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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Weight-in-Volume (W/V) Solutions

When we describe a concentration as a percentage without specifying the type of formula, we imply that the solution is to be made using the weight-in-volume (w/v) method. As with w/w, weight-in-volume is a simple type of formula for describing the preparation of a solution of solid material in a liquid solvent. This method can be used to describe any solution, but is commonly used for simple saline solutions and when the formula weight of the solute is unknown, variable, or irrelevant, which often is the case with complex dyes, enzymes or other proteins. Solutions that require materials from natural sources often are prepared w/v because the molecular formula of the substance is unknown and/or because the substance cannot be described by a single formula.

A one percent solution is defined as 1 gram of solute per 100 milliliters final volume. For example, 1 gram of sodium chloride, brought to a final volume of 100 ml with distilled water, is a 1% NaCl solution. To help recall the definition of a 1% solution, remember that one gram is the mass of one milliliter of water. The mass of a solute needed to make a 1% solution is 1% of the mass of pure water of the desired final volume. Examples of 100% solutions are 1000 grams in 1000 milliliters or 1 gram in 1 milliliter.

Let's make a 0.85% solution of sodium chloride-a solution often called physiological saline-by combining 0.85 gram of NaCl in a final volume of 100 ml of high quality water. We may have a use for this solution because sodium and chloride ions are the principal inorganic ions in vertebrate blood. This means they are the principal inorganic ions in the fluid in tissues that surrounds our cells. The concentration of NaCl in blood is about 0.85%, so if we are working with organs, tissues, or cells, we use such solutions (often containing other components in lower concentrations) to keep cell membranes from deteriorating and to maintain an osmotically neutral environment.

Solutions that we use for organs, cells, and tissues generally will be high in sodium, since sodium is the principal cation among the electrolytes in extracellular fluid, the liquid that is outside of cells. Solutions that we use for organelles are often high in potassium, since potassium is the principal cationic electrolyte in cytoplasm.