Now I know you’re going to think I’m crazy, but please bear with me as this is the best way, I know how to explain how organic acids pH and pKa in feed work.
You should read this if you don’t want your time and money you use on organic acids to go to waste by not getting the desired effects. Go ahead and get yourself a cup of coffee. It shouldn’t take longer than a quick cup of coffee to read through this and the coffee will aid in the understanding – go ahead I’ll wait.
Got your coffee? Great let’s get started.
Your coffee is a solution containing water, coffee and, if you are like me, a bit of milk. Now I don’t know if you use sugar or sweetener, but imagine you do. You can either add one of those tiny little sweetener pills to your coffee, a teaspoon of brown sugar or a sugar cube to sweeten your coffee.
Let’s say you get a phone call and your coffee has cooled a bit and is 75°C by the time you add the sweetener pill. Those little pills are so sweet that you can taste it immediately on your next sip. The pill rapidly dissolves and released its sweetness. Before you know it the whole little pill is dissolved, and your coffee has reached its equilibrium – it doesn’t get sweeter.
Your next cup of coffee is the same solution at the same temperature. This time you add a teaspoon of brown sugar. The sugar doesn’t dissolve as much as the little pill, in fact at this temperature exactly half of it dissolves. When you taste it, it is not as sweet as your first cup. It also has reached equilibrium, the only way you can make your coffee sweeter is by doubling the dose of sugar or by adding a different sweetener like the strong little pill. Doing this will bring your coffee solution to a new sweeter equilibrium. For the clever ones out there – no, you don’t have spoon.
By the time you have read all of this you are ready for your third cup. In this third cup you add a sugar cube, this dissolves even less than the other two sweeteners and of course the coffee is less sweet when it reaches equilibrium.
Now, if your coffee had been an iced coffee at a temperature of 15°C, less of each of the sweeteners would dissolve. The little pill would still have dissolved the most, but at this temperature only half of it will dissolve. The brown sugar and sugar cube will dissolve even less than before. Conversely, a hot cup of coffee at 100°C would result in a faster and a higher level of dissolution of each sweetener. Here the brown sugar dissolves more than half and the sugar cube dissolves exactly half.
Ok so what does all of this have to do with acids, pH and pKa?
Well we agreed your coffee is a solution and the temperature affects how each molecule of the sweeteners dissolves. To bring this back to acids think of the temperature concept as the pH of your solution. In other words, the pH of a solution is like the temperature of your coffee, it affects how your sweeteners, or acids, dissociate in your solution.
Now because the little pill has the strongest ability to sweeten it is like a strong acid. It dissociates rapidly and you taste the dissolved sweetness immediately. So, at a pH of “75°C” your acid is almost completely dissociated. Your taste buds are like a pH meter, they measures sweetness. In the case of acids, the sweetness are H+ molecules or protons released from the acid and pH is a measurement of the concentration of sweetness. pH is measurement of the H+ or more accurately the logarithm of H+ concentration.
Your coffee with brown sugar is a weaker acid, at “75°C” it was only half dissolved. pKa is exactly this, the pH at which half of an acid is dissociated. That means that pKa of brown sugar is “75°C“. Finally, the sugar cube doesn’t dissolve much at that pH and mostly remains intact. To dissolve more you need a higher temperature – a higher pH. So we can say that the sugar cube has a higher pKa than the brown sugar.
Now, what if we lower the pH to “15°C” like in the iced coffee. At this lower pH, less of each of the acids H+ molecules are released. For the pill there is still a lot of sweetness, a lot of H+. However, at this point we have lowered the pH to a level where only half of the pill is dissociated – this means that the pills pKa is at pH “15°C”. The same is true for sugar cube, but at a pH of “100°C” half of the cube is dissolved. The pKa of the sugar cube would be at pH “100°C”. At this pH all the H+ will be released for the pill, most of from the brown sugar and exactly half of the sugar cube.
One final step is to close the circle is to connect all of this to acids.
Your coffee is a solution that can be like anything from drinking water or liquid feed to feed in the stomach. In our coffee example the sweetener pill is phosphoric acid that has a pKa of 2,15. This means at a pH below 2,15 less than half of the acid is not dissociated. By increasing the pH more and more of the acid dissociates and the greater the concentration of H+ molecules. Brown sugar is formic acid that has a pKa of 3,75 and propionic acid has a pKa of 4,88.
All of this is important when we talk about using organic acids in feed, because acids have different properties and effects depending on whether they are dissociated or not. For example, it is only the undissociated acid that has an antibacterial effect, while the pH in the feed, drinking water or the stomach affects how they will work in the animal.
By now you have finished your coffee, but stay tuned for our next article that is about how acid binding capacity can kill the effect of your acidifiers.
Dissociation levels of three acids at the same pH
Propionic acid has a pKa of 4,88 so the majority of the acid is undissociated