Molar weight, determination

Calibrate a GPC column for molar weight determination analogously. Use a set of defined proteins or polypeptides as standards (see inset in Fig. 3.4). If the mass differences allow a complete separation, the proteins can be applied as a mixture. 

Abnormal Molar Weights.— In the case of a number of substances (very commonly in the case of organic acids and hydroxy-compounds in benzene), it is found that the molar weight determined by the cryoscopic method is greater than that calculated from the usual chemical formula of the substance, by an amount exceeding the experimental error. We are therefore led to the assumption that these substances associate in solution, i.e, two or more molecules combine to... 

Apparatus and Method.—The apparatus which is generally employed for the purpose of molar weight determinations by the boiling-point method is that designed by Beckmann, one form of which, together with the manner of using it, will be described here. 

We hope that this chapter on the molecular weight determination of synthetic polymers has illustrated that in the case of a complex polymer it is preferable to use several experimental methods for the molecular weight determination to obtain a full picture. Owing to the different sensitivity of the various methods some are blind for low molar masses while others are blind at low concentrations. As exemplified, often scaling laws can be utilized to compare results of different methods and different sensitivities. 

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

To get the final answer, we must divide the energy released by the moles of glucose. We find the moles of glucose from the mass given in the problem and the molar mass determined from the atomic weights of the elements. 

Various stated referenees are involved in a ehemieal measurement. In VIM the three different references possible are given with examples. For determining amount of substance several references can be involved e.g. we could weigh a material with known purity and from the known molar weight we could calculate the amount content. All the references used in a measurement should be stated and taken appropriately into account. 

Figure 6.11 Effect of ionic strength on (a) weight-average molar weight, Mw ( ), and second virial coefficient, A2 ( ), of p-casein in solution at pH = 5.5 and 22 °C, as determined by static light scattering and (b) average droplet diameter, c/43 (A.), and extent of gravity creaming ( ) of p-casein-stabilized emulsion (11 vol% oil, 0.6 wt% protein, pH = 5.5, 22 °C).

Another possible approach, which is broadly used, is to use high-purity substances (indirectly assayed) as standards. For use at the highest level, this approach requires the determination of all important impurities in the sample. This means not only metallic impurities, commonly stated in the manufacturers certificates, but also non-metals as oxygen, carbon, etc. The content of impurities is not always known in advance. If the total content of impurities is very low, the uncertainty of their determination does not affect the required uncertainty of the sample assay. Some other problems are discussed in Ref. [5], The need of determining the molar weight may equally apply here. 

The situation is seldom favourable enough to enable the use of a single method for establishing a link to the SI system. The information on the content of impurities (by means of relative methods) is needed in most cases. Except for coulometry, determination of molar weight... 

In the interest of conserving space in this handbook, a compact tabular presentation format has been adopted. Table 5.1.5.1 lists the chemical name, and its freon number (if applicable), molecular formula, molar weight and melting and boiling points. These data are available for virtually all substances in this group. Also shown in this table is the availability, expressed as a tick mark, of data on vapor pressure, solubility in water, octanol-water partition coefficient (Kqw) and the second order reaction rate constant with hydroxyl radicals. This rate constant is the critical determinant of persistence in the atmosphere. Tables 5.1.5.2 to Table 5.1.5.5 list the compounds and give the available property data with citations. 

Critical values y and (f> are shown in Table 7.5 for various degrees of polymerisation, x. It follows that for high molar weight polymer ycrit and determined from swelling experiments of cross-linked, i.e. nonsoluble, polymers. 

Molar mass determination requires the knowledge of the specific refractive index increment Ant Ac which in the case of complex polymers depends on chemical composition. Copolymer refractive index increments (dn/dc)copo can be accurately calculated for chemically monodisperse fractions, if comonomer weight fractions and homopolymer values are known ... 

As we did in earlier molecular weight determinations (Section 14-13), we must first find n, the number of moles that 0.500 grams of pepsin represents. We start with the equation it = MRT. The molarity of pepsin is equal to the number of moles of pepsin per liter of solution, n/K We substimte this for M and solve for n. 

However they are measured, the data are manipulated (with knowledge of the molar weight, and estimations of diamagnetic corrections for sample and holder) to give M, which is determined as a function of T and/or H. 

---