Acid-base problems

Faber MD, Kupin WL, Heligi CW, Narins RG. Common fluid-electrolyte and acid-base problems in the intensive care unit selected issues. Semin Nephrol 1994 14 8-22. 

We have reduced acid-base problems to just three types. When you encounter an acid or base, decide whether you are dealing with an acidic, basic, or intermediate form. Then do the appropriate arithmetic to answer the question at hand. 

Step 1 Solve the acid-base problem with the constants Kx, K2, and K3, which apply at pi = 0. Activity coefficients are considered to be unity in this first step. 

Step 3 Solve the acid-base problem again, this time with K, K2, K 3, and K w. 

Two graphical methods described here, a master variable (pC-pH) diagram and a distribution ratio diagram, are extremely useful aids for visualizing and solving acid-base problems. They help to determine the pH at which an extraction should be performed. Both involve the choice of a master variable, a variable important to the solution of the problem at hand. The obvious choice for a master variable in acid-base problems is [H+] [equations (2.9)—(2.12)], or pH when expressed as the negative logarithm of [H+]. 

Be able to solve acid-base problems involving amino acids, draw structures at various pH values, estimate isoelectric points and net charges, and construct and interpret their titration curves. 

All acid-base problems involving aqueous solutions of weak acids (HA) and/or their corresponding base forms (A-) fall into three distinct types type 1, Solutions of acid form only (e.g., HA) type 2, solutions of base form only (e.gM A-) type 3, solutions of both acid and base forms (e.g., HA and A"). For all three types of problems, two equilibrium conditions must be satisfied ... 

Strategy for Solving Acid-Base Problems A Summary... 

Now that we have considered all the fundamental definitions relevant to acid-base solutions, we can proceed to a quantitative description of the equilibria present in these solutions. The main reason that acid-base problems sometimes seem difficult is that, because a typical aqueous solution contains many components, the problems tend to be complicated. However, you can deal with these problems successfully if you use the following general strategies. 

Be systematic. Acid-base problems require a step-by-step approach. 

Be flexible. Although all acid-base problems are similar in many ways, important differences do occur. Treat each problem as a separate entity. Do not try to force a given problem to match any you have solved before. Look for both the similarities and the differences. 

Although these procedures may seem somewhat cumbersome, especially for simpler problems, they will become increasingly helpful as the aqueous solutions become more complicated. If you develop the habit of approaching acid-base problems systematically, the more complex cases will be much easier to manage. 

PROBLEMS Additional acid-base problems may be found at the end of Chapter 8. 

Acid-base problems may be discussed by referring to the three components of the bicarbonate buffer system. In practice, blood gas analyzers measure the H l of the sample and its PCO . There is no need to measure the third parameter, the bicarbonate. By the law of mass action ... 

Using the equilibrium equations in combination with mass and charge balances and the proton condition, it is now possible for us to proceed with the solution of acid-base problems and to arrive at the concentration of each species present in such systems. 

The H and OH" concentration lines are common to the graphical solution of all acid-base problems (and many other types of equilibrium problems) so that it would be handy to prepare pC-pH diagrams with these lines already plotted on them. The first step would then be to plot pCx on the diagram. 

The procedures for calculating species concentration in a solution containing metal ions and ligands are similar to that used for acid-base problems in Chapter 4 these procedures are illustrated in the following example. 

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