Analytical Chem istry - DePauw University

456 Analytical Chemistry 2.0microburetagar gel membranesample dropheptaneFigure 9.25 Experimental set-up for a diffusionalmicrotitration. The indicator is amixture of bromothymol blue and bromocresolpurple.Another approach carries out the acid–base titration in a single drop ofsolution. 9 The titrant is delivered using a microburet fashioned from a glasscapillary micropipet (Figure 9.25). The microburet has a 1-2 mm tip filledwith an agar gel membrane. The tip of the microburet is placed within adrop of the sample solution, which is suspended in heptane, and the titrantis allowed to diffuse into the sample. The titration’s progress is monitoredusing an acid–base indicator, and the time needed to reach the end pointis measured. The rate of the titrant’s diffusion from the microburet is determinedby a prior calibration. Once calibrated the end point time can beconverted to an end point volume. Samples usually consisted of picolitervolumes (10 –12 liters), with the smallest sample being 0.7 pL. The precisionof the titrations was usually about 2%.Titrations conducted with microliter or picoliter sample volumes requirea smaller absolute amount of analyte. For example, diffusional titrationshave been successfully conducted on as little as 29 femtomoles (10 –15moles) of nitric acid. Nevertheless, the analyte must still be present in thesample at a major or minor level for the titration to be performed accuratelyand precisely.Ac c u r a c yindicator’s color changeSee Figure 3.5 in Chapter 3 to review thecharacteristics of macro–major and macro–minorsamples.When working with a macro–major or a macro–minor sample, an acid–base titration can achieve a relative error of 0.1–0.2%. The principal limitationto accuracy is the difference between the end point and the equivalencepoint.9 (a) Gratzl, M.; Yi, C. Anal. Chem. 1993, 65, 2085–2088; (b) Yi, C.; Gratzl, M. Anal. Chem.1994, 66, 1976–1982; (c) Hui, K. Y.; Gratzl, M. Anal. Chem. 1997, 69, 695–698; (d) Yi, C.;Huang, D.; Gratzl, M. Anal. Chem. 1996, 68, 1580–1584; (e) Xie, H.; Gratzl, M. Anal. Chem.1996, 68, 3665–3669.