Physiologic Solutions, Buffers, Acid-Base Indicators, Standard Reference Materials, and Temperature Conversions

Physiologic Solutions

A physiologic solution is one that contains various salts in concentrations that closely approximate the composition of fluids in the human body. The simplest of these is physiologic saline, which has the same osmotic pressure as the blood. There are more elaborate solutions, for example, to maintain tissues in a metabolically active state for longer periods of time. Table A1.1 lists formulas of some solutions that are isotonic with respect to blood.

TABLE A1.1

Physiologic Solutions

	Saline	Locke’s Solution	Ringer’s Solution	Tyrode’s Solution
Sodium chloride	0.85 g	0.9 g	0.7 g	0.8 g
Calcium chloride		0.024 g	0.0026 g	0.02 g
Potassium chloride		0.042 g	0.035 g	0.02 g
Sodium bicarbonate		0.01–0.03 g		0.1 g
d -Glucose		0.01–0.25 g		0.1 g
Magnesium chloride				0.01 g
Monosodium phosphate				0.005 g
Distilled water	100 mL	100 mL	100 mL	100 mL

Buffers ∗

Buffers have the ability to resist changes in pH. Buffers usually consist of a weak acid and its salt or a weak base and its salt. The Henderson-Hasselbalch equation is useful in calculating the ratio of acid (or base) to salt required to establish a desired pH from a buffer system:

pH = pK + log [salt] / [acid]

$pH = pK + log [salt] / [acid]$

∗ For a comprehensive discussion, including preparation of buffer solutions of a definite ionic strength, consult Bates RG: Determination of pH—theory and practice, ed 2, New York, 1973, John Wiley and Sons.

Example 1

If the pH of a 0.1-M acetate buffer is known to be 4.90, calculate the concentration of acetic acid and sodium acetate in the buffer (pK for acetic acid = 4.76).

Substituting values of pH and pK in Equation A1.1 :

\begin{matrix} log [acetate] / [acetic acid] = 4.90 - 4.76 = 0.14 \\ [acetate] / [acetic acid] = 1.38 \\ [acetate] = 1.38 [acetic acid] \end{matrix}

$\begin{matrix} log [acetate] / [acetic acid] = 4.90 - 4.76 = 0.14 \\ [acetate] / [acetic acid] = 1.38 \\ [acetate] = 1.38 [acetic acid] \end{matrix}$

Because the total buffer/L concentrations is 0.1 M,

[acetate] + [acetic acid] = 0.1 M

$[acetate] + [acetic acid] = 0.1 M$

Substituting the value of acetate in Equation A1.2 :

1.38 [acetic acid] + [acetic acid] = 0.1 M

$1.38 [acetic acid] + [acetic acid] = 0.1 M$

Hence,

\begin{matrix} [acetic acid] = 0.042 M \\ [acetic acid] = 2.52 g / L \\ [acetate] = 0.058 M = 4.76 g / L \end{matrix}

$\begin{matrix} [acetic acid] = 0.042 M \\ [acetic acid] = 2.52 g / L \\ [acetate] = 0.058 M = 4.76 g / L \end{matrix}$

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