Electrolytes - Dissociation of Strong Ions

And the Role of Water in Acid-Base Regulation

(this post is the second in a 4-part series on electrolytes and how they influence acid-base chemistry)

A curious thing about electrolyte ingredients is that even though they are listed on the label, many of them don't end up in the solution we feed to the calf. There is no sodium chloride. There is no sodium bicarbonate, or sodium acetate, calcium lactate, potassium chloride or a number of other ingredients.

So...what happens to them? It's actually quite simple. They dissociate, or separate into their basic components when they dissolve in water -- or any aqueous medium such as blood plasma and other body fluids. As was pointed out in the previous post, not accounting for this dissociation can lead to sizable miscalculations when evaluating electrolyte products. This dissociation also has a profound effect inside the calf, which is the subject of this post.

A good indicator of whether or not an electrolyte ingredient will dissociate is the presence of one or more strong ions. Sodium, chloride, potassium, calcium and magnesium are all strong ions. It's pretty easy to identify which ingredients contain strong ions since their presence is indicated in the ingredient name.

Fig 1. Orientation of Water Molecules Around Strong Ion
Let's take a closer look at what happens when sodium chloride is mixed with water. Sodium and chloride provide the backbone of most electrolyte solutions - they are the major electrolytes in plasma and play a fundamental role in the metabolic regulation of acid-base balance.

Sodium has a strong positive charge associated with it while chloride has a strong negative charge. On their own, both ions would readily react with other substances - but water molecules prevent this from happening. Figure 1 shows how water molecules orient themselves around the dissociated sodium (Na+) and chloride (Cl-) ions. By orienting themselves in such a manner they isolate the ion and its charge, and prevent it from reacting with other substances. The negatively charged oxygen of the water molecule orients toward the positive sodium ion, while the positively charged hydrogens orient themselves toward the negative chloride ion.

By forming complexes with water, strong ions become chemically inert and don't enter into reactions within the body. It's easy to imagine the serious consequences that could result if strong ions were free to react and form compounds within plasma and cellular fluids -- it would certainly add a new dimension to kidney stones.

Fig 2. Attraction of OH- and H+ ions to Strong Ions In Solution
The orientation of water molecules around strong ions does more than just prevent unwanted reactions. The outward facing hydrogens of the water molecules around sodium have positive charges that attract hydroxyl ions (OH-). The water molecules around chloride have negative charges that attract hydrogen ions (H+). These attractive forces are illustrated in Figure 2.

Where do these OH- and H+ ions come from? They are the result of dissociation of nearby water molecules. Not only does water react to the presence of these strong ions by orienting themselves around each ion, water also responds to the attractive forces of the resulting strong ion/water complex by providing OH- or H+ ions depending upon which complex is present.

This is why sodium and chloride are so important to the kidneys as they work to regulate acid-base balance. By selectively removing sodium, for example, the kidneys lower the plasma OH- concentration. In effect, this increases the relative concentration of chloride and H+ in plasma, lowering pH. Conversely, by removing chloride, the kidneys lower plasma H+, which raises pH. The latter scenario is an important part of resolving acidosis.

Water readily dissociates into H+ and OH- and readily re-forms in response to changes in the solution, generating or destroying  H+ and OH- in the process. Water is the perfect medium for this purpose. No other substances are required to supply or to remove OH- or H+. No other reactions need to take place.

These characteristics of water are fundamental to acid-base chemistry. Much of the confusion and conceptual difficulties surrounding acid-base mechanisms arise from the mistaken idea that OH- and H+ must be added to or removed from the solution from the outside in order to bring about a change in acid-base status. Water is the perfect medium. It is an inexhaustible reservoir of OH- and H+ within the body and is constantly adjusting to changes within the system.

Sodium chloride is an important electrolyte ingredient, but its inclusion in the formula as the sole source of these two ions results in little change in the acid-base status of an animal. This is due to the counter-balancing effect of the two ions.  Each sodium ion added to the solution is offset by a chloride ion. To bring about a change in acid-base status, this relationship would need to change.

Electrolytes are formulated for a variety of purposes and functions. Calves that are dehydrated due to diarrhea may also suffer to some degree from acidosis, where plasma pH is below normal. To help remedy this situation and bring about an increase in plasma pH, the electrolyte should provide more sodium than chloride.

Unfortunately, strong ions cannot be added to a solution by themselves. In dry form, they are bound to some other substance which must be added to the solution along with the strong ion. In this case, where we are interested in replacing chloride, ingredients like sodium bicarbonate, sodium acetate and sodium lactate can be used to replace a portion of the sodium chloride in the formula. This provides sodium without the counterbalancing effect of chloride. Bicarbonate, acetate and lactate do not have the effect within the body that chloride does. They do not form complexes with water, they do not have the same attractive forces and they do not have the same effect on hydrogen ion concentration. They are also free to enter into reactions in the body. Sodium bicarbonate, sodium acetate and sodium lactate are sometimes referred to as alkalizing agents, which will be the subject of future posts.

Other posts in this series:
Jan 18, 2011, Electrolytes - Product Comparisons
Mar 23, 2011, Electrolytes - Alkalinizing Agents
May 4, 2011, Electrolytes - Dependent & Independent Variables