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Chapter 5 Errors In Chemical Analyses Mean, arithmetic mean, and average (x) are synonyms for the quantity obtained by dividing the sum of replicate measurements by the number of measurements in the set. where, x i represents the individual values of x making up a set of N replicate measurements. The median is the middle result when replicate data are arranged in order of size. Equal number of results are larger and smaller than the median. For an odd number of data points, the median can be evaluated directly. For an even number, the mean of the middle pair is used.

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Results from replicate determinations

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Precision Precision is the closeness of results to others that have been obtained in exactly the same way. Precision describes the reproducibility of measurements. Precision of a measurement is determined by simply repeating the measurement on replicate samples. Three terms are widely used to describe the precision of a set of replicate data: standard deviation, variance, and coefficient of variation. All these terms are functions of the deviation from the mean d i or just the deviation. d i = | x i – x |

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Accuracy Accuracy is the closeness of a measurement to the true or accepted value and is expressed by the error. Accuracy measures agreement between a result and its true value but precision describe the agreement among several measurements. We can never determine accuracy exactly because the true value of a measured quantity can never be known exactly. We need to use an accepted value. Accuracy is expressed in terms of either absolute or relative error.

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Illustration of accuracy and precision

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Absolute Error The absolute error of a measurement is the difference between the measured value and the true value. The absolute error E in the measurement of a quantity x i is given by the equation, E = x i - x t where, x t is the true or accepted value of the quantity. The negative sign indicates that the experimental result is smaller than the accepted value and the positive sign indicates that the result is larger than the accepted value.

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Relative Error Relative error of a measurement is the absolute error divided by the true value. The percent relative error is given by the expression, Relative error also expressed in parts per thousand (ppt).

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Absolute error in nitrogen determination

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Random or Indeterminate Errors The errors that affect the precision of measurement. This type of error causes data to be scattered more or less symmetrically around a mean value. Random error in a measurement is reflected by its precision. Systematic or Determinate Errors The errors that affect the accuracy of a result. This type of error causes the mean of a set of data to differ from the accepted value. A systematic error caused the results in a series of replicate measurements to be all high or all low.

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Gross Errors They usually occur only occasionally, are often large, and may cause a result to be either high or low. Gross error leads to outliers. This error causes the result differs significantly from the rest of the results. Bias measures the systematic error associated with an analysis. It has a negative sign if it causes the result to be low and a positive sign otherwise. Bias has a definite value, an assignable cause and are about the same magnitude for replicate measurements. Bias affects all the data in a set in the same way.

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How do Systematic Errors Arise? There are three types of systematic errors:- 1. Instrumental errors are caused by the imperfections in measuring devices and instabilities in their components. 2. Method errors arise from nonideal chemical or physical behavior of analytical systems. 3. Personal errors results from the carelessness, inattention, or personal limitations of the experimenter.

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Effects of Systematic Errors i) Constant Errors: Constant Errors does not depend on the size of the quantity measured ii) Proportional Errors: Proportional errors decrease or increase in proportion to the size of the sample taken for analysis. A common cause of proportional errors is the presence of interfering contaminants in the sample.

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Detecting Systematic Errors Systematic instrument errors are usually corrected by calibration. Periodic calibration of equipment is always desirable. Personal errors can be minimized by care and self-discipline. Errors that result from a known physical disability can usually be avoided by carefully choosing the method. Method errors or bias of an analytical method is estimated by analyzing standard reference materials.

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Standard reference materials are substances sold by the National Institute of Standard and Technology and Certified to contain specified concentrations of one or more analytes. The concentration of the components in these materials has been determined in one of three ways: 1. By analysis with a previously validated reference method. 2. By analysis by two or more independent, reliable measurement methods. 3. By analysis by a network of cooperating laboratories.

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Blank Determinations Blank determinations are useful for detecting certain types of constant errors. In blank determination all steps of the analysis are performed in the absence of a sample. A blank solution contains the solvent and all the reagents in an analysis but none of the sample. The results from the blank are then applied as a correction to the sample measurements. Blank determinations reveal errors due to interfering contaminants from the reagents and vessels employed in analysis.

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Home Reading Skoog et al. Fundamental of Analytical Chemistry. Chapters 5 and 6.

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