tailieunhanh - Modern Analytical Cheymistry - Chapter 14

Developing a Standard Method Trong chương 1 chúng tôi đã thực hiện một sự phân biệt giữa phân tích và phân tích hóa học. Các mục tiêu của hóa học phân tích là cải thiện phương pháp phân tích thành lập, mở rộng các phương pháp phân tích hiện có của các loại mới của mẫu, và để phát triển các phương pháp phân tích mới. Khi một phương pháp đã được phát triển và thử nghiệm, ứng dụng của nó là tốt nhất được mô tả như phân tích hóa học. Chúng tôi thừa nhận tình trạng của phương. | Developing a Standard Method I n Chapter 1 we made a distinction between analytical chemistry and chemical analysis. The goals of analytical chemistry are to improve established methods of analysis to extend existing methods of analysis to new types of samples and to develop new analytical methods. Once a method has been developed and tested its application is best described as chemical analysis. We recognize the status of such methods by calling them standard methods. A standard method may be unique to a particular laboratory which developed the method for their specific purpose or it may be a widely accepted method used by many laboratories. Numerous examples of standard methods have been presented and discussed in the preceding six chapters. What we have yet to consider however is what constitutes a standard method. In this chapter we consider how a standard method is developed including optimizing the experimental procedure verifying that the method produces acceptable precision and accuracy in the hands of a single analyst and validating the method for general use. 666 Chapter 14 Developing a Standard Method 667 14A Optimizing the Experimental Procedure In the presence of H2O2 and H2SO4 solutions of vanadium form a reddish brown color that is believed to be a compound with the general formula VO 2 SO4 3. Since the intensity of the color depends on the concentration of vanadium the absorbance of the solution at a wavelength of 450 nm can be used for the quantitative analysis of vanadium. The intensity of the color however also depends on the amount of H2O2 and H2SO4 present. In particular a large excess of H2O2 is known to decrease the solution s absorbance as it changes from a reddish brown to a yellowish Developing a standard method for vanadium based on its reaction with H2O2 and H2SO4 requires that their respective concentrations be optimized to give a maximum absorbance. Using terminology adapted by statisticians the absorbance of the solution is .