Buffer is a solid combination that can be used to control the fugacity of gas or liquid under high temperature and high pressure, or normal temperature and pressure. It can be solid or liquid. Buffers in chemical engineering are often called acid-base stabilizers, which are generally salts, such as strong acids and weak bases or weak acids and strong bases. During reaction or preservation, acids or bases in the salt are gradually released to maintain a stable acid-base value.
classification
Oxygen buffer
Oxygen buffer is a solid combination including water, which can produce constant oxygen fugacity under fixed temperature and total pressure, and is used in H-O system.
Graphite buffer
At a certain temperature and total pressure, the existence of graphite determines the various gas phase components and fugacity of the system, which is used in C-O system.
Oxygen and graphite combined buffer
For C-O-H system.
Acid base buffer
Acid base buffer is usually a solution, also known as acid-base buffer solution. It is a solution that can add a small amount of acid or alkali to the solution buffer solution, or stabilize the solution due to the acidity of the solution. If a small amount of acid or alkali is added to the solution, or a small amount of acid or alkali is produced due to the chemical reaction in the solution, or the solution is diluted slightly, the acidity of the solution is basically unchanged.
Buffer can inhibit the change of pH value of electroplating solution in electroplating. If boric acid is added to the nickel plating solution, its buffer range is close to the pH value of the plating solution of 5 ~ 6, so it can effectively inhibit the change of pH value of the solution near the cathode caused by the electroplating process. In addition, ammonium salt, acetate and citrate are also used in other electroplating solutions.
Action principle
Biological buffer
When a certain amount of acid and base is added to some solutions, it can hinder the change of pH of the solution, which is called buffering. Such a solution is called buffering solution. Mixed solutions of weak acids and their salts (such as HAC and NaAc), mixed solutions of weak bases and their salts (such as NH3 · H2O and NH4Cl) are buffer solutions. The buffering effect of buffer solution composed of weak acid HA and its salt NAA on acid is due to the existence of enough alkali a- in the solution. When a certain amount of strong acid is added to this solution, h+ ions are basically consumed by a- ions: therefore, the pH value of the solution is almost unchanged; When a certain amount of strong base is added, the weak acid HA in the solution consumes OH ions and hinders the change of pH.
Buffer capacity
When a small amount of strong acid or alkali is added to the buffer solution, the pH value of the solution changes little, but if a large amount of acid or alkali is added, the buffer solution will lose its buffering effect. This shows that its buffer capacity is limited. The buffering capacity of the buffer solution is related to the concentration of the components that make up the buffer solution. The buffer solution composed of 0.1mol · l-1hac and 0.1mol · l-1naac has greater buffer capacity than that of 0.01mol · l-1hac and 0.01mol · l-1naac. This point can be confirmed by calculation. However, the concentration of buffer solution components should not be too large, otherwise, the interaction between ions cannot be ignored. When the ratio of the two components constituting the buffer solution is 1:1, the △ pH is the smallest, and the buffer capacity is large. The farther the ratio of buffer components is from 1:1, the smaller the buffer capacity is, and it can not even play a buffer role. For any buffer system, there is an effective buffer range, which is roughly PKA φ (or PKB φ） Within one pH unit on each side. Weak acid and its salt (weak acid and its conjugated base) system ph=pka φ ± 1 weak base and its salts (weak base and its conjugated acid) system pooh=pkb φ ± 1 e.g. pKa of HAC φ It is 4.76, so HAC and NaAc are suitable for preparing buffer solution with pH of 3.76 ~ 5.76, which has a large buffer effect in this range. When preparing buffer solution with ph=4.76, the buffer capacity is the largest, at this time, C (HAC) /c (NaAc) =1.
Preparation and Application
In order to prepare a buffer solution with a certain pH, first select a weak acid, its PKA φ As close as possible to the pH value of the buffer solution to be prepared, and then calculate the concentration ratio of acid and alkali. According to this concentration ratio, the required buffer solution can be prepared. The above mainly takes the buffer solution composed of weak acid and its salt as an example to explain its action principle, pH calculation and preparation method. The same method can be used for buffer solutions composed of weak bases and their salts.
Buffer solution is widely used in material separation and component analysis. For example, when identifying mg2+ ions, the following reaction can be used: white magnesium ammonium phosphate precipitation is dissolved in acid, so the reaction needs to be carried out in alkaline solution, but the alkalinity is too strong, and white Mg (OH) 2 Precipitation may be generated, so the pH value of the reaction needs to be controlled within a certain range. Therefore, the buffer solution composed of NH3 · H2O and NH4Cl is used to maintain the pH value of the solution, Conduct the above reaction.