Basic indicator

Two simple invariants, U, and U5, were shown in the previous subsubsection to be the basic indicators of average laminate stiffnesses. For isotropic materials, these invariants reduce to U. =Qi. and U5 = Qqq, the extensional stiffness and shear stiffness. Accordingly, Tsai and Pagano suggested the orthotopic invariants U., and U5 be called the isotropic stiffness and isotropic shear rigidity, respectively [7-16 and 7-17]. They observed that these isotropic properties are a realistic measure of the minimum stiffness capability of composite laminates. These isotropic properties can be compared directly to properties of isotropic materials as well as to properties of other orthotropic laminates. Obviously, the comparison criterion is more complex than for isotropic materials because now we have two measures, and U5, instead of the usual isotropic stiffness or E. Comparison of values of U., alone is not fair because of the degrading influence of the usually low values of U5 for composite materials. 

The aqueous layer should be acidic to litmus paper. If it is basic, indicating that the ammonia was not completely removed from the reaction mixture, more hydrochloric acid should be added until an acidic test is obtained... 

To discuss the development of CRMs for the emerging use of microanalytical techniques, one has to be concerned chiefly with the degree of homogeneity of the components in the material at the designated sample size. Basic indications for the homogeneity properties of a CRM for microanalytical methods and the assessment of these properties can be derived from the general requirements ... 

Tanabe [78] defined a solid acid as a solid on whose surface a basic indicator changes its color or a base is chemically adsorbed. Several acidic properties... 

Titration method (consisting in the stndy of the interaction of indicator dyes with solids from solntions) is a techniqne for both qnalitative and qnantitative characterization of solid snrfaces. If a basic indicator B is nsed, the proton acidity of the snrface is expressed by the Hammett acidity fnnction. Similarly, the basicity can be defined when an acid is converted by its conjngated base. This allows defining acidity and basicity in the same scale. 

Color Plate 1 HC1 Fountain (Demonstration 6-2) (a) Basic indicator solution in beaker. (b) Indicator is drawn into flask and changes to acidic color. 

The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid (2). If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is expressed by the Hammett acidity function Ho, Ho = pKa + log[B]/[BH + ], where [B] and [BH + ] are, respectively, the concentrations of the neutral base (basic indicator) and its conjugate acid, and pK.d is p BH +. If the reaction takes place by... 

Basic Indicators Used for Measurement of Superacid Strength... 

The salt, when dried in air, is a dull white insoluble substance, containing one molecule of water. When heated at 100° C. it becomes anhydrous. On treatment with sodium hydrogen carbonate solution, both the monohydrate and the anhydrous salt cause effervescence to take place, a soluble double arsenate being produced. It would appear, therefore, that the hydrated salt is acidic, and determinations of its basicity indicate that its formula is Fe0.As02(0H)2.7... 

Chemical conditions. The occurrence of glauconites in sandstones (acid) and carbonates (basic) indicates that pH is not of great importance to their formation. They are found in sedimentary rocks ranging in age from Precambrian to the present day. Hence, their formation seems to be constrained by chemical and physical conditions rather than by specific events. There is no apparent time-dependent reaction which transforms them into new phases. 

Indicators, Hammett acidity. With a set of 17 basic indicators any acidity lying in the range from a dilute aqueous mineral acid to that of pure sulfuric acid may be determined in any solvent. The range is from -t-2.8 (4-aminoazobenzene) to —9.3 (2,4,6-trinitroaniline). Aldrich supplies a set of 500 mg. of each analytically pure indicator. L. P. Hammett and A. J. Deyrup, Am. Soc., 54,2721 (1932) L. P. Hammett and M. A. Paul, im,56,827 (1934)... 

This expression corresponds to the one derived for the acid indicators. The entire discussion concerning the transformation interval applies equally well to the basic indicators. 

Nitramine. The sensitivity of this indicator to hydroxyl ions does not change at higher temperatures. It may be concluded from this that nitramine acts as a basic indicator. 

Proof of this explanation is seen in the fact that a soap solution colored red with neutral red turns yellow upon addition of alcohol. The alcohol dissolves the colloidal fatty acid particles, thus allowing the neutral red to react in a normal manner. We may expect in general that, in colorimetric pH determinations, it is preferable to employ basic indicators in the presence of positive colloids and indicator acids when negative colloids are present. For example phenolphthalein shows approximately its normal reaction in a soap solution while the basic neutral red deviates considerably in the same medium. 

Analogous observation have been reported for the adsorption of basic indicators on some acidic adsorbents (64, 65). For example, p-dimethyl-aminoazobenzene, MegN—Ph— N=N—Ph (Me stands for CHg), is strongly adsorbed on silica-alumina, known to possess considerable proton acidity, with a color change from yellow to red. For a methanolic solution the spectral change is from an absorption band at 400 to one... 

Many types of solid acid and base catalysts are known.11 Superacids are those that are at least as strong as 100% sulfuric acid.12 The acid strengths are measured using basic indicators and are assigned a Hammett acidity function, H0- Table 6.1 lists some superacids, with the strongest at the top. 

The acidity function H0 determined by this method characterizes the tendency of the solution to protonate a neutral basic indicator... 

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