Lead, colorimetric analytical methods

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

Determination in Biological Fluids and Tissues All the advances in pharmacokinetics and drug metabolism described in Sections 7 and 8 would not have been possible without the availability of the proper analytical methods. The following is a tabulation of publications in this field, most of which have already been discussed in Section 5. It should be mentioned that a few publications talk about aspirin blood levels, but really mean salicylate levels. The following tabulation covers only those papers where aspirin was differentiated from other salicylates by chromatography or other means. It seems that the "workhorse" for serum salicylate levels is still the colorimetric (ferric-nitrate) method of Brodie, Udenfriend and Coburn153 published in 1944, or modifications thereof. Simplified versions (cf. 206) may lead to erroneous results under certain conditions.207 The method is also applicable for urinary metabolites after proper hydrolysis (cf. 208). For other methods restricted to salicylic acid, see Section 5.61. 

For many years the use of dithizone as a colorimetric reagent for lead was almost standard practice. Nowadays it is rather rarely used because of a range of disadvantages. These include the rather modest sensitivity and the possibility of interference from other metals [1]. In particular, the time-consuming nature of the method and the considerable experience necessary to achieve reliable results discourage many potential users. The method may still however be of use in laboratories lacking instrumentation for the other analytical methods. 

Analytical method. In order to measure colorimetrically the bulk concentration of lead ion in the aqueous suspension of y-AI2O3 containing lead nitrate, the interfering particles were filtered with Millipore filter (Millipore Filter Corp. GSWP 04700 with pore size of 0.22 ym). The colorimetric determination of lead ion was performed at X = 510 nm by the dithizone extraction method. 

Historically, lead concentrations were routinely measured with colorimetric methods using spectrophotometry. Currently, the primary spectrophoto-metric method for measuring lead is with dithizone (diphenylthiocarba-zone). It is a relatively simple and inexpensive technique, but it is relatively insensitive. The accuracy and precision of this, as well as other colorimetric measurements of lead, are also highly dependent upon the skills of the analyst. Moreover, colorimetric measurements of lead in biological materials require the same level of ultraclean techniques that are necessary for more sensitive analytical methods. 

Electropolymerization of a functionalized precursor represents a straightforward method for the realization of modified electrodes endowed with specific electrochemical or optical properties. Electrode materials based on electrogenerated functional conjugated polymers and their application as electrochemical sensors have been already reviewed [162-165]. On the other hand, the sensitivity of the optical properties of ir-conjugated polymers to conformational changes has led to the realization of colorimetric sensors for the detection of various analytes extending from alkali metal ions to anions and biomolecules [165-168]. In general, the realization of sensors based on functional PTs relies on the fact that complexation at a side chain may lead to perturbation of the polymer conformation, which can be read by either electrochemical or optical methods. 

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