Concentration of acids and bases

In the new polar organic mode, the ratio of acid/base in the mobile phase affects the selectivity and the concentration of acid and base controls the retention. It is suggested to start the method development with a medium concentration (0.1 %) for both acid and base. If retention is too long or too short, the concentration can be increased to 1 % or reduced to 0.01 %. If no selectivity is observed in this mode, reversed phase is recommended as the next step in the protocols. 

SOLUTION Because each acid molecule provides two protons, if the concentrations of acid and base were the same, the titration would require a volume of base equal to twice the volume of acid. The volume of base added to reach the stoichiometric point is less than twice the volume of acid so we can expect that the acid is less concentrated than the base. Proceed as in Toolbox L.2. 

The values of [HA] and [A ] in this expression are the equilibrium concentrations of acid and base in the solution, not the concentrations added initially. However, a weak acid HA typically loses only a tiny fraction of its protons, and so [HA] is negligibly different from the concentration of the acid used to prepare the buffer, [HA]initia. Likewise, only a tiny fraction of the weakly basic anions A- accept protons, and so [A-] is negligibly different from the initial concentration of the base used to prepare the buffer. With the approximations A ] [base]initia and [HA] [acid]initia, we obtain the Henderson-Hasselbalch equation ... 

From Table 10.1, the pKa for formic acid is 3.75. Assuming little change in the concentrations of acid and base due to the deprotonation of HCOOH, we write... 

The accuracy and precision with which the end-point can be determined potentiometrically solely depends upon the quantum of change in the observed e.m.f. in the vicinity of the equivalence point, which in turn entirely depends upon the strength and the concentration of acid and base employed. 

Decide what type and concentration of acid and base you will add to your systems. Dilute solutions of strong acids and bases work well. 

The pH of pure (and also not so pure) water is very sensitive to small concentrations of acids and bases. One drop of concentrated sulphuric acid added to a liter of water will change the pH by 4 pH nnits (from 7 to ca. 3). Solntion pH can be stabilized by a buffer (although there may be cases where a stable pH is not desirable) addition of (not too large) quantities of acid or base to a buffered solution will not affect the pH mnch. Buffers are usually mixtures of weak acids or bases and their salts. A common example in CD is the nse of an ammoninm salt (NH4X ) to control the pH of an ammonia solntion. The equilibrium of ammonia in water is given by... 

Beyond simple pH and pOH lie titrations and buffers. Titrations allow you to determine the concentration of acids and bases. Buffers maintain the pH of a solution by reacting to changes and neutralizing them. 

Acids and bases are two of the most common types of substances in the laboratory and the everyday world. We need to know how to recognize them, what their characteristic reactions are, and why they are such important chemicals. We shall see that keeping the concentrations of acids and bases in plant and animal cells within certain limits is necessary for the survival of individual organisms, and controlling the acidity of rain, of natural waters such as lakes and rivers, and of municipal water supplies is necessary to sustain human societies. 

Early work with aqueous solutions containing ionic solutes in a 1 1 mixture of water and glycerol showed that factors such as the pH of the solution and salt content had significant and reproducible effects on the distribution of ionic species measured by the mass spectrometer. Using the Henderson-Hasselbalch equation under simplifying conditions (at low ionic strengths with acid components whose pKa s lie between 3 and 10), it was shown that the pKa of an acid could be accurately determined knowing the pH of the solution and the concentrations of acid and base species (2). With respect to the measurement of this constant by FABMS,... 

Problems in the practical application of bipolar membrane electrodialysis In addition to the precipitation of multivalent ions in the base containing flow stream and the stability of the ions in strong acids and bases a serious problem is the contamination of the products by salt ions that permeate the bipolar membrane. In particular, when high concentrations of acids and bases are required the salt contamination is generally high [28] as illustrated in Figure 5.13 that illustrates the conversion of... 

Other substances. The concentrations of acids and bases and alkalis could be listed as follows acetic acid > 25% concentration, hydrochloric acid > 25% concentration, nitric acid > 20% concentration, chromic acid, hydrofluoric acid, perchloric acid > 10% concentration, sulfuric acid > 15% concentration, fuming sulfuric acid, ammonium hydroxide > 35% by weight of gas, potassium hydroxide (caustic potash), sodium hydroxide > 5% concentration, aluminum chloride, bromine, phosphorous trichloride, potassium bifluoride, sodium hypochlorite > 10% concentration, and zinc chloride. 

The rate of an acid- base catalyzed reaction depends on the concentrations of acid and base in the solution, viz. (S = substrate)... 

Concentration of Acids and Bases Common Commercial Strengths... 

In Examples 15.7, 15.8, and 15.9, the eqnilibrium concentrations of acid and base in the buffer systems were close to the initial concentrations. When this is the case, the calculation of pH is simplified greatly, becanse for either an acidic or a basic buffer. 

This equation constrains the system to a single location on the plot (where the lines for these two concentrations cross in Fig. 4.1), which uniquely fixes the pH and concentrations of acids and bases in the system. In this simple system the solution is acidic (pH = 4) because the concentration of the hydrogen ion and anion must be equal. 

Figure 19-3 (a) The titration curve for 100. mL of 0.100 M HCl with 0.100 M NaOH. Note that the vertical section of the curve is quite long. The titration curves for other strong acids and bases are identical with this one if the same concentrations of acid and bases are used and if both are monoprotic. (b) The titration curve for 100. mL of 0.100 M NaOH with 0.100 M HCl. This curve is similar to that in part (a), but inverted. 

Acid and base concentrations in living systems are carefully regulated to maintain conditions compatible with normal life. Biochemical reactions involving acids and bases occur in the body water, whereas buffer systems protect the body from significant variations in the concentrations of acids and bases. This chapter introduces basic concepts of the properties of water, acids, bases, and buffers, and Chapter 39 presents a detailed discussion of both normal and pathological aspects of acid-base metabolism. 

The answer is c. (Murray, pp 15-26.) The calculation of pH requires substitution of the concentrations of acid and base into the the Henderson-Hasselbalch equation. For the given ratio of acetate and acetic acid, the pH equals the pK of 4.8 plus the log of the concentration of base/acid (2 M acetate/0.2-M acetic acid). This simplifies to ... 

Solution (a) Because the concentrations of acid and base are equal, the volume has been doubled by the addition of the HCl solution. Therefore, the concentration would have been halved, to... 

The above figures for the concentrations of acids and bases were calculated to grams per liter, and these values with the corresponding I2S values for the solubility of hydrogen plotted on cross-section paper. From the resulting curves the following table was read. 

The concentration of acids and bases can be experimentally determined by neutralization reactions. The steps in the process involve careful measurement of a volume of, for example, an acid solution of unknown concentration. Then a basic solution of well-known concentration is slowly added until the acid is totally neutralized. This process is called titration. This analytical technique depends on the following conditions (a) The reaction must go to completion, (b) the reaction must be fast, and very importantly, (c) there must be a way of detecting when the reaction is complete. How would one know when to stop adding titrating reagent The reaction is stoichiometrically complete at what is called the equivalence point. The experimentally determined point of completion is called the end point. The goal is to get the theoretical equivalence point and the experimentally determined end point to be as close as possible. End points are determined by the use of dyes which change color at... 

Buffer (a) will be a more effective buffer because the concentrations of acid and base components are ten times higher than those in (b). Thus, buffer (a) can neutralize 10 times more added acid or base compared to buffer (b). 

Since the concentrations of acid and base are equal, equal volumes of each solution will need to be added to reach the equivalence point. Therefore, the solution volume is doubled at the equivalence point, and die... 

Concentrations of acids and bases are conventionally expressed in terms of molarity when the concentrations are greater then 0.1 M. However, for very dilute solutions of acids and bases where exponential numbers are required to describe [H" ] and [OH"] in a solution, it is more convenient to use a compressed logarithmic scale for [H" ] and to express pH mathematically as... 

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