Glassware, volumetric

As indicated in Section 4.1 (and as should be apparent from the discussion thus far in this chapter), titrimetric analysis methods heavily utilize solution chemistry, and therefore volumes of solutions are prepared, measured, transferred, and analyzed with some degree of frequency in this type of analysis. It should not be surprising that analytical laboratory workers need to be well versed in the selection and proper use of the glassware and devices used for precise volume measurement. 

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To ensure that S eas is determined accurately, we calibrate the equipment or instrument used to obtain the signal. Balances are calibrated using standard weights. When necessary, we can also correct for the buoyancy of air. Volumetric glassware can be calibrated by measuring the mass of water contained or delivered and using the density of water to calculate the true volume. Most instruments have calibration standards suggested by the manufacturer. 

I. 000 X 10- 1.000 X 10-k 1.000 X 10-k and 1.000 X 10- M from a 0.1000 M stock solution. Calculate the uncertainty for each solution using a propagation of uncertainty, and compare to the uncertainty if each solution was prepared by a single dilution of the stock solution. Tolerances for different types of volumetric glassware and digital pipets are found in Tables 4.2 and 4.4. Assume that the uncertainty in the molarity of the stock solution is 0.0002. 

Volumetric glassware used to deliver variable, but known volumes of solution. 

A suitable functional group is assayed in the same sample. In general chemistry and many polymer applications, this is merely the titration of acid groups with a base, or vice versa. Note that only volumetric glassware and a method for end point determination are required to do this. 

I = complex impedance, B = conductivity bridge, C = capillary viscometer, P = pycnometer or dilatometer, V = volumetric glassware, I = instrument, U = method unknown... 

Densities are perhaps the most straightforwardly determined and unambiguous physical property of ionic liquids. Given a quality analytical balance and good volumetric glassware the density of an ionic liquid can be measured gravimetrically (i.e., the sample can be weighed). 

Lam, R. B., and Isenhour, T. L., Minimizing Relative Error in the Preparation of Standard Solutions by Judicious Choice of Volumetric Glassware, Anal. Chem. 52, 1980, 1158-1161. 

It is important that any method chosen is scientifically sound under the conditions it will be applied. It is also necessary to demonstrate that the equipment, which will be used, is suitable and its use will not influence the results adversely. This includes all types of equipment, e.g. does the volumetric glassware have a suitable tolerance and do the instruments have sufficient sensitivity over the entire range of measurement The process for demonstrating equipment capability is called equipment qualification and is dealt with in Chapter 5. The staff carrying out validation need to be both qualified and competent in the tasks that they need to carry out. 

Instrument/Equipment Effects Examples include the calibration and precision of an analytical balance, the specified tolerance for volumetric glassware and a temperature controller that maintains a mean temperature which is different (within specification) from its indicated value. 

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. 

The three volumetric glassware products we will discuss are the volumetric flask, the pipet, and the buret. Let us study the characteristics of each type individually. 

Some volumetric glassware products have a large A imprint on the label. This designates the item as a class A item, meaning that more stringent calibration procedures were undertaken when it was manufactured. Class A glassware is thus more expensive, but it is most appropriate when highly precise work is important. This imprint may be found on both flasks and pipets. 

First, volumetric glassware should not be used for any volume measurement when the overall objective is strictly qualitative or when quantitative results are to be reported to two significant figures or less. 

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. 

Fourth, if a volume measurement does directly affect the numerical result of an accurate quantitative analysis (in a way other than entering directly into the calculation of the results), then volumetric glassware must be used. 

Specifications The design, construction and capacity of volumetric glassware must be in accordance with those laid down by the Indian Standards Institution (ISI). The tolerances on capacity for burettes, as specified in the relevant Indian Standards Institution, specifications are given in Table 2.1. 

Concentrations in the region of 0.1 mol 1 1 are often convenient but it obviously depends upon such factors as the amount of substance available, the cost, the solubility, etc. From this stock solution, a series of accurate dilutions are prepared using volumetric glassware and the absorbance of each dilution measured in a 1-cm cuvette at the wavelength of maximum absorbance for the compound. A plot of absorbance against concentration will give an indication of the validity of the Beer-Lambert relationship for the compound and a value for the molar absorption coefficient may be calculated from these individual measurements or from the slope of the linear portion of the graph ... 

Standardization of a Titrant For wet chemistry analytical methods, a titration is often used and the titrant, or the solution to which an unknown sample is compared, must be standardized. This can be done by comparing it with another standard. The important thing here is that the standard with which it is compared is ultimately traced to a SRM. The procedure utilizes volumetric glassware heavily, and thus the analyst must be assured that these are properly calibrated, as discussed above. Auto-titrators can be used (Figure 5.4). In this case, the automated equipment can be calibrated against manual equipment, i.e., volume readings obtained with the auto-titrator must match the volume readings obtained with a calibrated buret for the same sample. If they do not match (within accepted limits), the auto-titrator must be taken out of service and repaired, just like the defective balance. 

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