Mind-reading errors

On the other hand, it is possible to measure even in non-linear regions of the calibration curve if the optimal measuring conditions are carefully maintained, the relative error of measurement usually does not exceed about 20%. At very low concentrations, semiquantitative procedures can be employed for example, the sample is compared with standards and the direction of the drift of the unstabilized potential indicates whether the sample concentration is higher or lower than that in the standard [147, 162). It is necessary to bear in mind that the ISE response at very low concentrations is generally slow and the potential is unstable, so that potential values read after a certain, fixed time interval must often be used instead of stabilized values. 

With this definition of temperature in mind, let s return to mercury and measure its actual changes in volume with temperature. The result found is almost, but not quite, linear. If a mercury thermometer is calibrated to match the gas thermometer at 0°C and 100°C and if the scale in between is divided evenly into 100 parts to mark off degrees, a small error will result from using this thermometer. A temperature of 40.00°C on the gas thermometer will be read as 40.11°C on the mercury thermometer, because the volume of liquid mercury is not exactly a linear function of temperature. 

You should bear in mind the points made in this section when reading all the other sections. Remember there may not be much point in measuring and reporting results, for example, to three decimal places just because the instrument/computer generates them that way, especially if there are or could be gross errors in your sampling or sample preparation techniques. 

Suppose that one has, from chemical evidence, a fair idea of what the structure of the major component represented by peak 2 in Figure 13.11 may be. The identity of the compound then becomes established as soon as one finds which member of its class has the literature value of retention volume equal to C, within experimental error. But this is not so simple as that, if one bears in mind that the usefulness of retention volume as read from Figure 13.11 is limited, in that it applies only to a particular column packing at a given temperature. Values of retention volumes are very sensitive to temperature—a temperature change of 1°C frequently causes a 5% change in these values. For this reason it is now usual to deal with retention volumes measured relative to some given pure substance. This substance is then added to a mixture in which unknowns are desired to be studied, retention volumes of the unknowns are measured relative to the retention of the added substance. This eliminates the need for accurate control of column temperature and flow-rate. 

As we have just done, people often talk of breaches or violations of procedures. But it needs to be borne in mind, as noted in Chapter 11 that there are three types of error according to Rasmussen (see Further Reading at the end of this chapter) - skill-based, rale-based and knowledge-based (s, r and k) and so the reasons for the breaches of procedure can be quite different. As an example, we tend to use skill-based and rule-based decisions a thousand times more than knowledge-based decisions in driving a car. 

In reading the papers of Desormes and Clement after Priestley s contemporary publications we feel that we are in a new world. Cruickshank had just before them cleared up the confusion between the inflammable airs which was the main source of Priestley s numerous errors, but Priestley s mind was closed. He objected to Cruickshank s correct experiments and conclusions, and in a series of communications from 1798 to 1803 (to be mentioned later) to the Medical Repository he refused to accept a compromise by one of its editors, Mitchill, in which hydrogen was assumed to be a compound of phlogiston and heat (caloric), sulphur a phlogisture of sulphur , etc., and metals as elements. Priestley had said with pride that no person was ever more temperate, or more cautious, than I have been in the introduction of new terms considering the number of new facts that I have discovered , and inflammable air was an example of one term covering many different gases. 

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