Características de los sedimentos

ppm ppt soluble solute solution solvent sparingly soluble specific gravity (v/v) (w/v) (w/w)

A solution is a homogeneous mixture of two or more substances that do not chemically react with each other. Because the distribution of substances is uniform throughout a homogeneous mixture, a solution’s composition, appearance, and properties are the same in any one portion as they are in any other portion. The substance present in the largest amount is called the solvent, whereas every other component of the solution is referred to as a solute. As a mixture forms, the solute is said to dissolve in the solvent. In the clinical laboratory, the most common solvent is water, and a solution of anything in water is referred to as aqueous.

A solution may consist of a solid and a liquid, a common example being hot tea sweetened with sugar. An example from the clinical laboratory is normal saline, which is a well-defined solution of sodium chloride in water. The solids in these cases are soluble, meaning they dissolve in water. Insoluble solids do not dissolve in water, whereas sparingly soluble solids dissolve only to a small degree.

A solution may be composed of two liquids, common examples of which are (1) rubbing alcohol, which is a mixture of isopropyl alcohol and water, and (2) vinegar, which is a mixture of acetic acid and water. In such cases, the liquids are miscible with each other, meaning that they are capable of being mixed in any ratio without separating. Two immiscible liquids, such as gasoline and water, do not form solutions but instead separate on standing, with the less dense liquid rising to the top.

A solution may comprise a gas dissolved in a liquid. Two common examples are (1) sparkling water, which is just an aqueous solution of carbon dioxide gas, and (2) household ammonia, which is an aque- ous solution of ammonia gas.

Depending on the test we are conducting, serum and other body fluids can be regarded as any or all of the three kinds of solution outlined above. For example, when we quantify alcohol in blood, the serum is a liquid-in-liquid solution. However, we can view it as a solid-in-liquid solution when glucose is the analyte of interest or as a gas-in-liquid solution when the partial pressure of oxygen is being determined. This chapter focuses on the calculation and expression of concentration, which is a measure of how much solute and solvent are present in a solution. It is difficult to exaggerate the importance of master- ing this material because the consequences of reporting an incorrect concentration on a patient sample can be grave, as, for example, when it leads to the misdiagnosis of an illness or the administration of an inappropriate drug. Nearly every liquid in a clinical laboratory—from reagents to control solutions, from serum to spinal fluid, from disinfectants to cleaning agents—has a concentration that we must specify correctly and unambiguously.

Quantifying an analyte in a patient sample requires that the concentration of every reagent used in the assay be accurate to an acceptable degree. Although some reagents do come ready to use from the manufac- turer, others must be prepared in the laboratory directly before use. Erroneous concentrations can even have repercussions not directly related to patient results. Consider two examples. First, the concentration of an unconsumed reagent that has expired may determine whether the solution is poured down the drain or con- signed to hazardous waste—a decision that affects both the environment and the laboratory’s budget. Second, there is the issue of laboratory hygiene. We commonly use bleach as a general disinfectant for surfaces in the laboratory. The required concentration, however, depends on the purpose; it may be as high as 10% for surface disinfection or as low as 0.5% for decontaminating the tubing in an automated instrument. If the concentra- tion is too low, the bleach will fail to disinfect thoroughly; if too high, it might damage expensive parts. When bleach is an unsuitable disinfectant, alcohol serves as an alternative, most effective when its concentration is 70%; it is markedly less bactericidal at higher or lower concentrations. Thus, a solution labeled “70%” that was improperly prepared will lack the expected disinfecting power, possibly creating a risk to laboratory personnel.

ExprEssing ConCEntration

There are many ways to express concentration, some deriving from convenience and others from tradi- tion; it can be based on mass, volume, or number of moles.

Percentage

The following are the three common systems for expressing concentration as a mass or volume percentage.

• Weight of solute per volume of solution (w/v). In this system, the value is the number of grams of solute in 100 mL of solution. For example, “10% NaCl (w/v)” describes an aqueous solution of 10 grams of sodium chloride in every 100 mL of solution. Understand that this is not the same as 10 grams of NaCl in 100 mL of water (see “Molality”). Preparation of 10% NaCl entails dissolving 10 g NaCl in a small volume of water in a volumetric flask and then adding water until the volume is 100 mL.

• Weight of solute per weight of solution (w/w). In this system, the value is the number of grams of solute in 100 grams of solution. Thus, 6% KOH (w/w) is a solution of 6 grams of sodium hydroxide in 100 grams of solution. In simple terms, it is prepared by dissolving 6 g KOH in a small volume of water in a suitable vessel resting on a balance and then adding water until the weight of the solution is 100 g.

• Volume of solute per volume of solution (v/v). Used for liquid solutes, this system gives the num- ber of milliliters of solute in 100 mL of solution. Therefore, 70% ethanol (v/v) is a solution of 70 mL of ethanol in 100 mL of solution, prepared by transferring 70 mL of ethanol into a volumetric flask and then adding water until the volume is 100 mL. Because we can weigh liquids, of course, we may also express the concentration of a liquid-in-liquid solution in the other two systems (w/v, w/w). At this point, it should be clear that expressing concentration just as a percentage (e.g., “10% C6H12O6” ) is inadequate because it does not give the basis for the ratio. Therefore, it is necessary

to specify the system in a suffix: “10% C6H12O6 (w/w).”