Fonte: Laboratorio del Dr. Neal Abrams – SUNY College of Environmental Science and Forestry
La solubilità di un composto ionico può essere determinata tramite analisi qualitativa. L’analisi qualitativa è una branca della chimica analitica che utilizza proprietà chimiche e reazioni per identificare il catione o l’anione presente in un composto chimico. Mentre le reazioni chimiche si basano su regole di solubilità note, quelle stesse regole possono essere determinate identificando i prodotti che si formano. L’analisi qualitativa non viene tipicamente eseguita nei moderni laboratori di chimica industriale, ma può essere facilmente utilizzata sul campo senza la necessità di strumentazione sofisticata. L’analisi qualitativa si concentra anche sulla comprensione delle reazioni ioniche e ioniche nette, nonché sull’organizzazione dei dati in un diagramma di flusso per spiegare le osservazioni e trarre conclusioni definitive.
Molti cationi hanno proprietà chimiche simili, così come le controparti anioniche. Una corretta identificazione richiede un’attenta separazione e analisi per identificare sistematicamente gli ioni presenti in una soluzione. È importante comprendere le proprietà acido/base, gli equilibri ionici, le reazioni redox e le proprietà del pH per identificare con successo gli ioni.
Mentre esiste un test qualitativo per praticamente ogni ione elementare e poliatomico, il processo di identificazione inizia tipicamente con la conoscenza di una “classe” di ioni analizzati; cationi o anioni, elementari o poliatomici, gruppi o periodi, transizione o gruppo principale. In questo esperimento, vengono identificati entrambi i tipi di ioni, cationi e anioni. I cationi includono anche ioni poliatomici.
The reactions shown here can be used to identify the presence of a class of cations or anions or be used very specifically for a certain ion. Because two reagents are used in the analyses, either reagent can be typically detected using the other. For example, instead of analyzing for the presence of chloride using silver ion, silver ion can be identified using chloride. A combination of common rules of precipitation followed by specific colorimetric or precipitation tests can be used to positively identify nearly every ion, atomic or polyatomic, available. At the same time, most of those same rules can be established by reacting anions and cations together systematically to generate a set of rules for cation and anion solubility.
Qualitative analysis and rules related to solubility are common experiments in the general chemistry laboratory. This is due, in part, to the ease, speed, and inexpensive nature of the tests. It is for these reasons that qualitative tests are also used in field-based analyses and confirmatory lab tests. For example, a geology firm may wish to know if significant quantities of nickel exist in stream runoff from a mine. A simple test by adding the water to dimethylgloxime is selective for nickel ion. Similarly, water-quality authorities can use barium (or some other group 2 metals) to detect carbonate in water, thus detecting the level of water hardness. Advanced instrumentation is used, however, where quantitative results are required or multiple ions need to be identified at very low levels. This includes various forms of mass spectroscopy as well as ion chromatography and light spectroscopy.
Trends in the solubility properties of ionic compounds can be used for the qualitative analysis of ionic solutions. When a compound is added to a mixture of ionic solutions, many products can form, each with different solubility properties. If only one product is insoluble, then it alone will leave the solution. By performing sequential reactions, ions in a solution can be systematically identified and isolated.
While a variety of analytical instruments exist for elemental analysis, the techniques are often time-consuming or require transporting samples between laboratories. Qualitative analytical techniques such as examining solubility properties are fast, accessible pre-screening methods for analysis.
This video will introduce the solubility properties of ionic compounds, demonstrate procedures for selectively precipitating ionic compounds, and introduce a few applications of qualitative analysis using solubility trends in industrial settings.
Ionic compounds are composed of a cation and an anion. When a reaction occurs between two different ionic compounds, the cation of one compound is electrostatically attracted to the anion of another, forming a new compound. The ions that do not participate in the reaction are called spectator ions, and are omitted from the net ionic reaction. When an ionic compound dissolves, they reversibly interact with solvent molecules, and the ions dissociate. If the interaction between an ion and the new counter-ion is stronger than between the ion and the solvent molecules, it will be more favorable for the product to be in the solid phase. The formation of solid product from solution is known as precipitation, and the solid is called the precipitate.
Ions can be selectively isolated from solution by inducing reactions with insoluble precipitates. To design these reactions, cations and anions are assigned to broad categories based on solubility trends. Cations are grouped by identifying the anion common to their insoluble reaction products, and anions are likewise grouped by common cations. Solutions of these common ions are used to test for these groups.
When separation is desired for ions belonging to the same group, specialized reagents or concentrated solutions can be used to induce selective reactions once the ions in that group have been isolated. These specialized reagents can also be used to confirm the identity of an isolated ion. Now that you understand the principles behind the qualitative analysis of ions, let’s go through a technique for analyzing a solution for phosphate, followed by a procedure for separating a mixture of metal cations.
To analyze a solution for phosphate, first prepare dilute test solutions of aqueous calcium, ammonium orthomolybdate, and concentrated nitric acid. Then, place 5 mL of the unknown solution in a test tube.
Add the calcium solution dropwise to the unknown solution. The formation of a white precipitate could indicate the presence of calcium phosphate, or calcium carbonate. To verify the presence of phosphate, slowly add nitric acid to the test tube. Dissolution of the precipitate indicates that hydrogen phosphate has formed. The lack of gas bubbles indicates that no carbonate is present, as carbonate would have reacted with the acid to form carbon dioxide and water.
Finally, slowly add the ammonium orthomolybdate to the test tube. Ammonium phosphomolybdate forms as a yellow precipitate, confirming the presence of phosphate in the solution.
First, prepare dilute test solutions as listed in the text protocol. Obtain four test tubes and caps suitable for use in a centrifuge. Place a mixture of aqueous zinc, nickel, silver, and iron nitrates in one test tube. To begin separation, first slowly add dilute hydrochloric acid to the mixture, swirling gently. The white precipitate that forms is silver chloride. Continue adding chloride solution until no more precipitate forms.
Separate the liquid, or supernatant, and the solid silver chloride by centrifugation. Decant the supernatant into the second test tube. Wash the silver chloride three times with water and decant each wash into the second test tube. Next, add the sodium hydroxide solution dropwise to the second test tube. Three precipitates will form: white zinc hydroxide, yellow iron hydroxide, and green nickel hydroxide. Continue adding sodium hydroxide until the solid white zinc compound dissolves, forming the soluble zincate ion. Separate the zinc solution and the solid nickel and iron compounds by centrifugation, and then decant the solution into the third test tube. Wash the solids with water three times and decant each into the zinc solution.
Slowly add hydrochloric acid to the zinc solution in the third test tube until zinc hydroxide precipitates and then dissolves.
Then, add potassium hexacyanoferrate dropwise to the zinc solution to form potassium zinc hexacyanoferrate as a white precipitate. Now, to the test tube containing solid nickel hydroxide and iron hydroxide, slowly add ammonia to form the soluble blue nickel hexammine ion. Separate the nickel solution from the solid iron hydroxide by centrifugation and decant the nickel solution into the fourth test tube. Wash the iron hydroxide three times with water and decant the washes into the nickel solution. Then, slowly add dimethylglyoxime to the nickel solution to form nickel dimethylglyoxime as a red precipitate. To the solid iron hydroxide, carefully add concentrated hydrochloric acid to form a solution of ferric chloride. To confirm the presence of iron, add thiocyanate to form the deep red thiocyanatoiron cation.
The simplicity and speed of performing qualitative analysis of ions in solution makes this technique widely used in environmental chemistry and industry.
When water contains a high concentration of metal cations such as calcium or magnesium, it is called hard water. These metal cations can react with anions in the water such as carbonate to form chalky deposits that clog pipes or hot water heaters. Water hardness can be assessed by adding a carbonate solution to a water sample. White precipitate indicates high levels of calcium.
Phosphate is an important nutrient for many forms of life and is therefore used in both industrial and garden fertilizers, but an excess of phosphate can be detrimental, particularly in freshwater environments. Wastewater in residential and commercial areas can be tested for phosphates by adding nitric acid and ammonium orthomolybdate. Yellow precipitate indicates high levels of phosphates.
You’ve just watched JoVE’s introduction to solubility rules for ions. You should now be familiar with the principles of ionic reactions, a few procedures for qualitative analysis of solutions, and some applications of qualitative analysis using solubility.
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