Stoichiometry volumetric analysis

Here are some examples to illustrate stoichiometry calculations in volumetric analysis. The key step is to relate moles of titrant to moles of analyte. We also introduce the Kjeldahl titration as a representative volumetric procedure. 

I n volumetric analysis, the volume of a known reagent required for complete reaction with analyte by a known reaction is measured. From this volume and the stoichiometry of the reaction, we calculate how much analyte is in an unknown substance. In this chapter we discuss general principles that apply to any volumetric procedure, and then we illustrate some analyses based on precipitation reactions. Along the way, we introduce the solubility product as a means of understanding precipitation reactions. 

The volumetric expansion parameter S may thus be taken as 0.9675. The product distribution will vary somewhat with temperature, but the stoichiometry indicated above is sufficient for preliminary design purposes. (We should also indicate that if one s primary goal is the production of ethylene, the obvious thing to do is to recycle the propylene and ethane and any unreacted propane after separation from the lighter components. In such cases the reactor feed would consist of a mixture of propane, propylene, and ethane, and the design analysis that we will present would have to be modified. For our purposes, however, the use of a mixed feed would involve significantly more computation without serving sufficient educational purpose.)... 

TABLE II. Results of Volumetric Gas-Phase Analysis for Determination of the CO/Fe Stoichiometry on Titania Samples After Decomposition of Fe (CO)5... 

General principles of calibration of course apply to speciation analysis (Griepink, 1993 Quevauviller et al., 1996a). All efforts made to obtain a good sample and perform the extraction under the proper conditions are spoiled if the calibration is wrong. Basic principles of QA apply here, including calibration of balance and volumetric glassware, verification of the calibrant purity and stoichiometry, verification of the stability of stock solutions, etc. 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

The accuracy of analysis is only comparable to the volumetric measurement accuracy if a) the titer of the titrating solution can be determined correspondingly accurately, b) the stoichiometry of the titration reaction is known as accurately and remains constant, and c) the titration end point can also be determined with comparable accuracy ... 

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