Calcium complexes buffers

For deliming, ammonium salts and acids are used. The proportion of ammonium salts to acids and the type of acids employed is a matter of the tanner s choice. The acid neutralizes the lime, Ca(OH)2, thereby adjusting the pH. The ammonium salts have two functions to buffer the solution to a pH required for bating, and to form calcium ammonium complexes. The acidity and the complex formation solubilize the calcium and serve to bring the hide to the desired pH. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

Fig. 4.1.8 Influence of various calcium chelators on the relationship between Ca2 " concentration and the luminescence intensity of aequorin, at 23-25°C (panel A) in low-ionic strength buffers (I < 0.005) and (panel B) with 150 mM KC1 added. Buffer solutions (3 ml) of various Ca2+ concentrations, pH 7.05, made with or without a calcium buffer was added to 2 pi of 10 pM aequorin solution containing 10 pM EDTA. The calcium buffer was composed of the free form of a chelator (1 or 2mM) and various concentrations of the Ca2+-chelator (1 1) complex to set the Ca2+ concentrations (the concentration of free chelator was constant at all Ca2+ concentrations). The curves shown are obtained with 1 mM MOPS (A), 1 mM gly-cylglycine ( + ), 1 mM citrate (o), 1 mM EDTA plus 2mM MOPS ( ), 1 mM EGTA plus 2 mM MOPS ( ), 2 mM NTA plus 2 mM MOPS (V), and 2 mM ADA plus 2 mM MOPS (A). In the chelator-free buffers, MOPS and glycylglycine, Ca2+ concentrations were set by the concentration of calcium acetate. Reproduced with permission, from Shimomura and Shimomura, 1984. the Biochemical Society.

The calcium phosphate method was first used in 1973 to introduce adenovirus DNA into mammalian cells [3]. DNA-Calcium-phosphate complexes are formed by mixing DNA in a phosphate buffer with calcium chloride. These complexes adhere to the cell membrane and enter the cytoplasm by endocytosis. Disadvantages of DEAE-dextran and calcium phosphate transfection are a certain level of cytotoxicity, a complicated transfection procedure, and the fact that not all cell types can be transfected using these methods. 

The presence of normal concentrations of sodium, magnesium, and strontium have no net effect on the determination of calcium above the approximate level of accuracy of about 0.1% so that no correction factor seems necessary. A sufficient amount of titrant must be added to complex at least 98% of dissolved calcium before the buffer is added this apparently reduces the loss of calcium by coprecipitation with magnesium hydroxide. 

Controlled metal complexation — While uncontrolled metal complex -ation can be a major source of problems, controlled complexation is a potentially useful selectivity modifier. The key is to evaluate the effects of only one metal at a time, which in turn requires that your sample be stripped of metals in preparation for your experimental treatment. The EDTA-imidazole treatment described above can be used for this purpose. For evaluating the effects of ferric iron or calcium, buffer exchange the treated sample into 0.05 M MES, pH 6, then add the metal salt of choice to a concentration of 5 mM. For other metals, buffer exchange the treated sample into 0.05 M Tris, pH 8, then add the metal salt of choice to 5 mM. 

The selectivity is severely impeded if phosphate-based buffers are employed, since calcium-phosphate complexation occurs. In principle, the selectivity of a membrane-based ISE is constant, but contamination and degradation of the membrane s polymer component means that the selectivity decreases with usage. Contamination with biological samples can also be problematic. 

A second way to improve resolution is the modification of mobility by complexation of the analyte. Many buffers for analysis of cations use HIBA or 18-crown-6 to improve the resolution between sodium, potassium, calcium, magnesium, etc. as well as some aliphatic amines. By diluting an existing validated buffer, one can change the concentration of the complexation agent and thus also the selectivity of the system. 

These titrations arc used in the estimation of metal salts. Ethylenediamine tetracetic acid (EDTA) shown in Figure 3.10 is the usual titrant used. It forms stable 1 1 complexes with all metals except alkali metals such as sodium and potassium. The alkaline earth metals such as calcium and magnesium form complexes which are unstable at low pH values and are titrated in ammonium chloride buffer at pH 10. The general equation for the titration is ... 

Alkaline earths Distribution coefficients for Sr(II) and Ba(II) as a function of concentration of Ca(II) are summarized in Figures 13a and 13b. The loadings in the case of Sr(It) (Figure 2) are in the linear isotherm range or only slightly out of it. Slopes of log D vs log calcium concentration are close to the ideal value, -1. Invariance of between 0.01 M and 0.1 M acetate buffer indicates that there is no interference from acetate complexing. At the same Ca(II) concentration, is a factor of... 

In principle, it would be logical to combine plots of the buffer index curves of each of the buffer components of milk and thus obtain a plot which could be compared with that actually found for milk. It is not difficult, of course, to conclude that the principal buffer components are phosphate, citrate, bicarbonate, and proteins, but quantitative assignment of the buffer capacity to these components proves to be rather difficult. This problem arises primarily from the presence of calcium and magnesium in the system. These alkaline earths are present as free ions as soluble, undissociated complexes with phosphates, citrate, and casein and as colloidal phosphates associated with casein. Thus precise definition of the ionic equilibria in milk becomes rather complicated. It is difficult to obtain ratios for the various physical states of some of the components, even in simple systems. Some concentrations must be calculated from the dissociation constants, whose... 

Three approaches have been used in attempting to account for the buffer behavior of milk in terms of the properties of its components. These are calculation, fractionation, and titration of artificial mixtures. Whittier (1933A.B) derived equations for dB/dpH in calcium phosphate and calcium citrate solutions, taking into account available data on dissociation constants and solubility products. Presumably this approach could be extended to calculate the entire buffer curve. It demands precise knowledge of the dissociation constants of the several buffers, the dissociation of the calcium and magnesium complexes, and the solubility products of the calcium and magnesium phosphates under the conditions of a titration of milk. 

Charley and Saltman studied the migration of radioactive Ca2ffi in the presence and absence of lactose in aqueous solution at pH 7.0. The solutions were buffered with sodium hydrogen carbonate. The inability of calcium to migrate in the presence of lactose indicated that Ca2ffi had reacted with the sugar to form a soluble, uncharged complex. 

Based on this observation, calcium released from this complex requires high-temperature heating in combination with a calcium chelating and/or precipitating agent such as EDTA, EGTA, citrate buffer, or urea. Because these reagents are chelators of divalent... 

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