Glassware, accuracy

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

The analyte in a sample may be too concentrated or too dilute for the chosen method. If it is too concentrated, a dilution with a compatible solvent may be performed. The dilution should be performed with volumetric glassware and with good analytical technique so that the dilution factor is known and accuracy is not diminished. 

Second, if the results of a quantitative analysis are to be reported to three or more significant figures, then volume measurements that enter directly into the calculation of the results should be made with volumetric glassware so that the accuracy of the analysis is not diminished when the calculation is performed. 

Accuracy in the determination depends on the care with which the glassware is cleaned, how solvents are purified, how solutions are prepared and clarified, the calibration of the photometer, the determination of the specific refraction increment (dn/dc), etc. Consequently, the accuracy is often not better than 10%. 

For greatest accuracy, we calibrate volumetric glassware to measure the volume actually contained in or delivered by a particular piece of equipment. We do this by measuring the mass of water contained or delivered by the vessel and using the density of water to convert mass into volume. 

In addition, the reliability of the stripping data strongly depends on the degree to which contamination can be minimized. Hence, as in other trace analytical methods, all principles of good laboratory practice (glassware cleanliness, sample collection and storage, etc.) must be observed to obtain high accuracy and low detection limits. 

The graduated cylinder is a piece of glassware used for measuring the volume of a liquid. Graduated cylinders come in various sizes with different degrees of accuracy. 

Ware. This glassware is calibrated not only to Class A tolerances, but to each mark on the glass. This calibration ensures high accuracy for every measurement. It needs to be specially ordered and calibrated. 

Accuracy. Most glasswork is done within an accuracy of 1.5 mm (V16th of an inch). If you require greater accuracy, say so. Similarly, if the glassware fits into some other device, or some other device fits into the glassware, say so. For an example, see Fig. A.5. If item A is ordered, and it is required to fit into item B, it is important to provide the following specifications of item A to the glassblower ... 

Before measuring a volume it is important to choose the most appropriate equipment in order to achieve the greatest accuracy. The volume and level of accuracy will help determine which piece of equipment should be used. The most common equipment includes various pieces of glassware (e.g. volumetric flask, measuring cylinder, burette, pipette), mechanical micropipette (or pipettor) and syringes (Figure 2.3). 

You wish to prepare 1 L of a 0.02 M potassium iodate solution. You require that the final concentration be within 1 % of 0.02 M and that the concentration must be known accurately to the fourth decimal place. How would you prepare this solution Specify the glassware you would use, the accuracy needed for the balance, arid the ranges of acceptable masses of KI03 that can be used. 

The procedures for weighing and the glassware used in the preparation of solutions differ according to the level of accuracy required. 

The accuracy of the dimensions of machine-drawn tubing is inadequate for such applications as high quality graduated glassware, syringe barrels, interchangeable manometers and many electronic devices which must have metal parts accurately located with respect to the glass-tube envelope. 

There is at present no generally accepted criterion for the accuracy of instruments, although tolerances for the calibration of volumetric glassware and thermometers have been published. Manufacturers claims of accuracy within 1% are diflScult to assess without knowledge of the samples tested or method used to obtain the value. It should be possible for manufacturers to specify the accuracy of calibration of many instruments. As a general rule, it seems desirable that any inaccuracy in an instrument should not contribute significantly to the total inaccuracy of the result. 

Highest Accuracy. The volumetric glassware items with the highest accuracy are bulb pipettes and volumetric flasks (see Figure 4.4). 

Figure 4.4 Laboratory glassware with graduation marks of the highest accuracy...

Using information on the purity of the material used to prepare the spiked sample, and the accuracy and precision of the volumetric glassware and analytical balance used, the uncertainty in the concentration of Cl solvent red 24 in the sample, (CRM), was estimated as 0.05 mg l-1.1 The uncertainties associated with the concentration of quinizarin and Cl solvent yellow 124 were estimated as 0.025 mg 1 1 and 0.062 mg 1 1, respectively. The relevant values are ... 

The most important part of any spectroscopic assay is not the performance of the spectrophotometer (although the accuracy of the instrument is checked periodically). The crucial part of any experiment is the accurate preparation of the test and standard solutions. This often involves the accurate dilution of a stock solution using the volumetric glassware introduced in Chapter 6, namely the pipette and the volumetric flask. 

Volumes are often measured in liters or milliliters in the metric system. One liter (1 L) is one cubic decimeter (1 dm ), or 1000 cubic centimeters (1000 cm ). One milliliter (1 mL) is 1 cm In medical laboratories, the cubic centimeter (cm ) is often abbreviated cc. In the SI, the cubic meter is the basic volume unit and the cubic decimeter replaces the metric unit, liter. Different kinds of glassware are used to measure the volume of liquids. The one we choose depends on the accuracy we desire. For example, the volume of a liquid dispensed can be measured more accurately with a buret than with a small graduated cylinder (Figure 1-13). Equivalences between common English units and metric units are summarized in Table 1-7. 

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