Unknowns preliminary tests

In many cases, the results of the IR and mass spectrum interpretation are sufficient to allow a complete molecular structure to be deduced. In preliminary tests on 12 unknown compounds of molecular weight 100-200, the author, using the results reported by the program but without access to the original spectra, was able to correctly identify 9 of the unknowns. 

VI.5 TESTING FOR ANIONS IN SOLUTION ON THE SEMIMICRO SCALE The preliminary tests 7 and 8 with dilute sulphuric acid and with concentrated sulphuric acid will have provided useful information as to many anions present. For more detailed information, it is necessary to have a solution containing all (or most) anions free from heavy metal ions. This is best prepared by boiling the substance with concentrated sodium carbonate solution double decomposition occurs (either partially or completely) with the production of the insoluble carbonates of the metals (other than alkali metals) and the soluble sodium salts of the anions, which pass into solution. Thus, if the unknown substance is the salt of a bivalent metal M and an acid HA, the following reaction will occur ... 

NOTES.—(d) When aromatic compounds bum they produce a large amount of soot. Soot is also formed when unsaturated compounds and paraffin derivatives which contain alkyl radicals with four or more carbon atoms are burned. The simpler paraffin derivatives do not produce soot on burning. The behavior of an unknown substance on ignition is frequently determined as a preliminary test in its identification. The test is best made by putting some of the liquid or solid on a small roll of copper gauze, to which a piece of wire is attached to serve as a handle. 

A great deal of useful information can be obtained from simple tests on unknown coatings. Mere visual examination, touching and smelling the polymer can determine the type of polymer or eliminate some others. For example, PVC and many rubber coatings have distinctive colour, texture and smell. Preliminary tests are shown in Table 7.1. 

Final Test. Mix 40-50 mg of the unknown. 1 mL of 0.5 A4 hydroxylamine hydrochloride in 95% ethanol, and 0.2 mL of 6 A4 sodium hydroxide in a test tube. Heat the mixture to boiling and, after cooling it slightly, add 2 mL of hydrochloric acid. If the solution is cloudy, add about 2 mL more of 95% ethanol. Add 1 drop of 0.6 A4ferric chloride, and observe any color. If needed, add more ferric chloride solution until the color persists. Compare the color obtained here with that from the preliminary test. If the color is burgundy or magenta, as compared to the yellow color in the preliminary experiment, the presence of an ester group is indicated. 

Preliminary tests help you select a route to follow to ultimately identify the unknown material at hand. These tests frequently consume material, so, given the amounts of material generally available at the micro- or semimicroscale level, judicious selection of the tests to perform must be made (in some tests, the material analyzed may be recovered). Each preliminary test that can be conducted with little expenditure of time and material can offer valuable clues as to which class a given compound belongs. 

If the preliminary tests outlined above indicate that the unknown in question contains impurities, it may be necessary to carry out one of several purification steps. These techniques are discussed in earlier chapters and are summarized below for correlation purposes ... 

The separation techniques mentioned above can be rationally employed onfy when suitable preliminary tests have established at least roughly the nature of the polymer and, in favourable cases, even the class of polymer. Such tests, especially important in large scale industrial chemical practice, include simple qualitative chemical tests for particular heteroelements and functional groups ). A certain number of jiiysical data, mainly of interest for quality and production control, can also yield criteria for identification of an unknown polymer " " These... 

A thorough preliminary examinatioji should always precede any attempt made to separate a mixture.—To the experienced analyst, certain short-cuts will always be apparent, but for the beginner and usually for the experienced chemist also, a thorough preliminary examination is by far the best short-cut to be found. In the case of a liquid unknown, there is always the temptation to proceed immediately to a fractional distillation and in the case of a solid mixture we find too often that the first attempt at analysis has been a resort to the use of the wrong solvents. It is only after the preliminary examination that one can decide upon the most logical and satisfactory method for the final separation. Although these preliminary tests are usually similar for different mixtures, the final methods of separation will be different in every case, since it will then be possible to dispense with all unnecessary steps. 

To this day, we still hear people claim that in vitro testing of materials alone shows that they are suitable for use in chronically implanted devices. Others continue to say that I proved the materials are biocompatible and biostable, so I don t have to do any device testing. This statement can be very far from the truth. In vitro testing has its place, primarily to screen materials and processes for further testing. In some cases where no suitable in vitro test exists, one may be forced to develop accelerated in vivo materials tests. Once the preliminary testing is accomplished, however, one must test the device per se in animals. A biocompatible material does not necessarily make a biocompatible device. The same may be said about biostability. These statements are true because shape, size, surface finish, interactions between the materials in the device, etc., all can affect its biocompatibility and biostability. But even well-performed animal studies may not unveil previously unknown mechanisms, because animals do not perfectly mimic the human in vivo environment. An excellent example of this is the subclavian crush in humans (clamping a lead between the clavicle and first rib), which is impossible to discover in animals with no clavicles. With the right protocol for the device in question, only postmarket surveillance appropriate for the device in question can determine for certain that the device does or does not meet expectations. 

The detection of anions in the original sample does not proceed in the same way in that the material is usually subjected to a series of preliminary tests. Moreover, the cation analysis described above can often be assessed to infer the presence of certain anions. A sample solution is separately treated with AgNOs and BaCh, and the various precipitates of silver and barium are used to verify the presence of anionic components. This procedure is then followed by the addition of concentrated cold H2SO4 to the unknown solid, which results in the liberation of characteristic gases (e.g., 1 gives the odor of H2S and violet fumes of I2), and by a series of confirmatory tests for each anion. 

The toxicity of both feeds and products must be carefully considered during the preliminary design stages (53,54) especially if the feeds or products contain known or suspected carcinogens. Special testing may even be requked prior to operation to determine the toxicity of unknown materials. 

Measurements should be carried out with two different indicators, unless routine analyses are involved. When nothing at all is known of the acidity of the solution, the approximate pH must first be found in order to permit a choice of proper indicators. Of course only those indicators may be used which impart a distinct intermediate color to the unknown solution. An indicator is of no value if its color in the unknown solution is due to the pure acid or pure basic form. The preliminary examination for acidity may be performed with the use of various indicator papers such as congo, litmus, phenolphthalein, and turmeric papers, or by treating small portions of liquid (on a spot plate) with various indicators. If it happens, for example, that a solution remains colorless in the presence of phenolphthalein and is alkaline towards methyl orange, its pH must lie between 8 and 4.5. Should further tests show the unknown to be alkaline also to methyl red, we would know that the pH being measured lies between 6 and 8, and that a suitable indicator could be chosen from the group which includes bromthymol blue, phenol red, neutral red, and cresol red. Universal indicators are especially useful for making such estimates of pH values. 

A set of solutions of the various inorganic ions is very desirable so that control tests may be made. Equally important is a stock of typical representatives of the various classes of organic compounds which should be kept in readiness for comparison purposes. A collection of plant products, standard preparations of chemicals, minerals and different technical products will be found of great value in the preliminary examination of unknown materials. It is also well to provide a card index with simple working directions in the immediate neighborhood of the necessary reagents. 

The International Union of Pure and Applied Chemistry (lUPAC) explains the extrapolation by SAM in more detail. lUPAC recommends adding amounts of analyte x equimolar to that expected for the test solution, so that xj xq X2 2-xj . .Xp p-Xi p is the number of levels of the addition frequently p = 3 or 4). A preliminary measurement of xq would be needed. Following this, they established the relationship between the unknown test solution and the signal as yg = br Xq, where yo is the signal yielded by the unspiked test solution, whose analyte concentration is Xq, and is the slope. 

Providing tests are performed at low strain amplitude, small enough for the complex modulus to exhibit no strain dependency, then dynamic testing yields in principle linear viscoelastic functions. This implies that, with an unknown material, a preliminary strain sweep test is performed in order to experimentally detect the maximum strain amplitude for a linear response to be observed [i.e. G lo, f(Y)]-As illustrated in Fig. 6 with data from Dick and Pawlowsky [20], such a requirement is practically never met within the available experimental window with filled rubber materials, whose linear region tends to move back to a lower and lower strain range as the filler content increases. 

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