Unknowns solubility tests

Polymers degrade before they vaporize, so indirect methods must be used to determine their solubility parameters. The simplest of these methods is a two-valued bar graph, as is illustrated in Figure 2.24. A series of solvents of known solubility parameter are used. Each is tested to determine if it is a solvent for the polymer. The yes or no answers are then plotted at different heights along the <5, abscissa. The value of corresponding to the midpoint of the yes bar is taken to be the solubility parameter S2 of the polymer. This is a reverse application of equation (24). The argument is now that solvents that are found to dissolve the polymer must have solubility parameters close to the unknown solubility parameter of the polymer. 

In any attempt to determine the structure of an unknown biological compound, researchers must deal with two fundamental problems (1) If you don t know what it is, how do you know if it is pure (2) If you don t know what it is, how do you know that your extraction and purification conditions have not changed its structure Morgan addressed problem 1 through several methods. One method is described in his paper as observing constant analytical values after fractional solubility tests (p. 312). In this case, analytical values are measurements of chemical composition, melting point, and so forth. 

You are to purify 2.0 g of an unknown provided by the instructor. Conduct tests for solubility and ability to crystallize in several organic solvents, solvent pairs, and water. Conserve your unknown by using very small quantities for solubility tests. If only a drop or two of solvent is used the solvent can be evaporated by heating the test tube on the steam bath or sand bath and the residue can be used for another test. Submit as much pure product as possible with evidence of its purity (i.e., the melting point). From the posted list identify the unknown. 

The object of the present experiment is to identify an unknown amine or amine salt. Procedures for solubility tests are given in Section 1. 

Carry out the tests according to the scheme of Fig. 2 and the following Notes to Solubility Tests and tentatively assign the unknown to one of the groups I-X. 

After the unknown is assigned to one of the solubility groups (Fig. 2) on the basis of solubility tests, the possible type should be further narrowed by application of classification tests, e.g., for alcohols, or methyl ketones, or esters. 

Confirm the identity of your fabric by performing a burning or a solubility test on your unknown materials. 

Solubility tests such as these are rapid and useful ways to classify unknown compounds. 

Do not perform the chemical tests either haphazardly or in a methodical, comprehensive sequence. Instead, use the tests selectively. Solubility tests automatically eliminate the need for some of the chemical tests. Each successive test will either eliminate the need for another test or dictate its use. You should also examine the tables of imknowns in Appendix 1 carefully. The boiling point or the melting point of the unknown may eliminate the need for many of the tests. For instance, the possible compounds may simply not include one with a double bond. Efficiency is the key word here. Do not waste time performing nonsensical or unnecessary tests. Many possibilities can be eliminated on the basis of logic alone. 

Solubility tests should be performed on every unknown. They are extremely important in determining the nature of the main functional group of the imknown compound. The tests are very simple and require only small amounts of the unknown. In addition, solubility tests reveal whether the compoimd is a strong base (amine), a weak acid (phenol), a strong acid (carboxylic acid), or a neutral substance (aldehyde, ketone, alcohol, ester). The common solvents used to determine solubility types are... 

This set of 5 imknowns will help you make proper observations. They will not be used further in your organic laboratory course. Note that almost all organic compounds, except inert ones, will be soluble in sulfuric acid. This reagent should always be the last one tried, as shown in the Solubility chart. Using solubility tests, distinguish these unknowns by type. Verify your answer with the instructor. Read the discussion sections that follow for details on solubility behavior. A more general discussion of solubility behavior is provided in Technique 10, Section 10.2... 

Dissolution If instructed to do so, position an inverted funnel connected to a vacuum source above the mouth of the test tube being used for recrystallization (Fig. 2.71b) to minimize release of vapors into the laboratory. Place 100 mg of the unknown in the test tube and dissolve it in a minimum amount of boiling solvent you selected on the basis of solubility tests. Caution Because the sample may... 

The solubility of an organic compound in water, dilute acid, or dilute base can provide useful, but not definitive, information about the presence or absence of certain functional groups. In reality, however, the assignment of an unknown to a formal solubility class may be arbitrary because a large number of compounds exhibit borderline behavior. We recommend that the solubility tests be done in the order presented here. 

Hydrochloric acid. You should determine the solubility of the unknown in 1.5 M hydrochloric acid. If the unknown is soluble, the presence of an amino group in the compound is indicated because amines are organic bases that react with dilute acids to form ammonium salts that are usually water-soluble (Eq. 25.6). However, this solubility test does not permit the distinction between weak and strong organic bases. 

The research chemist usually obtains IR and NMR spectra of an unknown substance even before obtaining an elemental analysis or performing solubility tests because these types of spectra are easily and quickly measured on a small amount of sample. Only after analyzing them does the researcher undertake other experimental approaches for determining the structure. This process can save many hours of unnecessary laboratory work. Other useful information about a compound may be obtained from mass spectrometry (Sec. 8.5), which provides the molar mass of the compound and, if high-resolution data are available, its elemental composition. 

Solubility Tests.—Determine the solubility of the unknown in water, dilute alkali, dilute acid, ether, and cold concentrated H2SO4. For details and discussion see Chapters II and VIII. Finally consult the Solubility Table at the end of this text. 

A given unknown appeared to be pure since the boiling-point was fairly constant at 198°-199° while preliminary examination and solubility test gave no indication of a mixture. By means of the usual systematic tests the unknown was limited to the class of primary aromatic amines, and consultation of the tables (page 200) suggested the following individual possibilities ... 

Amount of Material Required in Solubility Tests.—The quantity of the unknown used in a solubility test will naturally depend upon the amount available. Usually it is convenient to use 0.10 g. 

IV. Solubility Behavior.—The solubility tests differ from those applied to individual compounds in one essential point it is necessary to determine whether any part of tlie mixture has dissolved. This is done by separating the solvent and examining it for dissolved material by precipitation, extraction, or distfllation methods, or by combinations of such methods. Diminution of volume in liquid unknowns is occasionally of value. The following scheme is of value in connection with the application of solubility tests on a water-insoluhle mixture. A one-gram sample will usually serve for these tests and the suction pipette, page 112, will be found of particular value in connection with the separations and extractions. All fractions are to be retained for later use. 

The plan for a Solubility Table was presented on page 24 of this text and will be found illustrated in more detail in the chart on the inside rear cover. In connection with a systematic identification of an unknown, the chart may be consulted after the completion of the solubility tests since it may prove of aid in the choice of suitable classification reactions. The tables of individual compounds, however, should not be consulted until the completion of the tests and the. systematic elimination of a considerable number of subgroups. 

Solubility in 5 per cent, hydrochloric acid. Add the acid to 0 10 g. of the solid or 0 20 ml. of the liquid in quantities of 1 0 ml. until 3 0 ml. have been introduced. Some organic bases (e.g., p-naphthylamine) form hydrochlorides that are soluble in water but are precipitated by an excess of acid if solution occurs at any time, the unknown is assigned to Group IV. If the compound appears insoluble, remove some of the supernatant liquid by means of a dropper to a semimicro test-tube (75 X 10 mm.), and add 5 per cent, sodium hydroxide solution until basic and observe whether any precipitate is produced the formation of a precipitate will place the compound in Group IV. 

Alkali or alkaline-earth salts of both complexes are soluble in water (except for Ba2[Fe(CN)g]) but are insoluble in alcohol. The salts of hexakiscyanoferrate(4—) are yellow and those of hexakiscyanoferrate(3—) are mby red. A large variety of complexes arise when one or more cations of the alkah or alkaline-earth salts is replaced by a complex cation, a representative metal, or a transition metal. Many salts have commercial appHcations, although the majority of industrial production of iron cyanide complexes is of iron blues such as Pmssian Blue, used as pigments (see Pigments, inorganic). Many transition-metal salts of [Fe(CN)g] have characteristic colors. Addition of [Fe(CN)g] to an unknown metal salt solution has been used as a quaUtative test for those transition metals. 

The preparations of luciferin (Ln, an electron acceptor) and soluble enzyme used were crude or only partially purified. The luciferase was an insoluble particulate material, possibly composed of many substances having various functions. Moreover, the luciferin-luciferase reaction was negative when both luciferin and luciferase were prepared from certain species of luminous fungus. It appears that the light production reported was the result of a complex mechanism involving unknown substances in the test mixture, and probably the crucial step of the light-emitting reaction is not represented by the above schemes. 

For example, adding silver nitrate solution to test for Cl (aq) is effective due to the very low solubility of silver chloride. You can use the precipitation of an insoluble salt to remove almost all of a particular ion from a solution and, as a result, cause a shift in the position of equilibrium of the original solution. The common ion effect is important in the solubility of salts. The precipitation of insoluble salts is used to identify the presence of unknown ions. You will learn more about the common ion effect in Chapter 9. 

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