Buffer mixtures, acetic acid-acetate bicarbonate-carbonate

Living systems depend on buffers for pH control most of these are complex buffers that contain a mixture of various acids and bases. Buffers can be made from weak acids/bases and the salts of other weak acids/bases. For example, we could make an acidic buffer from acetic acid and sodium bicarbonate (NaHC03, a salt of the weak acid carbonic acid). Or, we could make a basic buffer from NH3 and NH2(CH3)2Br (the salt of the weak base di-methylamine, HN(CH3)2). But it is more difficult to calculate pH s or other concentration values from these mixed buffer systems. We have limited our discussion and detailed examples to buffers that contain a weak acid/base and the salt of icfiX. same acid/base. 

Bj Pivaloyloxymethyl D(—)-Ot-aminobenzylpenicillinate. hydrochloride To a solution of pivaloyloxymethyl D(—)-a-azidobenzylpenicillinate (prepared as described above) in ethyl acetate (75 ml) a 0.2 M phosphate buffer (pH 2.2) (75 ml) and 10% palladium on carbon catalyst (4 g) were added, and the mixture was shaken in a hydrogen atmosphere for 2 hours at room temperature. The catalyst was filtered off, washed with ethyl acetate (25 ml) and phosphate buffer (25 ml), and the phases of the filtrate were separated. The aqueous phase was washed with ether, neutralized (pH 6.5 to 7.0) with aqueoussodium bicarbonate, and extracted with ethyl acetate (2 X 75 ml). To the combined extracts, water (75 ml) was added, and the pH adjusted to 25 with 1 N hydrochloric acid. The aqueous layer was separated, the organic phase extracted with water (25 ml), and the combined extracts were washed with ether, and freeze-dried. The desired compound was obtained as a colorless, amorphous powder. 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

To extract the liberated fatty acids, 1.5 ml of 0.1 mM carbonate-bicarbonate buffer, pH 10.5 is added and the mixture is shaken for 10 s. Centrifugation for 45 min at 1500 xgin a swing-out rotor will separate the water from the lower organic phase. In a scintillation vial, 2 ml of the upper water phase are mixed with 50 pi of glacial acetic acid containing 500 pg ferric acid before the scintillation liquid is added (16 ml of Ecoscint toluene (7 1 volume). Liquid scintillation counting is done for 5 min and the LPL activity is calculated from the difference in counts between the blank and the sample vials [40]. 

Racemic amides 4 and 5 were then independently subjected to the PGA-cata-lyzed deacylation reaction. Because of the selective preference of PGA towards the (R)-enantiomer, the rate of deacylation of the (R)-enantiomer is significantly higher than the (S)-enantiomer for both 4 and 5. The result is the selective deacylation of the (R)-enantiomers. The racemic ethyl 3-(N-phenylacetyl)-5-(trimethylsilyl)-4-pentynoate 4 was placed in the phosphate buffer (pH 7.4). The pH of the medium was adjusted to 7.5. At room temperature, immobilized enzyme PGA was added to the reaction medium and the pH of this medium was maintained at approximately 7.4 using dilute potassium bicarbonate solution. The reaction was usually complete in approximately 20 hours. After completion of the reaction, the reaction mixture was extracted with ethyl acetate. The ethyl acetate extract was then extracted with dilute hydrochloric acid. The organic phase contained the (S)-amide 6a (96% yield). The pH of the aqueous phase was adjusted to 8 using potassium carbonate and the aqueous phase was extracted with ethyl acetate to obtain (R)-amine 7a (91% yield). Similarly, starting with the racemic ethyl 3-(N-phe-... 

Some titration curves have a long region of relatively flat slope. The addition of reagent in this portion of the titration cun e has little effect on the pH. Tliis flatness of the curve can be due to buffering, which occurs for mixtures of a weak acid or a weak base and its salt. It is also important for keeping solutions used for calibration at a known pH despite contamination from residue on the electrode. Some of the more commonly encountered buffer systems are acetic acid-acetate, carbonic acid-bicarbonates, and citric acid-citrates. 

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