Chemistry
Sodium Hydroxide Test
The Sodium Hydroxide Test is a chemical test used to identify the presence of certain metal ions in a solution. Sodium hydroxide is added to the solution, and the resulting precipitate is observed to determine which metal ions are present. The test is commonly used in analytical chemistry.
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eBook - ePubPharmaceutical Chemical Analysis
Methods for Identification and Limit Tests
- Ole Pedersen(Author)
- 2006(Publication Date)
- CRC Press(Publisher)
A larger number of cations, especially the ones placed in the periodic table around the border between the metals and the nonmetals, form amphoteric hydroxide. That is, they form insoluble hydroxides in alkaline environments, which dissolve in excess sodium hydroxide, and this fact is used in the test. To enhance the selectivity of the identification, aluminum’s reaction toward sulfide and ammonium ions is tested as well.In the last three steps of the analysis, we test whether the cation present forms insoluble hydroxides upon the addition of sodium hydroxide, which dissolves in excess, and that the reaction is reversed by ammonium chloride. Since a fairly large number of cations (Be2+ , Pb2+ , Sn2+ , Sn4+ , and Sb3+ ) have the same characteristics, a false positive reaction by one of these is initially eliminated by precipitation with S2- .In this first step of the analysis, the 2 ml test solution is added to 0.2 ml dilute hydrochloric acid R and about 0.5 ml thioacetamide reagent R. In the acidic environment, thioacetamide will liberate sulfide ions by hydrolyze. The thioacetamide reagent is preferred to SH2 gas and Na2 S solutions as a source of S2- ions, since it is safer to handle.Aluminum’s sulfide salt shows a high degree of solubility, so no precipitation occurs. But at the low pH of the test solution, where the concentration of the sulfate ion is very limited, cations with very small solubility products, e.g., lead sulfide, will precipitate.In this way, false positive reactions by Pb2+ , Sn2+ , Sn4+ , and Sb3+ should be eliminated.In the second step, dilute sodium hydroxide solution R is added dropwise. This leads to the formation of a white gelatinous precipitate of aluminum hydroxide.In case any of the above mentioned and hopefully eliminated ions did not precipitate with S2- at low pH, they will do so now at high pH where the concentration of S2- is much greater. But the precipitates most likely will be very different in appearance from the white gelatinous aluminum hydroxide, because of their color and colloidal characteristics. Owing to aluminum’s ability to form amphoteric hydroxides, the aluminum hydroxide precipitate dissolves on the further addition of dilute sodium hydroxide solution R
- Thomas J. Bruno, Paris D.N. Svoronos(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
Sodium hydroxide evolution of ammonia gas upon heating 2. Potassium tetraiodomercurate (Nessler’s reagent) brown–yellow color, or brown precipitate of mercury (II) amidoiodide; high sensitivity; all other metals (except Na and K) interfere 3. Tannic acid: silver nitrate precipitate of black elemental silver, from neutral solution; very sensitive 4. p-Nitrobenzene-diazonium chloride red colored solution results in the presence of sodium hydroxide; sensitivity: 0.7 μg NH 4 + Note: Ammonium ions will cause a similar reaction to that of potassium in the presence of: sodium hexanitroco- baltate (III) sodium hydrogen tartrate. QUALITATIVE TESTS 687 ORGANIC PRECIPITATION REAGENTS FOR INORGANIC IONS The following table lists the most important organic reagents used for precipitating various inorganic species from solution [1,2]. Many of these reagents are subject to the serious disadvantage caused by lack of selectivity. Thus, many of the listed reagents will precipitate more than one spe- cies. The selectivity of some of the reagents can be controlled to a certain extent by adjustment of pH, reagent concentrations, and the use of masking reagents. The first two factors, pH and concen- tration, are the most critical. A number of these reagents form rather large, bulky complexes. While this can serve to enhance sensitivity (especially for gravimetric procedures), it can also impose rather stringent concentration limits. The reader is referred to several excellent “recipe” texts for further guidance [3–10]. REFERENCES 1. Kennedy, J. H. Analytical Chemistry. 2nd ed. New York: Saunders College Publishing, 1990. 2. Christian, G. D. Analytical Chemistry. 6th ed. New York: John Wiley and Sons, 2003. 3. Barber, H. H., and T. I. Taylor. Semimicro Qualitative Analysis. New York: Harper and Brothers, 1953. 4. Greenfield, S., and M. Clift. Analytical Chemistry of the Condensed Phosphates. Oxford: Pergamon Press, 1975. 5. Ryabchikov, D. I., and E.
eBook - ePubThe Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2.
With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.
- Julius Stieglitz(Author)
- 2014(Publication Date)
- Perlego(Publisher)
[p171] TOC The chemistry of the analytical reactions of the alkalies and alkaline earths is extremely simple,—it is essentially the chemistry of well-defined bases and their salts,—and the separations and identifications, as we have seen, depend almost entirely on physical differences rather than on chemical contrasts. In the aluminium and zinc groups, which are precipitated together and which will be discussed together, the chemistry of the reactions becomes very much more complex. Therefore, we shall not, as yet, consider the groups as a whole, but shall first discuss the important analytical reactions of some compounds of aluminium. Aluminium Hydroxide an Amphoteric Hydroxide. —Whereas the hydroxides of the alkali and alkaline earth metals are bases, pure and simple, aluminium hydroxide shows the properties both of a base and of an acid; it is an amphoteric hydroxide, the term "amphoteric" indicating the combination of acid with basic properties in any compound. Aluminium hydroxide dissolves in acids. From its solution in hydrochloric acid, an aluminium salt, aluminium chloride AlCl 3, 6 H 2 O, is obtained. It also combines with strong bases, dissolving for instance in a solution of sodium hydroxide and forming an aluminate, NaAlO 2. The two salts mentioned are typical representatives of the two series of salts, which aluminium hydroxide is capable of forming. This dual character of the hydroxide raises a number of interesting questions, which one meets with quite frequently in the study of analytical reactions
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