Capacity buffer

Buffer capacity measures how well a solution resists changes in pH when acid or base is added. The greater the buffer capacity, the smaller the pH change. We will find that buffer capacity is maximum when pH = pKafor the buffer. 

When 0.010 0 mol of OH is added to this mixture, the concentrations and pH change in a manner that you are now smart enough to calculate  

In choosing a buffer, you should seek one whose pK is as close as possible to the desired pH. The useful pH range of a buffer is approximately pK 1 pH unit. Outside this range, there is not enough of either the weak acid or the weak base to react with added base or acid. Buffer capacity increases with increasing buffer concentration. To maintain a stable pH, you must use enough buffer to react with the acid or base expected to be encountered. 

A buffer is a mixture of a weak acid and its conjugate base. The buffer is most useful when pH pAa. Buffer pH is nearly independent of concentration. A buffer resists changes in pH because it reacts with added acid or base. If too much acid or base is added, the buffer will be consumed and will no longer resist changes in pH. 

The higher bicarbonate ion concentration in blood makes the buffer capacity of blood greater for acid than for base, which is necessary because the products of metabolism that enter blood are mostly acidic. For example, when we exercise, our bodies produce lactic acid (HC3H5O3). The lactic acid enters the bloodstream and must be neutralized. 

The bicarbonate ion neutralizes the lactic acid according to the equation  

An enzyme called carbonic anhydrase then catalyzes the conversion of carbonic acid into carbon dioxide and water  

We eliminate the carbon dioxide from our blood when we breathe. When large amounts of lactic acid are produced, we must breathe faster to keep up with the need to eliminate carbon dioxide. This is why we pant when we exert ourselves. 

A 70 kg person has a total blood volume of about 5.0 L. Given the carbonic acid and bicarbonate concentrations stated, what volume (in mL) of 6.0 M HCl can be neutralized by blood without the blood pH dropping below 7.0 (which would result in death)  

If HCl is added to a weak acid there is a considerable change in pH showing that a weak acid on its own has no buffering capacity. The effect, however, is not as great as that observed when a strong acid or base is added to water. A solution of a weak acid consists of both the undissociated weak acid, HA(aq) and its conjugate base A (aq). 

After the strong acid has been added, nearly all of the weak acid will be in the form of unionised HA - suppression of the ionisation - and the pH will be given approximately by the concentration of the strong acid. A large decrease in pH will result, but this will not be so large as is found when strong acid is added to water. 

In effect, the term buffer capacity means the range of pH over which a buffer is effective. The commonly held view is that this range lies within p T 1- The table and graph below illustrate the buffering capacity of the ethanoic acid/sodium ethanoate buffer, and allow a judgement to be made as to whether the accepted range, 1, is reasonable. pK for ethanoic acid is 4.76 at 25°C. 

The graph shows that, around pH = p/ifa, the change in pH for a given fraction of HA converted to A is smallest and that there is good buffering capacity in this region. As the 

Note when L. = 1 then pH = pTiTa, and this corresponds to half-way to the end point in a 

Buffer solutions work best at controlling pH at pH values roughly equal to the pi Ca of the component acid or base, i.e. when the [SALT] is equal to the [ACID]. This can be shown by calculating the ability of the buffer to resist changes in pH, which is the buffer capacity. 

The buffer capacity is defined as the number of moles per litre of strong monobasic acid or base required to produce an increase or decrease of one pH unit in the solution. When the concentrations of salt and acid are equal, the log term in the Henderson-Hasselbalch equation becomes the logarithm of 1, which equals 0. To move the pH of the buffer solution by one unit of pH will require the Henderson-Hasselbalch equation to become 

It will require addition of more acid or base to move the pH by one unit from the point where pH = pKa than at any other given value of the ratio. This can be neatly illustrated by the following example. 

Suppose one litre of buffer consists of 0.1 M CH3COOH and 0.1 m CH3COO Na+ the pH of this buffer solution will be 4.7 (since the log term in the Henderson-Hasselbalch equation cancels). Now, if 10 mL of 1 M NaOH is added to this buffer, what will be the new pH  

Clearly, the 10 mL of NaOH will ionise completely (strong alkali) and some of the 0.1 M acetic acid will have to convert to acetate anion to compensate. The new pH will be 

We saw in the previous two examples how buffer systems damper external pH actions. In example 5-G the initial concentration is 10 times lower than in example 5-H. This means that by addition of equivalent amounts of and OH, the change in pH becomes different. The larger the initial concentration of the buffer components are, the less pH will change by the addition of equivalent amounts of strong acid and strong base respectively. In other words the buffer capacity of the solution increases with the concentration of the buffer components. 

Calculate the change in pH that occurs when 0.010 mol of gaseous HC1 is added to 1.0 L of each of the following solutions. 

For both solutions the initial pH can be determined from the Henderson-Hasselbalch equation  

In each case [C2H302 ] = [HC2H302]. Thus, for both A and B, 

After the addition of HCl to each of these solutions, the major species before any reaction occurs are 

Will any reactions occur among these species Note that we have a relatively large quantity of H+ that will readily react with any effective base. We know that Cl- will not react with H+ to form HCl in water. However, C2H302-will react with H+ to form the weak acid HC2H302  

Given the relations that the sum of [A ] + [HA] remains constant at some concentration, c, and that the sum of the 

The quantity, d[B]/dpH, is a quantitative measure of the buffering ability of a solution. It was introduced by Van Slyke (1922) as the buffer unit or buffer value , and is also known as the buffer capacity . It is the reciprocal of the slope of the pH-neutralisation curve. 

Example Compare the buffer capacities at pH 7.4 of the Tris buffer given in Table 10.32 with the HEPES buffer given in Table 3.6. 

For HEPES, pATg l.SS and c = 0.05M. A similar calculation gives d[B]/dpH = 0.028. Hence HEPES is a more effective buffer than Tris at this pH. 

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Phosphoric Acid. This acid is the primary acidulant in cola beverages. Phosphoric acid is stronger than most organic acids and weaker than other mineral acids. The dibasic properties of phosphoric acid provide minor buffering capacity in the beverage. Food-grade phosphoric acid is commercially available in concentrations of 75%, 80%, and 85% and is one of the most economical acidulants. 

Flue Ga.s Desulfuriza.tion. Citric acid can be used to buffer systems that can scmb sulfur dioxide from flue gas produced by large coal and gas-fired boilers generating steam for electrical power (134—143). The optimum pH for sulfur dioxide absorption is pH 4.5, which is where citrate has buffer capacity. Sulfur dioxide is the primary contributor to acid rain, which can cause environmental damage. 

The filter media should have buffering capacity in order to maintain a pH of at least 3. This is especially a concern when inorganic compounds are targeted for reduction by the biofilter. 

When a forest system is subjected to acid deposition, the foliar canopy can initially provide some neutralizing capacity. If the quantity of acid components is too high, this limited neutralizing capacity is overcome. As the acid components reach the forest floor, the soil composition determines their impact. The soil composition may have sufficient buffering capacity to neutralize the acid components. However, alteration of soil pH can result in mobilization or leaching of important minerals in the soil. In some instances, trace metals such as Ca or Mg may be removed from the soil, altering the A1 tolerance for trees. 

Between various wood species great differences can occur in pH as well as in the buffer capacity. Even within the same wood species, differences might occur due to seasonal variations, portion of the wood substance under investigation, pH of the soil, age of the tree, time span after cutting, drying and processing parameters. 

The water extracts from particles made from freshly harvested wood have higher pH-values, but lower buffer capacities than surfaces made from stored chips. The former might lead to prehardening using the usual amount of hardener with the consequence of a decrease of the board strength. 

Process Conceptual Design Equipment selection and sizing Inventory of process Single vs. Multiple trains Utility requirements Overdesign and flexibility Recycles and buffer capacities Instrumentation and control Location of plant Preliminary plant layout Materials of construction As above plus equipment suppliers data, raw materials data, company design procedures and requirements... 

Holma, B. (1985). Influence of buffer capacity and pH-dependent rheological properties of respiratory mucus on health effects due to acidic pollution. Set. Total Environ. 41, 101-123. 

A second way to achieve constancy of a reactant is to make use of a buffer system. If the reaction medium is water and B is either the hydronium ion or the hydroxide ion, use of a pH buffer can hold Cb reasonably constant, provided the buffer capacity is high enough to cope with acids or bases generated in the reaction. The constancy of the pH required depends upon the sensitivity of the analytical method, the extent of reaction followed, and the accuracy desired in the rate constant determination. 

FIGURE 2.15 A buffer system consists of a weak acid, HA, and its conjugate base, A. The pH varies only slightly in the region of the titration curve where [HA] = [A ]. The unshaded box denotes this area of greatest buffering capacity. Buffer action when HA and A are both available in sufficient concentration, the solution can absorb input of either H or OH, and pH is maintained essentially constant. 

The pH then remains relatively constant. The components of a buffer system are chosen such that the of the weak acid is close to the pH of interest. It is at the that the buffer system shows its greatest buffering capacity. At pH values more than one pH unit from the buffer systems become ineffec-... 

The buffer capacity of the pit fluid is equal to the change in alkalinity of the system per unit change of pH. Figure 4-491 shows the buffer intensity (capacity) of a 0.1 M carbonate pit fluid. Calculating the initial buffer capacity of the pit fluid allows for prediction of the pH change upon introduction of live acid and also any addition of buffer, such as sodium bicarbonate, required to neutralize the excess hydrogen ions. 

The reasons why some anions exhibit strong inhibitive properties while others exhibit strong aggressive properties are not entirely clear. The principal distinction seems to be that inhibitive anions are generally anions of weak acids whereas aggressive anions are anions of strong acids. Due to hydrolysis, solutions of inhibitive anions have rather alkaline pH values and buffer capacities to resist pH displacement to more acid values. As discussed... 

The buffer capacity indicates how much OH or H+ ions a buffer can react with. What is the buffer capacity of the buffers in Problem 9 ... 

Buffer capacity Amount of strong acid or base that can be added to a buffer without causing a drastic change in pH, 390 Buret, 7... 

A solution containing equal concentrations of acid and its salt, or a half-neutralised solution of the acid, has the maximum buffer capacity . Other mixtures also possess considerable buffer capacity, but the pH will differ slightly from that of the half-neutralised acid. Thus in a quarter-neutralised solution of acid, [Acid] = 3 [Salt] ... 

In general, we may state that the buffering capacity is maintained for mixtures within the range 1 acid 10 salt and 10 acid l salt and the approximate pH range of a weak acid buffer is ... 

The concentration of the acid is usually of the order 0.05-0.2 mol L" Similar remarks apply to weak bases. It is clear that the greater the concentrations of acid and conjugate base in a buffer solution, the greater will be the buffer capacity. A quantitative measure of buffer capacity is given by the number of moles of strong base required to change the pH of 1 litre of the solution by 1 pH unit. 

The monoethanolamine-hydrochloric acid buffer has a buffering capacity equal to the ammonia-ammonium chloride buffer commonly employed for the titration of calcium and magnesium with EDTA and solochrome black (compare Section 10.54). The buffer has excellent keeping qualities, sharp end points are obtainable, and the strong ammonia solution is completely eliminated. 

R FS. 0.05m Potassium hydrogenphthalate. Dissolve 10.21 g of the solid (dried below 130 °C) in water and dilute to 1 kg. The pH is not affected by atmospheric carbon dioxide the buffer capacity is rather low. The solution should be replaced after 5-6 weeks, or earlier if mould-growth is apparent. 

Buffer action 46 Buffer capacity 48 Buffer mixture universal, (T) 831 Buffer solutions 46, (T) 831 acetic acid-sodium acetate, 49 for EDTA titrations, 329 preparation of IUPAC standards, 569 Bumping of solutions 101 Buoyancy of air in weighing 77 Burette 84, 257 piston, 87 reader, 85 weight, 86... 

Fruit and vegetable juices packed with 21-26 in. of vacuum and stored in uncoated aluminum cans caused severe corrosion as shown in Table III. The corrosion rate brought about by the juices depends more on the nature of the organic acid present and the buffering capacity of the juice than on the total titratable acidity (11). The use of coated aluminum containers considerably minimized corrosion problems. Product control under extended storage conditions may be achieved by using specific chemical additives. However, more work is needed in this area before final conclusions can be reached. 

Low pH corrosion includes Low pH general corrosion Results from acid breakthrough into boiler water with only limited buffering capacity... 

Where process industries operate very variable steam demand operations, the use of a steam accumulator may provide adequate buffering capacity and minimize steam quality problems. Good operational practices include ... 

NOTE Where coordinated phosphate programs are employed, the entire rationale is designed to inhibit the presence of free caustic. As a consequence, the alkalinity buffering capacity is severely reduced and the tolerance for silica is likewise diminished. 

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