Preparation of buffer solutions

A mixture of a weak acid and its salt (that is, a conjugate base), or a weak base and its conjugate acid, has the ability to reduce the large changes in pH which would otherwise result from the addition of small amounts of acid or alkali to the solution. The reason for the 

Added OH ions are removed by combination with the weak acid to form undissociated water molecules  

The buffering action of a mixture of a weak base and its salt arises from the removal of H ions by the base B to form the salt and removal of OH ions by the salt to form undissociated water  

The concentration of buffer components required to maintain a solution at the required pH may be calculated using equation (3.70). Since the acid is weak and therefore only very slightly ionised, the term [HA] in this equation may be equated with the total acid concentration. Similarly, the free A ions in solution may be considered to originate entirely from the salt and the term [A ] may be replaced by the salt concentration. 

By similar reasoning, equation (3.72) may be modified to facilitate the calculation of the pH of a solution of a weak base and its salt, giving 

Materials used in the preparation of buffer solutions should be good quality laboratory chemicals, purified if necessary as described in Chapter 8 and dried to constant composition. The distilled water used as solvent should have been recently boiled to remove dissolved carbon dioxide and have been protected from contamination by atmospheric carbon dioxide while cooling. (This precaution is unnecessary in preparing buffer solutions having pH values less than 5.) The water should have a specific conductivity of less than 2 X 10 ohm cm at 25°C. Solutions should be stored in stoppered Pyrex (or similar borosilicate glass) or pure polyethylene bottles. 

It is important that specified concentrations be adhered to in preparing buffer solutions. Attempts to use stronger or more dilute solutions than specified will introduce error, even when the concentration ratio of the buffer species remains constant. This is because of changes in the ionic strength and, consequently, a change in the practical pA a value of the buffer acid. Where buffer solutions must be diluted, the effect on the pH of the solution can be calculated to within a few hundredths of a pH unit from the Debye-Hiickel equation, as described in Chapter 2. 

By appropriately varying the volume ratios of these solutions the pH of the buffer system can be varied over the useful buffer range while maintaining a constant ionic strength. (The total moles of NaOH must not exceed the amount of amine hydrochloride present.) 

For alkaline solutions, including amines, borax and some phosphates, it is necessary to guard against absorption of carbon dioxide. Detailed procedures for preparing and handling carbon dioxide-free solutions of alkalis are described by Albert and Seijeant (1971). 

The primary buffer standards — phthalate, phosphate and borax solutions — are prepared by direct solution of weighed amounts in water and dilution to a known volume. Solutions of acid or alkali used in the preparation of buffers should be standardized by titration. Even so, to guard against any inadvertent error, the user is strongly advised to check with a pH meter the pH values of buffer solutions at the temperature for which they are required. 

When a strong base such as NaOH is added to the original buffer solution, it is neutralized by the more acidic component, NH4 (the conjugate add of ammonia). 

Because the [NH ] is high, this can occur to a great extent. The result is the neutralization of OH by NH,,  

Summary Changes in pH are minimized in buffer solutions because the basic component can react with added H30 ions (producing additional weak acid), while the acidic component can react with added OH ions (producing additional weak base). 

Buffer solutions can be prepared by mixing other solutions. When solutions are mixed, the volume in which each solute is contained increases, so solute concentrations change. These changes in concentration must be considered. If the solutions are dilute, we may assume that their volumes are additive. 

Buffer Preparation by Mixing of a Conjugate Acid-Base Pair 

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The method of preparation of buffer solutions with the certain acidity value from HMTA and HCl aqueous solutions is offered. It is recommended to use the equations ... 

Reagents. Cyclic nucleotides (3, 5 -cyclic adenosine monophosphate (c-AMP), 3, 5 -cyclic guanosine monophosphate (c-GMP), and 3, 5 -cyclic inosine monophosphate (c-IMP)) sodium tetraborate hydrochloric acid and potassium hydroxide were purchased from Sigma Chemical Company, St. Louis, Missouri). Millex disposable filter units (0.22 pm) were obtained from Millipore Corporation (Bedford, Massachusetts). Triply distilled and deionized water was used for the preparation of buffer solutions. Both buffers and samples were routinely degassed with helium after filtration (using microfilter units). 

The isolation of human hepatocytes is described according to Hengstler et al. (Hengstler 2000). Preparation of buffer solutions ... 

It is unfortunate that there has been so little work devoted to quantitative measurements of cation-pseudobase equilibria in methanol and ethanol since these media have several advantages over water for the determination of the relative susceptibilities of heterocyclic cations to pseudobase formation. The enhanced stability of the pseudobase relative to the cation in alcohols compared to water is discussed earlier this phenomenon will permit the quantitative measurement of pseudobase formation in methanol (and especially ethanol) for many heterocyclic cations for which the equilibrium lies too far in favor of the cation in aqueous solution to allow a direct measurement of the equilibrium constant. Furthermore, the deprotonation of hydroxide pseudobases (Section V,B) and the occurrence of subsequent irreversible reactions (Sections V,C and D), which complicate measurements for pKR+ > 14 in aqueous solutions, are not problems in alcohol solutions. Data are now available for the preparation of buffer solutions in methanol over a wide range of acidities.309-312 An appropriate basicity function scale will be required for more basic solutions. The series of -(substituted phenyl)pyridinium cations (163) studied by Kavalek et al.i2 should be suitable for use as indicators in at least some of the basic region. The Hm and Jm basicity functions313 should not be assumed90 to apply to methoxide ion addition to heterocyclic cations because of the differently charged species involved in the indicators used to construct these scales. 

Preparation of Buffer Solutions.—The buffer capacity of a given acid-base system is a maximum, according to equation (77), when there are present equivalent amounts of acid and salt the hydrogen ion concentration is then equal to and the pH is equal to pfca. If the ratio of acid to salt is increased or decreased ten-fold, i.e., to 10 1 or 1 10, the hydrogen ion concentration is then lOfca or O.IAto, and the pH is pA a — 1 or pfca + 1, respectively. If these values for cn are inserted in equation (76), it is found that the buffer capacity is then... 

For details concerning the preparation of buffer solutions, see Clark, The Determination of Hydrogen Ions, 1928, Chap. IX Britton, Hydrogen Ions, 1932, Chap. XI Kolthoff and Rosenblum, Acid-Base Indicators, 1937, Chap. VIII. 

The Common Ion Effect and Buffer Solutions 19-2 Buffering Action 19-3 Preparation of Buffer Solutions 19-4 Acid-Base Indicators... 

Fasman, G. D., ed. Handbook of Biochemistry and Molecular Biology Physical and Chemical Data Section, 2 vols., 3rd ed. Cleveland, OH Chemical Rubber Company, 1976. [Includes a section on buffers and directions for preparation of buffer solutions (vol. 1, pp. 353-378). Other sections cover all important types of biomolecules.]... 

This equation is commonly used for the preparation of buffer solutions at a given pH. When ionic strength effects are important, should be used instead of in Eqs, 4-78 to 4-80. 

Stock solutions of ca 10 M of the drugs were made up weekly in the appropriate solvent and stored in the dark and under refridgeration to minimise decomposition. Analar chemicals were used in the preparation of buffer solutions, mobile phases etc and L.C. grade solvents employed in the liquid chromatographic separations. 

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