that can be detected when tasting wines. From The Oxford Companion to Wine, Third Edition we learn that the acids in wine are an important component in both winemaking and the finished product of wine. A variety of acids can be found in both grapes and in wine, and they have direct influences on the color, balance, and taste of the wine, as well as the growth and vitality of yeasts during fermentation and help protect the wine from bacteria.
The pH of the soil has an optimum range for Vitis vinifera (Common Grape Vine), hybrids and native American varieties. Natives tolerate, and even prefer, the slightly higher acids of a low pH soil. Our local soils, which are relatively acidic and well drained, are one of the reasons that Vitis labrusca or “fox grape” does so well on the islands (I wrote about our local grape species in 2008 and 2011 www.yesterdaysisland.com/2008/features/foxgrape.php and www.yesterdaysisland.com/2011/science/2.php).
At an optimum range pH range, soil nutrients are more accessible. In researching this article, I found a very comprehensive yet readable book on the science of wine called Wine Science: Principles and Applications (Third Edition) by Ronald Jackson. Essentially it is a distillation, if you’ll pardon the pun, of the science behind grape culture, wine production, and sensory evaluation. Jackson believes that the pH of soil is not nearly as critical to wine quality and that other soil factors including drainage, mineral content, and organic matter concentration matter more in the quality of the final product. Other researchers consider soil pH extremely important when it comes to nutrient uptake such as the authors of this 2003 Cornell paper “Improving Wine Grape Production in Acid Soils with Rootstocks and Soil Management” (http://lergp.cce.cornell.edu/Bates/BatesWine_Grape_Prod.htm accessed May 6th 2012).
The science and chemistry of wine and wine making and fermentation of all types is complex and fascinating. From Jackson’s book we learn that in wine tasting, the term “acidity” refers to the fresh, tart and sour attributes of the wine which is evaluated in relation to how well the acidity balances out the sweet and bitter components of the wine such as sugars and tannins. A well balanced wine contributes just enough acid to the palette to give it some “oomph”. There are three primary acids found in wine grapes: tartaric, malic and citric. According to the article “The acidity of Wine” by Alexander J. Pandell, Ph.D. (http://www.wineperspective.com/the_acidity_of_wine.htm, accessed May 6, 2012): “Tartaric and malic acids are produced by the grape as it develops. In warm climates, these acids are lost through the biochemical process of respiration. Therefore, grapes grown in warmer climates have lower acidity than grapes grown in cooler climates. For example, Chablis (France) produces grapes with high acid because the climate is very cool, while Napa Valley produces grapes with lower acidity because the climate is warmer.”
During the course of winemaking and in the finished wines, acetic, butyric, lactic and succinic acid can play significant roles. A low pH (more acidity) also helps control microorganisms by inhibiting their growth. Most of the acids involved with wine are fixed acids or non-volatile with the notable exception of acetic acid, mostly found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. Sometimes additional acids are used in winemaking such as ascorbic, sorbic and sulfurous acids to act as antioxidants, prevent color changes, and to stabilize the sugars (more on wine chemistry at http://www.erictheviking.com/Chemistry/Wine%20Chemical%20Dictionary.pdf accessed May 6th, 2012)
As a chemical oceanographer I love some of the simple things that make ocean water so unique, resilient and nourishing for plants and animals. I also love to teach classes about the salinity, pH, and oxygen profiles of different types of water. Here at the field station it is easy to compare the saltiness and pH and amount of oxygen present in our pond, the marsh, and the harbor. In the next Yesterday’s Island, we’ll talk about pH in different types of water around the world and about a worrisome change in ocean pH. The oceans are becoming more acidic (actually less basic) as the result of excess carbon dioxide entering the atmosphere. We’ll learn the implications for our shellfish and how ocean life like coral reefs is adapting. We’ll also discuss our island soils, groundwater, tannic ponds, bogs, and swamps. Stay tuned.
Each week I’d like to highlight an interesting nature or science blog around the country in case you want to learn more about the habitats and creatures that populate your backyard. This week’s blog is by Brooklyn naturalist Matthew Wills and a Nantucket entry can be found at http://matthewwills.com/2012/03/17/squam-swamp/
J. Robinson (ed) “The Oxford Companion to Wine” Third Edition pg 387 Oxford University Press 2006
http://www.grapestompers.com/articles/measure_acidity.htm