Citric Acid-based Systems

To widen the application of PEGMC, our lab set out to develop an injectable, porous, and strong citric acid based-composite, which could be used as a delivery vehicle for cells and drugs in bone tissue engineering applications. PEGMC was combined with various wt.-% of HA to create PEGMC/HA composites. 

Biodegradable Injectable Systems for Bone Tissue Engineering 

HA hydrogel after 7 days. (B) Mineralization in SBF for PEGMC/HA composite with 40 wt.% HA at 7 days. (C) 10 pm section of PEGMC scaffold showing the porosity created from a gas foaming technique. 

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The pH of the micellar mobile phase is usually buffered using the phosphoric or citric acid-base systems. Potassium ion caimot be used with SDS, as potassium dodecyl sulfate precipitates from aqueous solutions due to its high Krafft point (see Chapter 2). The column should be equilibrated by purging with the mobile phase until the pH before and after the column is identical. Only one peak is observed in the chromatograms of weak acids and bases because prototropic equilibria are much faster than the solute-micelle or solute-stationary phase dynamics. 

Aqueous solutions of citric acid make excellent buffer systems when partially neutralized because citric acid is a weak acid and has three carboxyl groups, hence three p-K s. At 20°C pifj = 3.14, pi 2 4.77, and = 6.39 (2). The buffer range for citrate solutions is pH 2.5 to 6.5. Buffer systems can be made using a solution of citric acid and sodium citrate or by neutralizing a solution of citric acid with a base such as sodium hydroxide. In Table 4 stock solutions of 0.1 Af (0.33 N) citric acid are combined with 0.1 Af (0.33 N) sodium citrate to make a typical buffer solution. 

Acidulants. The other component of any system of chemical leavening based on sodium hydrogen carbonate is an acid. The original acidulants were sour milk (lactic acid), vinegar (acetic acid), lemon juice (citric acid) and cream of tartar (potassium acid tartrate). All of these will react immediately on mixing so that the carbon dioxide is released straight away. The product had to be baked before the carbon dioxide escaped from the batter or product. The only delay possible was that allowed by the batter viscosity. 

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

There have been several investigations of plutonium uptake by plants and several authors have reviewed the various articles (124-126). Based on studies with plant-soil systems direct root uptake into the plant appears to be low. Concentration factors for (pCi/g dry acceptor)/(pCi/g donor) are of the order 10-5 to 10 4 (127). However, variations in the chemical form of plutonium and the presence of chelating agents in the soil can result in the concentration of more plutonium in the plant. Over five years Romney et aL (33) observed that the plutonium concentration in ladino clover increased from 3.1 dpm/g for the first year to 22.6 dpm/g in the fifth year. It should be noted that clovers release citric acid into the soil (113). 

The conditions used in the hydrochloride formation/crystallization step were somewhat different when MIBK/water was used in place of ethyl acetate. In particular, hydrochloride formation needed to be carried out by adding concentrated hydrochloric acid to the MIBK solution of dilevalol base at ca. 55°C (versus ca. 25°C for ethyl acetate). In this way, oiling out of the hydrochloride salt was avoided. A small amount of citric acid was included in the crystallization system to chelate any traces of iron which may be introduced. The amount of water in the system is more than sufficient to dissolve the small amount of citric acid—in early versions of the process, using much less water, precipitation of some citric acid caused a slight discoloration of the dilevalol hydrochloride. The crystallization conditions were carefully chosen to produce a crystal which filtered and washed well, which dried well (to MIBK <0.5%) and which gave a bulk density (ca. 0.3 g/ml) which met Pharmaceutical Development s criteria for operation of their tabletting process. 

A better understood acid/base sensing reaction can be found in the detection and monitoring of ammonia either in air or in water. Two systems have been explored in great detail one using citric acid as the sensing moiety and the other using poly (4-vinyl phenol). 

Acid-base reactions between alkali metal bicarbonate and citric or tartaric acid have been used for many years to produce pharmaceutical preparations that effervesce as soon as water is added. In such systems, it is practically impossible to achieve much more than an atmospheric saturation of the solution with respect to the released carbon dioxide. If the acid dissolves first, then the bulk of the reaction takes place in the saturated solution in close proximity to the undissolved bicarbonate particles. If the bicarbonate dissolves faster, the reaction essentially takes place near the surface of the undissolved acid. Such suspension systems do not favor supersaturation with respect to carbon dioxide because the particulate solids act as nuclei for bubble formations. 

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