Concentration Determination through Standard Solution Addition

2 Concentration Determination through Standard Solution Addition (Positions A B) 

Here again the electrode potential at position A is determined by the measured ion activity in the sample stream. The electrode potential at point B in the flow diagram is determined by the sum of the measured ion activities in the sample and comparison solutions, provided that a constant indicated ion activity is supplied in the comparison stream and that the dilution upon combining the two streams is taken into account. 

Naturally, in place of a comparison solution with constant measured ion activity, a reagent solution can be added which lowers the measured ion activity by a specific amount through precipitation or complexation of the indicated ion (subtraction method). 

This procedure allows the determination of species which are not directly indicated by an electrode, but which react quantitatively with some indicated ion. The comparison solution contains an ion indicated by an ion-selective electrode in position C The ion to be measured quantitatively combines (precipitate, complex) with this indicator ion. A lower activity of this indicator ion results at point B, Examples of this are the determination of Al ions as already mentioned, of sulfate ions by means of two lead-selective electrodes and a Pb(C104)2 standard solution or phosphate ions with two fluoride-selective electrodes and a L2l(NOs)s standard solution. The calculation corresponds to that of the subtraction method. 

The accuracy of all these more or less direct potentiometric procedures can amount to within 2 to 5% for monovalent and 4 to 10% for divalent ions, depending on the interfering ion levels. 

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Distillation ofhydrazoic acid from strong acid solutions is the most common method of separation [12, 17], For small quantities of azide ion in relatively large volumes of solvent, evaporation in alkaline media or carrier precipitation is necessary for the preliminary concentration of azide ions prior to distillation. The distillations are usually made from perchloric acid solutions. A 50-ml round-bottom flask, with a side-arm attached (so that a stream of inert gas passes through the solution) and an air condenser is preferable for distilling hydrazoic acid. The addition of a diluent carrier gas provides added safety. Absorbing solutions for the hydrazoic acid include known, excess quantities of ceric ion. standard base, or a known quantity of ferric perchlorate for colorimetric determination of small quantities of azide ion. The distillation separation is complex and lengthy, but the method is reliable and has universal applicability. 

Standard curves were constructed by plotting the change in absorbance with time (dA/dt) versus concentration of ATI or ATF. Standards for the assay were prepared by dilution of stock solutions of ATI or ATF to the appropriate concentrations (0,25,50,100,175, and 250ng/mL for ATI 0,5,10,20,50, and lOOng/mL for ATF) with PBS (pH 7.4), with the addition of 1% (v/v) blank rat plasma. Assays were validated with respect to precision and accuracy, by analysis of QC samples at 25,100,250ng/ml for ATI and 5,20, lOOng/ml for ATF, respectively. Intra-assay and inter-assay variability were determined through the analysis of QC samples. 

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

The conventional sodium distillation method(sodium distilled off in vacuum and the residue after suitable dissolution analysed for the impurities) was foimd to be suspect for the determination of boron. Standard addition studies did not yield quantitative recoveries. Hence an alternative method was standardised which involves dissolution of sodium in high purity water under argon atmosphere and the resulting sodium hydroxide solution was passed through an ion exchange column to selectively remove the B and Sb from the sodium. The eluate was concentrated and analysed by ICP-MS. The method was standardised using die standard addition technique. The detection limits of the method for B and Sb was found to be 0.1 pg/g of sodium. Primary sodium samples received from FBTR were analysed using the above method. 

MDI method For determination of 4,4 -diphenylmethane diisocyanate in the atmosphere, 5 litres of air should be drawn through 3 ml of 0-4n hydrochloric acid. After the addition of three drops of a 0-6% (w/v) sodium nitrite solution six drops of 10% (w/v) sulphamic acid should be added and the mixture transferred to a separating funnel containing 2 ml of 1 n sodium hydroxide solution and six drops of a suspension of 0-5 g of 2-hydroxy-3-naphthoic anilide (Brenthol AS) in 50 ml water. This mixture is then shaken and 1 ml of 6n sulphuric acid followed by 3 ml of chloroform are added. This is shaken and the chloroform layer separated off. The colour of this chloroform layer is then compared with standard colour solutions prepared by mixing a solution of cobaltous chloride (as above) with a solution of 32 g anhydrous ferric chloride, 25 ml concentrated hydrochloric acid (SG 20°C 1-18) and 975 ml water. The proportions of these two solutions equivalent to different levels of atmospheric diisocyanate are given in Table 11.3. 

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