Ionisation buffer

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

For procedure (ii) an ionisation buffer is required and this involves preparing a potassium stock solution (10000 mg L ). Dissolve 9.6 g of potassium chloride in de-ionised water and make up to 500 mL in a graduated flask. 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

The addition of an easily ionisable element at relatively high concentration as an ionisation buffer allows one to reduce the effect of shifts in ionisation equihbria. The ionisation buffer (often an alkali metal salt such as potassium chloride) creates a high concentration of electrons in the flame, resulting in suppression of the ionisation of the analyte. 

To prevent ionisation interferences, cesium is added to act as an ionisation buffer. 

Borate-gluconate eluant. Prepare a buffer concentrate by dissolving the following substances in water and making up to 1 L with distilled, de-ionised... 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

Procedure. Prepare an ammonia-ammonium chloride buffer solution (pH 10), by adding 142 mL concentrated ammonia solution (sp. gr. 0.88-0.90) to 17.5 g ammonium chloride and diluting to 250 mL with de-ionised water. 

Pipette 25 mL hickel solution (0.01 M) into a conical flask and dilute to 150 mL with de-ionised water. Add about 15 drops of the indicator solution, 10 mL of the buffer solution and titrate with standard EDTA solution (0.01 M) until the colour changes from blue to claret red. 

Buffer solution. Add 55 mL of concentrated hydrochloric acid to 400 mL de-ionised water and mix thoroughly. Slowly pour 310 mL of redistilled monoethanolamine with stirring into the mixture and cool to room temperature (Note 2). Titrate 50.0 mL of the standard magnesium chloride solution with standard (0.01M) EDTA solution using 1 mL of the monoethanolamine-hydrochloric acid solution as the buffer and solochrome black as the indicator. Add 50.0 mL of the magnesium chloride solution to the volume of EDTA solution required to complex the magnesium exactly (as determined in the last titration), pour the mixture into the monoethanolamine-hydrochloric acid solution, and mix well. Dilute to 1 litre (Note 3). 

Procedure. Prepare an EGTA solution (0.05M) by dissolving 19.01 g in 100 mL sodium hydroxide solution (1M) and diluting to 1 L in a graduated flask with de-ionised water. Prepare the indicator by dissolving 0.065 g zincon in 2 mL sodium hydroxide solution (0.1M) and diluting to 100 mL with de-ionised water, and a buffer solution (pH 10) by dissolving 25 g sodium tetraborate, 3.5 g ammonium chloride, and 5.7 g sodium hydroxide in 1 L of de-ionised water. 

Pipette 25 mL of the solution containing magnesium, manganese and zinc ions (each approx. 0.02M), into a 250 mL conical flask and dilute to 100 mL with de-ionised water. Add 0.25 g hydroxylammonium chloride [this is to prevent oxidation of Mn(II) ions], followed by 10 mL of the buffer solution and 30-40 mg of the indicator/potassium nitrate mixture. Warm to 40 °C and titrate (preferably stirring magnetically) with the standard EDTA solution to a pure blue colour. 

Details for the preparation of the solutions referred to in the table are as follows (note that concentrations are expressed in molalities) all reagents must be of the highest purity. Freshly distilled water protected from carbon dioxide during cooling, having a pH of 6.7-7.3, should be used, and is essential for basic standards. De-ionised water is also suitable. Standard buffer solutions may be stored in well-closed Pyrex or polythene bottles. If the formation of mould or sediment is visible the solution must be discarded. 

For most purposes it is not necessary to follow the procedures given above for the preparation of standard buffer solutions the buffer tablets which are available from laboratory suppliers, when dissolved in the specified volume of distilled (de-ionised) water, produce buffer solutions suitable for the calibration of pH meters. 

Total Ionic Strength Adjustment Buffer (TISAB). Dissolve 57 mL acetic acid, 58 g sodium chloride and 4g cyclohexane diaminotetra-acetic acid (CDTA) in 500 mL of de-ionised water contained in a large beaker. Stand the beaker inside a water bath fitted with a constant-level device, and place a rubber tube connected to the cold water tap inside the bath. Allow water to flow slowly into the bath and discharge through the constant level this will ensure that in the... 

Ionic strength adjuster buffer 565, 570 Ionisation constants of indicators, 262, (T) 265 of acids and bases, (T) 832, 833, 834 see also Dissociation constants Ionisation suppressant 793 Iron(II), D. of by cerium(IV) ion, (cm) 546 by cerium(IV) sulphate, (ti) 382 by potassium dichromate, (ti) 376 by potassium permanganate, (ti) 368 see also under Iron... 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

C. N. Partitioning of ionising molecules between aqueous buffers and phospholipid vesicles. J. Pharm. Sci. 1995, 84, 1180-1183. 

Buffer solutions are used to control retention and selectivity in the chromatography of ionisable solutes, and in addition the chemical nature of the buffering agent can affect secondary equilibria, eg interaction of the solute with silanol groups (see Section 4.2). 

The acids are both partly ionised in aqueous solution. What would be the effect on the dissociation of the acids if they were made up in (c) an acid buffer (b) an alkaline buffer ... 

Adding a common ion (eg H + ) shifts the equilibrium to the left, so if the pH of the buffer is low enough the weak acids will be present as neutral molecules. Similarly, we can force the acids to ionise completely if the pH is high enough. In between, both forms will be present. 

We want the acids to be present only as ions, as the two forms of each acid will have different retentions on the ion-exchange stationary phase. As a rough guide, to suppress ionisation completely, we want to buffer at pH = (pKa - 1.5) and to cause complete ionisation we need pH = (p/Ca + 1.5). 

In a pH 5.7 ethanoate buffer, the two weak acids will be fully ionised. In this buffer, separation will occur because of competition for the stationary phase exchange sites between the ethanoate ions and the acid anions ... 

The pyrophosphate buffer is of particular technical interest as it can be used over the relatively wide range of pH 3-9. Unlike the orthophosphate titration curve, that for the tetrabasic pyrophosphate system is almost straight [6]. This linearity (Figure 10.3) means that effective buffering action is available across the whole pH range simply by using various pairs of ionised components and varying their proportions even so, however, it does not seem to be widely used. 

TSI. The liquid is converted into a vapour jet and small droplets are generated with the help of a heated vapouriser tube. A buffer dissolved in the eluent assists the ionisation process through the formation of adduct ions, which are produced via statistical charging of individual droplets. Due to the softness of the procedure, no structurally characteristic fragments, which could aid identification of unknown compounds, are formed. 

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