tailieunhanh - Modern Analytical Cheymistry - Chapter 5

Calibrations, Standardizations, and Blank Corrections Trong chương 3, chúng tôi giới thiệu một mối quan hệ giữa các tín hiệu đo, Smeas, và số lượng tuyệt đối của chất phân tích Smeas = knA + Sreag 5,1 hoặc số lượng tương đối của chất phân tích trong một mẫu Smeas = KCA + Sreag 5,2 nơi nA là mol của chất phân tích, CA là nồng độ của chất phân tích, k là độ nhạy của phương pháp, và Sreag là sự đóng góp Smeas từ những sai lầm liên tục được giới thiệu bởi các chất phản ứng được sử dụng trong. | Calibrations Standardizations and Blank Corrections In Chapter 3 we introduced a relationship between the measured signal 5meas and the absolute amount of analyte Smeas knA Sreag 5 1 or the relative amount of analyte in a sample Smeas kCA Sreag 5 2 where nA is the moles of analyte CA is the analyteÕs concentration k is the methodÕs sensitivity and 5reag is the contribution to 5meas from constant errors introduced by the reagents used in the analysis. To obtain an accurate value for nA or CA it is necessary to avoid determinate errors affecting 5meas k and 5reag. This is accomplished by a combination of calibrations standardizations and reagent blanks. 104 Chapter 5 Calibrations Standardizations and Blank Corrections 105 5A Calibrating Signals Signals are measured using equipment or instruments that must be properly calibrated if Smeas is to be free of determinate errors. Calibration is accomplished against a standard adjusting Smeas until it agrees with the standard s known signal. Several common examples of calibration are discussed here. When the signal is a measurement of mass Smeas is determined with an analytical balance. Before a balance can be used it must be calibrated against a reference weight meeting standards established by either the National Institute for Standards and Technology or the American Society for Testing and Materials. With an electronic balance the sample s mass is determined by the current required to generate an upward electromagnetic force counteracting the sample s downward gravitational force. The balance s calibration procedure invokes an internally programmed calibration routine specifying the reference weight to be used. The reference weight is placed on the balance s weighing pan and the relationship between the displacement of the weighing pan and the counteracting current is automatically adjusted. Calibrating a balance however does not eliminate all sources of determinate error. Due to the buoyancy of air an object s weight in