(this post is the first in a 4-part series on electrolytes and how they influence acid-base chemistry)
Electrolyte comparisons show up in articles and publications from time to time, and are typically based on information found on the label of each product. Although probably unintentional, these comparisons usually contain a fair amount of incorrect or misleading information. These profiles may be convenient and are intended to be informative, but caution needs to be exercised when using these comparisons.
To begin with, the product profiles in these comparisons use values that have been calculated from the actual numbers of molecules of the different ingredients that make up each electrolyte product. What?! For many of us who ventured into high school chemistry class, thinking in terms of molecules can lead to flashbacks of Avogadro's number and the incomprehensible "mole". Talk about a panic attack. But don't fret, you are in good company.
It's actually the use of these principles that leads to problems with electrolyte evaluations and comparisons - not just in published comparisons, but across the board. The intent of this post is to shed some light on how these problems are generated, and how to avoid the pitfalls they create.
The term or unit of measure most commonly used in electrolyte evaluations is millimole, abbreviated mmol. Simply put, this is a measure of the concentration of a substance, such as sodium, chloride, or glucose that is dissolved in a solution. The term osmolarity is often used when talking about mmol concentrations.
To evaluate an electrolyte product, we need to work with the solution the calf actually consumes, not just what's in the package on the shelf. We start with the dry product, add water according to the label instructions and then evaluate. This is all figuratively speaking, of course -- we do this on paper, not in an actual bucket. The label provides a list of ingredients and a guaranteed analysis. The Ingredient List is an accounting of the ingredients used to make the product, whereas the Guaranteed Analysis states the concentrations of various ingredients or specific nutrients provided in the dry product. Sometimes the details in the Guaranteed Analysis can be a bit skimpy, especially if a manufacturer is protecting a proprietary formula. That's understandable since they may have a significant investment in the product and don't want to give the formula away. In such cases, a few phone calls or emails to manufacturers may be necessary to provide sufficient detail for each product.
The example electrolyte product shown below contains six ingredients which are listed in the left-hand column. The right-hand column shows the mmol/liter concentration of each ingredient in the final solution. The other columns are mathematical steps along the path to our mmol objectives. (If you're interested in the math, a sample calculation is provided at the end of this post.)
This is the standard approach to evaluating electrolytes. There is, however, one slight problem. Several of the ingredients (sodium chloride, sodium citrate, sodium bicarbonate and potassium chloride) don't exist in the electrolyte solution that the calf drinks. Once these substances come into contact with water, they dissociate into their base components. In this case, we're left with sodium, chloride and potassium (which are called strong ions) as well as citrate and bicarbonate.
This dissociation has profound effects on acid-base balance in the body (and will be the subject of future posts) as well as having profound effects on the osmolarity of the solution. For example, the actual impact of the sodium chloride in this formula on osmolarity is 114 mmol/L, not 57 mmol as shown in the table. Sodium citrate, sodium bicarbonate and potassium chloride actually contribute 16, 154 and 44 mmol/L, respectively. Dissociation of sodium citrate yields four molecules (three sodium and one citrate), whereas sodium chloride, sodium bicarbonate and potassium chloride dissociate into two molecules each. Glucose and glycine stay the same since they don't dissociate.
Some electrolyte comparisons go further and provide a total mmol/L value for the different electrolyte products being compared. These values can be quite inaccurate and misleading. Our example electrolyte shows a total mmol/L value of 354. The actual osmolarity of the product is 522 mmol/L. So be cautious about using these numbers.
The following table accounts for ingredient dissociation and provides a more accurate description of the electrolyte solution consumed by the calf. It allows for correct assessments of osmolarity, and results in a higher degree of accuracy when evaluating electrolytes.
g/L: (% formula/100) x (oz. powder/(liquid vol. x 0.95)) x 28.35
mmol/L: (g/L/molecular wt.) X 1000
glycine: (4/100) x (3.2/(2 x 0.95)) x 28.35/75.07 x 1000 = 25.4mmol/L
Note: The molecular weight of dextrose/glucose listed in the table is greater than the actual molecular weight of this sugar, which is typically listed as 182. The larger number accounts for the high moisture level of commercially available dextrose/glucose used in electrolyte products. Using the higher molecular weight provides a more accurate assessment of the osmolarity of this ingredient in the final solution.
Other posts in this series:
Feb 15, 2011, Electrolytes - Dissociation of Strong Ions
Mar 23, 2011, Electrolytes - Alkalinizing Agents
May 4, 2011, Electrolytes - Dependent & Independent Variables
Labels: electrolyte series