The phosphate buffer system

In this system the weak acid is sodium dihydrogenphosphate (NaH2P04) and the weak base is disodium hydrogenphosphate (Na2HP04). The action of the buffer system is similar to that of the hydrogencarbonate system. 

These two buffer systems are normally sufficient to maintain the blood pH between 7.35 and 7.45 however, there are certain organic compounds, such as amino acids and haemoglobin, that are still more effective as they are in higher concentrations in the body. 

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An alternative approach to the microbial deracemization of secondary alcohols is to use two different microorganisms with complementary stereoselectivity. Fantin et al. studied the stereoinversion of several secondary alcohols using the culture supernatants of two microorganisms, namely Bacillus stearothermophilus and Yarrowia lipolytica (Figure 5.18) [31]. The authors tested three main systems for deracemization. First, they used the supernatant from cultures of B. stearothermophilus, to which they added Y. lipolytica cells and the racemic alcohols. Secondly, they used the culture supernatant of Y. lipolytica and added B. stearothermophilus cells and the racemic alcohols. Finally, they resuspended the cells of both organisms in phosphate buffer and added the racemic alcohols. The best results were obtained in the first system with 6-penten-2-ol (26) (100% ee and 100% yield). The phosphate buffer system gave... 

Moreover, several buffer systems exist in the body, such as proteins, phosphates, and bicarbonates. Proteins are the most important buffers in the body. Protein molecules contain multiple acidic and basic groups that make protein solution a buffer that covers a wide pH range. Phosphate buffers (HPO T /H2P07) are mainly intracellular. The pK of this system is 6.8 so that it is moderately efficient at a physiological pH of 7.4. The concentration of phosphate is low in the extracellular fluid but the phosphate buffer system is an important urinary buffer. Bicarbonate (H2C03/HC0 3) is also involved in pH control but it is not an important buffer system because normal blood pH 7.4 is too far from its pK 6.1 [144],... 

Two especially important biological buffers are the phosphate and bicarbonate systems. The phosphate buffer system, which acts in the cytoplasm of all cells, consists of H2POT as proton donor and HPOf as proton acceptor ... 

The phosphate buffer system is maximally effective at a pH close to its pKa of 6.86 (Figs 2-16, 2-18) and thus tends to resist pH changes in the range between about 5.9 and 7.9. It is therefore an effective buffer in biological fluids in mammals, for example, extracellular fluids and most cytoplasmic compartments have a pH in the range of 6.9 to 7.4. 

In the case of cesium no significant variations in [mJ0 were observed for (HA)W at least up to 1 g/L, neither in the acetate nor in the phosphate buffer system. Assuming the existence of 1 1 complexes only and taking the original sodium content in the sodium humate (27) as a measure for sites available for cesium complexa-tion, we find log 8 <1.2,... 

Since biological systems are dynamic, with many different processes taking place and many different substances present, buffers are necessary to prevent the kind of wide variation of pH that can inhibit proper enzyme catalysis. Thus, a proper pH aids in regulating the reaction rates associated with certain enzymes and maintaining them at levels appropriate for their particular functions. Two important biological buffers are the phosphate buffer system that regulates pH for the fluid inside cells and the carbonic acid buffer system that regulates pH for blood plasma. The chemical equations for these buffers are shown below for an aqueous solution. 

The phosphate buffer system consists of serum inorganic phosphate (3.5 to 5 mg/dL), intracellular organic phosphate, and calcium phosphate in bone. Extracellular phosphate is present only in low concentrations, so its usefulness as a buffer is limited however, as an intracellular buffer, phosphate is more useful. Calcium phosphate in bone is relatively inaccessible as a buffer, but prolonged metabolic acidosis will result in the release of phosphate from bone. 

An average rate of metabolic activity produces roughly 22,000 mEq acid per day. If all of this acid were dissolved at one time in unbuffered body fluids, their pH would be less than 1. However, the pH of the blood is normally maintained between 7.36 and 7.44, and intracellular pH at approximately 7.1 (between 6.9 and 7.4). The widest range of extracellular pH over which the metabolic functions of the liver, the beating of the heart, and conduction of neural impulses can be maintained is 6.8 to 7.8. Thus, until the acid produced from metabolism can be excreted as CO2 in expired air and as ions in the urine, it needs to be buffered in the body fluids. The major buffer systems in the body are the bicarbonate-carbonic acid buffer system, which operates principally in extracellular fluid the hemoglobin buffer system in red blood cells the phosphate buffer system in all types of cells and the protein buffer system of cells and plasma. 

The reaction of HIL and methylglyoxal at pH 5 performed in water yielded 2 8 mol% sotolone compared to 7.4 mol% when using the phosphate buffered system This suggests a catalytic effect of phosphate on the formation of sotoione from HIL. 

The phosphate buffer system is common in the laboratory in vitro, outside the living body) as well as in living organisms (in vivo). The buffer system based on TRIS [tm(hydroxymethyl)aminomethane] is also widely used in vitro. Other buffers that have come into wide use more recently are zwitterlons, which are compounds that have both a positive charge and a negative charge. Zwitterions are usually considered less likely to interfere with biochemical reactions than some of the earlier buffers (Table 2.8). 

The concentration of phosphate is low in the extracellular fluid but the phosphate buffer system is an important urinary buffer. Bicarbonate (H2C03/HC0 3) is also involved in pH control but it is not an important buffer system because normal blood pH 7.4 is too far from its pK 6.1 [144]. 

There are also several mechanisms by which our body maintains the pH around 7.4. Some of these mechanisms use simple standard chemistry, some are more complex. These mechanisms are (i) the carbonic acid-bicarbonate buffer system, (ii) the protein buffer system, and (iii) the phosphate buffer system. Apart from these buffers, the pH of our body is also maintained by exhalation of carbon dioxide, elimination of hydrogen ions via the kidneys, etc. 

The presence of the phosphate buffer system H2PO4 /HPO4 usually keeps urine from going much below pH 6, but too great an excess of protons from carbonic acid can exceed the buffer capacity of the system and result in quite acidic urine. 

The body uses the phosphate buffer system shown in the following reaction (a). 

Fig. 16E shows an inset of the magnitude of the pulse signal as a function of the chosen pH point for sensing (the pH point is the pH value where the analyte redox system exhibits a reversible potential consistent with the applied sensor potential). It can be seen that the signal is higher at pH 9.5 and a decrease occurs toward pH 7 and towards pH 12. This is caused by the phosphate buffer system. At the point of lowest buffer capacity (= pH 9.5) the maximum sensor response is detected. This establishes a secondary filter where the choice of the sensor potential can enhance or eliminate redox responses dependent on the type of buffer medium and the reversible potential of the analyte. 

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