Error buret

Given the care with which laboratory equipment (balances, burets, instruments, etc.) is calibrated at the factory, why should the chemical analyst worry about errors ... 

When reading the liquid level in a buret, your eye should be at the same height as the top of the liquid. If your eye is too high, the liquid seems to be higher than it really is. If your eye is too low, the liquid appears too low. The error that occurs when your eye is not at the same height as the liquid is called parallax. 

Error can be caused by failure to expel the bubble of air often found directly beneath the stopcock (Figure 2-8). If the bubble becomes filled with liquid during the titration, then some volume that drained from the graduated portion of the buret did not reach the titration vessel. The bubble can be dislodged by draining the buret for a second or two with the stopcock wide open. You can expel a tenacious bubble by abruptly shaking the buret while draining it into a sink. 

Another systematic error arises from an uncalibrated buret. The manufacturer s tolerance for a Class A 50-mL buret is 0.05 mL. When you think you have delivered 29.43 mL, the real volume could be anywhere from 29.38 to 29.48 mL and still be within tolerance. One way to correct for an error of this type is to construct a calibration curve, such as that in Figure 3-3, by the procedure on page 38. To do this, deliver distilled water from the buret into a flask and weigh it. Determine the volume of water from its mass by using Table 2-7. Figure 3-3 tells us to apply a correction factor of —0.03 mL to the measured value of 29.43 mL. The actual volume delivered is 29.43 — 0.03 = 29.40 mL. 

A key feature of systematic error is that it is reproducible. For the buret just discussed, the error is always —0.03 mL when the buret reading is 29.43 mL. Systematic error may always be positive in some regions and always negative in others. With care and cleverness, you can detect and correct a systematic error. 

Summary. Most of the automatic burets which have been discussed have been primarily refinements of models that were introduced many years ago. In the adaptations of automatic burets to microchemistry, some burets have eliminated major drainage errors and contact of titrant with mercury and have provided readily readable volumes on dials or mechanical counters. These improvements are reflected in increased... 

Procedure of Kline and Acree. For the determination of the aldose present it is preferable to take an aliquot of the sugar solution, or a weighed amount of the solid substance, which will react with approximately 20 ml. of 0.1 A iodine. Titrate this solution with 0.1 A sodium hydroxide or hydrochloric acid until it is exactly neutral to phenolphthalein. Add the phenolphthalein at this point only when it is necessary to bring the solution to neutrality and use only one drop, as the alcohol introduces a potential source of error involving a loss of iodine. A water solution of this indicator or of phenol red or thymol blue might be used for this titration. Add 5 ml. of 0.1 A iodine from a buret then add drop by drop from a buret 7.5 ml. of 0.1 A sodium hydroxide. Repeat this process until 22 ml. of iodine and 35 ml. of alkali have been added. This operation takes about five to six minutes. Allow a two-minute interval for the completion of the oxidation. Acidify with 0.1 A (or 0.2 A) hydrochloric acid to free the iodine from any sodium iodate present and titrate the liberated iodine with... 

All measuring devices are potential sources of systematic errors. For example, pipets, burets, and volumetric flasks may hold or deliver volumes slightly different from those indicated by their graduations. These differences arise from using glass-... 

Parallax Apparent change in position of an object as a result of the movement of the observer results in systematic errors in reading burets, pipets, and meters with pointers. 

Note. Should the volume of liquid go over the cahbration mark, it is still possible to save the solution as follows. Paste against the neck of the flask a thin strip of paper and mark on it with a sharp pencil the position of the meniscus, avoiding parallax error. After removing the thoroughly mixed solution from the flask, fill the flask with water to the calibration mark. Then by means of a buret or small volume graduated pipet, add water to the flask until the meniscus is raised to the mark on the strip of paper. Note and record the volume so added and use it to mathematically correct the concentration calculation. 

Avoid parallax error in reading buret or pipet volumes. 

Your CD has the Table 2.4 spreadsheet, with formulas as indicated in the table. You can substitute specific weights of water in air, obtained from a flask, pipet, or buret, in cell B at the temperature of measurement to obtain the calculated calibration volume at temperature, T, and for 20°C. We describe the use of spreadsheets in Chapter 3. The CD also has a table and figure of the percent error for weight in vacuum as a function of sample density. 

Two main classes of errors can affect the accuracy or precision of a measured quantity. Determinate errors are those that, as the name implies, are determinable and that presumably can be either avoided or corrected. They may be constant, as in the case of an uncalibrated weight that is used in all weighings. Or, they may be variable but of such a nature that they can be accounted for and corrected, such as a buret whose volume readings are in error by different amounts at different volumes. 

To minimize evaporation errors, your instructor may direct you to refill the buret for each volume delivery (0-5, 0-10,... 0-50 mL) and deliver it into an empty (dry) fiask. 

The results of the trials show very good precision (for a graduated cylinder). The student has good technique. However, note that the average value measured using the buret is significantly different from 25 mL. Thus this graduated cylinder is not very accurate. It produces a systematic error (in this case, the indicated result is low for each measurement). 

Random error is associated with every measurement. To obtain the last significant figure for any measurement, we must always make an estimate. For example, we interpolate between the marks on a meter stick, a buret, or a balance. The precision of replicate measurements (repeated measurements of the same type) reflects the size of the random errors. Precision refers to the reproducibility of replicate measurements. 

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