Everything is made of different kind of atoms, that make elements that react together in many types of chemical reactions (single replacement, double replacement, combination, decomposition, redox, combustion…). You may not know it, but you witness reactions on a daily basis. One type of reaction that is very important for us, because it happens in any of our bodily fluids and in the soils in which we grow our food, is the neutralization reaction, that occurs when an acid reacts with a base.
What are acids, bases, and neutralization reactions?
Acids and bases were first defined by Svante Arrhenius in 1884 as being respectively “a material that can donate a proton H+” and “a material that can donate a hydroxide ion OH-.” This definition was limited by the fact that sometimes an acid and a base will react, but the base will not actually lose an hydroxide ion, it will gain the proton lost by the acid instead. That is why, in 1923, Lowry and Brönsted redefined an acid as a material that can donate a proton and a base as a material that can accept one. This is important to know because we can then define a neutralization reaction as a reaction involving an acid that donates a proton H+ to a base.
After an acid donates its proton, it becomes its conjugate base, which is the base it is paired to, and the same goes for the base, after it gains the proton it becomes its conjugate acid. In other words, AH+ is an acid, it can donate a proton to base. When reaction, AH+ is now just A (because it lost a proton when reacting). A is the conjugate base of the acid AH+. We can write them in pairs:
AH+/A (AH+ is the acid form, and A is the base form) or B/B- (B being the acid and B- the base).
Example: Reaction HNO3 with H2O
The conjugate pairs here are HNO3/NO3- and H2O/H3O+
Now that you know how an acid and a base react together, you might be wondering what the consequences of this reaction are. As you now know, when an acid reacts it becomes a base, and when a base reacts it becomes an acid. The total amount of acid and base in the solution changes as the reaction occurs. A way to measure the amount of acid and base is the pH (potential Hydrogen). It quantifies the amount of protons H+, to express how acidic an aqueous solution is. The pH of an aqueous solution is approximately the negative of the base 10 logarithm of the molar concentration of protons H+ in the solution.
[H+] being the protons concentration in the solution.
The neutral pH is 7, it is the point at which the concentration of protons is equal to the concentration of hydroxide ions. 7 is the pH of pure water. A pH less than 7 is acidic, the concentration of protons is greater than the concentration of hydroxide ions, and a pH higher than 7 is basic (or alkaline), the concentration of hydroxide ions is greatest than the concentration of protons.
What role does all of this play in our body?
Acids and bases play a major role in the human body. In fact, the bodily fluids (blood, saliva…) have a set pH: saliva has a pretty neutral pH of approximately 7.4 in the mouth, but it has a pH more acidic in the stomach, between 1.5 and 6.5 depending on which part of the stomach. Your pH can sometimes shift a little bit to a more acidic or alkaline level without any consequences, it actually probably happened to you many times. If you eat too much animal meat, your body pH is going to shift to a more acidic level, but your kidney should regulate that. A shift in pH is usually identifiable because you can feel acidic reflux from your stomach, a good thing to do in that case is to eat alkaline foods: vegetables, fruits, nuts, sprouts, breads… to help your body regulating your pH. In fact, each food has a different pH, that can go from very acidic to very basic.
When the pH cannot be maintained constant, an acidosis/alkalosis occurs. This is usually caused by a disease, a bad nutrition over a long period of time, or an excessive alcohol/drug use… and the consequences can be felt over your general health: allergies, fatigues, frequent diseases, headache, skin problems, muscular pain.
The question you might be asking yourself at this point is How does our body regulate its pH? Most fluid from our body regulates its pH on its own. All of them regulate it the same way: they are buffer solutions. A buffer solution is a solution containing a very high quantity of both an acid and a base, so that when you add any acid or base to the solution, it immediately reacts with the acid or base in the solution. That way, the solution’s pH does not change (or extremely slightly) when adding an acid or a base.
As an example, let’s consider the blood. The pH of the human’s blood is approximately 7.4. It might change slightly, but if it changes by more than 0.6, it is likely that you die. Don’t worry though, your body has been keeping it right since you were born! Two of the chemicals that keep your blood’s pH constant are the carbonic acid (H2CO3) and its conjugate base, the bicarbonate (HCO3-). The carbonic acid is produced through the cellular respiration. Once in your blood, some of it dissociates to bicarbonate, its conjugate base. Your blood contains a lot of these two chemicals, so much than whenever an acid or a base is added to the blood, it immediately reacts with either the carbonic acid or the bicarbonate, just like in any other neutralization reaction involving an acid and a base.
Here is the equation of the reaction:
The reaction also produces water and carbon dioxide, that are evacuated from your blood just like if you had drunk it (for the water) or breathed it in (for the carbon dioxide).
What about agriculture?
Knowing about acids and bases and neutralization reactions will be helpful in any domain related to agriculture. That is because in any type of agriculture, the first thing you need is a good soil specific for what you might grow on that soil. In fact, not all plants can grow on any soil, because not all plants need the same nutriments, and not every nutriments are going to be found on any soil. Now you might think that this has nothing to do with acids and bases, we use fertilizer to give the right nutriments to the plant. That’s right, but fertilizer also has a lot of acids or bases in it. In fact, the point of using fertilizer is mainly to raise or lower the pH of the soil.
Each plant needs a set pH to grow, as Steve Turner says, “Most people understand [pH] as whether the soil is acidic (less than 7.0), alkaline (more than 7.0), or neutral (7.0), and […] different plants prefer different ranges, but why? In different pH’s, nutrients move at different rates through the soil. The more acidic the soil, the faster the rate. If a plant is acid-loving and is in alkaline soil, it will not get nutrients at a fast enough rate.”
As you now know, the pH of an aqueous solution is set by the amount of acid and base reacting together (through the neutralization reaction). Knowledge of this chemistry concept will be quite helpful for a soils 101 course for example, it is a course you will take if you study agriculture.
A nice example of the effect of the soil’s pH on the plants are Hydrangeas. Their blooms’ color change depending on the soil’s pH, more specifically on the quantity of Aluminum cations in the soil are directly related. In an acidic soil, Aluminum cations will be present in higher quantity, and they will be less present in an alkaline soil. These aluminum cations are responsible for the Hydrangea bloom color: if there is a lot of aluminum cations, the color of the blooms will be blue. If not, it will be pink.
This concludes this post, now remember acids and bases react together in a neutralization reaction. The quantity of acid and base present define the pH of your blood, a soil, or so much more things or solutions, and pH defines wether your blood will keep you alive, or wether a plant will grow or die…
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Long, Franklin A. and Boyd, Richard H. (2014). Acid and base. In AccessScience. McGraw-Hill Education. Retrieved from LINK
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Tripp, Williams. (2012). Hydrangea blooms turn colors based on soil pH levels. In Media Newswire. Retrieved from LINK
Stehouwer, Richard. (2016). Soils 101 Syllabus. In PennState College of Agricultural Sciences. Retrieved from LINK
Turner, Steve. (2011). Soil Basics 101: From pH to microbes, learn about the lifeblood for your plants. In Michigan Gardener. Retrieved from LINK