Analytical procedures volumetric calibrations

Take the 250-mi volumetric flask contg material extracted by carbon tetrachloride as shown here under H- Analytical Procedures, step 1, and transfer from it 25 ml (using a calibrated pipet, previously rinsed with CC14 and dried, to the titration flask F, Evaporate the CC14 to dryness with a slow stream of air and then sweep the air off with a stream of C02 from a cylinder (or from a Kipp generator). While continuing to pass C02 thru the flask, introduce... 

The analyte concentration in the portion of the sample zone yielding the analytical signal should match the dynamic concentration range of the flow-based analytical procedure. To this end, in-line sample dilution is often required, and this step is efficiently accomplished in a flow analyser. As emphasised in Chapter 1, calibrated devices such as burettes, volumetric flasks and pipettes are not required, as the sample and reagent volumetric fractions are maintained for all of the assayed sample and standard solutions. 

Determinate errors can arise from uncalibrated balances, improperly calibrated volumetric flasks or pipettes, malfunctioning instrumentation, impure chemicals, incorrect analytical procedures or techniques, and analyst error. 

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. 

Coulometric methods are performed by measuring the quantity of electrical charge required to convert a sample of an analyte quantitatively to a different oxidation state. Coulometric and gravimetric methods share the common advantage that the proportionality constant between the quantity measured and the analyte mass is derived from accurately known physical constants, which can eliminate the need for calibration with chemical standards. In contrast to gravimetric methods, coulometric procedures are usually rapid and do not require the product of the electrochemical reaction to be a weighable solid. Coulometric methods ai-e as accurate as conventional gravimetric and volumetric procedures and in addition are readily automated. ... 

For each above Set 1, 3, 5, 10, IS, and 20 mL aliquots of each component are pipetted into respective 100 mL volumetric flasks or vials while accurately recording the masses. For example, for Set 1, into flask one add 1.0 mL MTBE, 1.0 mL DIPE, 1.0 mL ETBE, 1.0 mL TAME into flask two add 3.0 mL MTBE, 3.0 mL DIPE, 3.0 mL ETBE, 3.0 mL TAME and so forth. Add the oxygenate in reverse order of their boiling points. The above procedure produces six calibration solutions for each set with the concentrations of each analyte at 1, 3, S, 10,15, and 20 volume %. 10.0 mL of DME (internal standard) is then added at constant volumes to each flask or vial while recording its mass. The flasks or vials are then filled to 100 mL total volume with toluene. It is not necessary to weigh the amount of solvent added since the calculations are based on the absolute masses of the calibration components and the internal standard components. 

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