Name: cation identification by precipitation tests
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 3 min 46 s
Channel: JoVE

K12 Standards

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Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Interactions Of Matter

Chemical reactions

Qualitative Analysis

Gdy nieznany roztwór zawiera mieszaninę kilku różnych jonów metali, kationy można zidentyfikować za pomocą systematycznej serii selektywnych wytrąceń, zwanej analizą jakościową.
Na każdym etapie analizy dodawany jest inny odczynnik strącający. Odczynniki te selektywnie wytrącają niektóre kationy w postaci nierozpuszczalnych soli, które można usunąć, podczas gdy inne pozostają w roztworze.
W roztworach wodnych występują 22 powszechnie występujące kationy, które można podzielić na pięć grup na podstawie produktów rozpuszczalności ich nierozpuszczalnych soli.
Kationy grupy 1 to jony metali, które tworzą nierozpuszczalne chlorki.
Większość soli chlorkowych jest rozpuszczalna w wodzie. Tak więc, gdy wodny roztwór jonów metali jest traktowany 6 M kwasem solnym, osad wskazywałby na jon metalu z grupy 1, taki jak srebro, ołów lub rtęć.
Jeśli nie tworzy się osad, oznacza to, że w roztworze nie ma kationów grupy 1. Mieszanina ta jest następnie odwirowywana lub filtrowana w celu oddzielenia stałego osadu i wodnego supernatantu.
Następnie gazowy siarkowodór jest przepuszczany przez kwaśny supernatant. W wyniku reakcji między jonami metali a siarkowodorem powstaje siarczek metalu i protony.
Ze względu na dodanie kwasu solnego w poprzednim etapie, wysokie stężenie protonów przesuwa równowagę w kierunku reagentów.
Tak więc w warunkach kwaśnych wytrąca się tylko jony metali z grupy 2, które tworzą wysoce nierozpuszczalne sole siarczkowe, podczas gdy inne siarczki metali, które są nieco lepiej rozpuszczalne, pozostają w roztworze.
Następnie wytrąca się kationy grupy 3, które składają się z nierozpuszczalnych w zasadach siarczków i wodorotlenków.
Wodorotlenek sodu dodaje się do supernatantu z poprzedniego etapu w celu ustalenia warunków podstawowych. Dodatek ten zubaża protony z reakcji wytrącania siarczku metalu, a równowaga przesuwa się w kierunku produktów.
W rezultacie wiele siarczków metali, które były rozpuszczalne w środowisku kwaśnym, teraz staje się nierozpuszczalnych i tworzy osady.
Dodatkowo jony metali, które tworzą nierozpuszczalne wodorotlenki, takie jak żelazo, aluminium i chrom, wytrącają się z roztworu.
Po oddzieleniu tej mieszaniny w roztworze pozostają tylko jony metali alkalicznych i ziem alkalicznych.
Metale ziem alkalicznych, które stanowią kationy grupy 4, tworzą nierozpuszczalne fosforany.
Dodanie wodorofosforanu dwuamonu do zasadowego supernatantu powoduje wytrącanie się jonów magnezu, wapnia, baru i strontu.
Ciecz zdekantowana z tego etapu zawiera kationy z grupy 5. Kationy te nie tworzą nierozpuszczalnych soli i muszą być identyfikowane indywidualnie.
Jeśli dodanie wodorotlenku sodu do roztworu w poprzednich etapach uwolniło gaz o charakterystycznym zapachu amoniaku, oznacza to, że w mieszaninie obecne były jony amonowe.
Jony sodu i potasu można zidentyfikować za pomocą testu płomieniowego. Jony sodu wytwarzają jasnożółty płomień, podczas gdy fioletowy płomień wskazuje na jony potasu.

<ol>
	<li>Cole'a Jr., G. Mattneya i Williama H. Waggonera. "Analiza jakościowa". Journal of Chemical Education 60, nr 2 (1983): 135.</li>
	<li>Lo, Glenn V. "Powiązanie analizy jakościowej z zasadami równowagi." Journal of Chemical Education 78, nr 11 (2001): 1557.</li>
	<li>Ricketts, John A. i Eugene P. Schwartz. "Analiza pierwiastkowa stopu: schemat analityczny podkreślający chemię kationów". W: Proceedings of the Indiana Academy of Science, tom 101, nr 1-2, s. 67-74. Rok 1992.</li>
</ol>

analiza jakościowa

W przypadku roztworów zawierających mieszaniny różnych kationów tożsamość każdego kationu można określić na podstawie analizy jakościowej. Technika ta polega na serii selektywnych wytrąceń różnymi odczynnikami chemicznymi, przy czym w każdej reakcji powstaje charakterystyczny osad dla określonej grupy kationów. Jony metali w grupie są dalej rozdzielane poprzez zmianę pH, ogrzewanie mieszaniny w celu ponownego rozpuszczenia osadu lub dodanie innych odczynników w celu utworzenia jonów złożonych.

Na przykład kationy grupy IV, które składają się z nierozpuszczalnych węglanów i fosfataz, takich jak Ba2+, Ca2+ i Mg2+, wszystkie tworzą białe osady w obecności wodorofosforanu diamonu (NH4)2HPO4 w roztworze zasadowym. Osad rozpuszcza się w rozcieńczonym kwasie octowym. Aby zidentyfikować każdy kation, przeprowadza się test potwierdzający.

Wszystkie trzy kationy tworzą jasnożółte sole chromianowe po dodaniu chromianu potasu (K2CrO4); jednakże tylko chromian baru (BaCrO4) jest nierozpuszczalny w kwasie octowym. Roztwór można przesączyć, a filtrat zawiera Ca2+ i Mg2+.

Przesącz można teraz podzielić na dwie części w celu zbadania pozostałych kationów. Jeżeli w obecności roztworu szczawianu amonu (NH4)2C2O4 z roztworu wytrąci się biały osad, można potwierdzić obecność jonów Ca2+. Biały osad to szczawian wapnia, który jest nierozpuszczalny zarówno w wodzie, jak i w kwasie octowym.

Mg2+ identyfikuje się za pomocą testu wnękowego na węglu drzewnym. W tym teście węglany metali rozkładają się na odpowiedni tlenek metalu w komorze węgla drzewnego. Kolor pozostałości wskazuje na możliwy kation. Tlenek magnezu MgO pozostawia białą pozostałość w komorze węgla drzewnego. Pozostałość tę traktuje się kilkoma kroplami roztworu azotanu kobaltu Co(NO3)2. Pod wpływem ciepła azotan kobaltu rozkłada się na tlenek kobaltu (II), który tworzy różowy amalgamat, CoO-MgO, co potwierdza obecność Mg2+.

Analiza jakościowa

W przypadku roztworów zawierających mieszaniny różnych kationów, tożsamość każdego kationu można określić za pomocą analizy jakościowej. Ta technika obejmuje ...

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Acid-base and Solubility Equilibria

Równowagi kwasowo-zasadowe i rozpuszczalności

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

cation identification by precipitation tests

Cation identification by precipitation tests uses selective reactions to find metal ions in a mixture. In qualitative analysis, different reagents are added step by step. Each reaction can produce a characteristic precipitate for a specific group of cations. Other steps, such as changing pH, heating the mixture to dissolve a precipitate again, or adding reagents that form complex ions, help separate ions within the same group.

Group IV cations such as Ba2+, Ca2+, and Mg2+ are identified through a series of tests. In a basic solution, diammonium hydrogen phosphate, (NH4)2HPO4, gives each of these ions a white precipitate. The precipitates are then dissolved in dilute acetic acid. A confirmatory test follows for each cation.

Potassium chromate, K2CrO4, forms bright yellow chromate salts with all three ions. Even so, only barium chromate, BaCrO4, stays insoluble in acetic acid. The solution is filtered after this step. The filtrate then contains Ca2+ and Mg2+, which can be tested separately.

If ammonium oxalate, (NH4)2C2O4, produces a white precipitate, Ca2+ is confirmed. That precipitate is calcium oxalate, and it does not dissolve in water or acetic acid. Mg2+ is identified by a charcoal cavity test. In this test, metallic carbonates decompose into the matching metal oxide in a charcoal cavity, and the residue color gives a clue to the cation.

Magnesium oxide, MgO, leaves a white residue in the charcoal cavity. The residue is then treated with a few drops of cobalt nitrate, Co(NO3)2, solution. When heated, cobalt nitrate decomposes to cobalt(II) oxide and forms a pink CoO-MgO amalgam. This pink result confirms the presence of Mg2+.

Cation Identification by Precipitation Tests

cation-identification-by-precipitation-tests

common ion effect

Common Ion Effect on Solubility

The common ion effect reduces the solubility of an ionic compound in a solution that already contains one of its ions. In aqueous solutions, this means less of the solid dissolves than it would in pure water. The effect follows the law of mass action and can be explained with Le Châtelier’s principle. Silver iodide shows this equilibrium clearly. When silver(I) ions or iodide ions are added, the dissolution equilibrium shifts left. More AgI precipitates, and the concentrations of dissolved Ag+...

buffers

Buffer Solutions and pH Stability

Buffer solutions are mixtures that help keep pH steady. A buffer contains an appreciable amount of a weak conjugate acid-base pair. It resists a change in pH when small amounts of a strong acid or a strong base are added. One common buffer uses a weak acid and its salt. Acetic acid and sodium acetate form this type of system, written as CH3COOH(aq) + CH3COONa(aq). Another buffer uses a weak base and its salt. A solution of ammonia and ammonium chloride, NH3(aq) + NH4Cl(aq), is an example. The...

calculating ph changes in a buffer solution

calculating-ph-changes-in-a-buffer-solution

Buffer pH After Acid or Base Addition

A buffer solution can resist sudden pH changes when a strong acid or strong base is added. It does not stop the pH from changing completely. The shift is usually small, as long as the addition stays within the buffer’s capacity. To find the new pH, first determine how the added acid or base changes the amounts of the buffer components. A buffer contains a weak acid and its conjugate base, or the reverse pair. These amounts are found with stoichiometric calculations, which track how much of...

buffer effectiveness

Buffer Capacity and pH Stability

Buffer solutions help keep pH fairly constant, but they do not work forever. Their resistance to pH change depends on having enough of a weak acid and its conjugate base, or a weak base and its conjugate acid, in the mixture. When too much strong acid or strong base is added, one part of the buffer pair is used up and the buffer action becomes weaker. Buffer capacity describes how much acid or base can be added to a fixed volume of buffer before the pH changes noticeably, usually by about one...

titration calculations: strong acid - strong base

titration-calculations-strong-acid-strong-base

pH at Key Points in Acid-Base Titration

pH at key points in a strong acid and strong base titration can be calculated from the amounts of acid and base present. The example uses 25.00 mL of 0.100 M HCl, a strong acid, and 0.100 M NaOH, a strong base. The pH changes as base is added, and each stage follows a different calculation. At 0 mL of titrant, the pH comes from the ionization of HCl. Because HCl is a strong acid, its ionization is complete. The hydronium ion molarity is 0.100 M, and the pH is found from that value. At 12.50...

titration calculations: weak acid - strong base

titration-calculations-weak-acid-strong-base

pH at Key Points in a Weak Acid Titration

pH at key points in a weak acid titration can be calculated for an acetic acid solution as sodium hydroxide is added. The example uses 25.00 mL of 0.100 M CH3CO2H titrated with 0.100 M NaOH. The reaction is shown as the weak acid reacts with the strong base. The initial pH is found from the acetic acid solution using the usual ICE approach. ICE means initial, change, and equilibrium, and it is a common way to set up equilibrium problems. The solution pH is calculated before any titrant is...

indicators

Acid-Base Color Changes

Acid-base indicators show color changes that help identify the pH of a solution. These are often weak organic acids or weak organic bases, and they respond to changes in hydronium ion concentration in dilute aqueous solution. Phenolphthalein is one example. It is colorless when the hydronium ion concentration is greater than 5.0 × 10^-9 M, which means the pH is below 8.3. In more basic solutions, where the hydronium ion concentration is lower, it turns red or pink. Methyl orange shows how an...

titration of a polyprotic acid

Polyprotic Acid Equivalence Points

Polyprotic acids have more than one ionizable hydrogen, so they lose protons in steps during titration. When the acid dissociation constants for those protons are far enough apart, the titration curve shows a separate equivalence point for each hydrogen that can ionize. Diprotic acids have two ionizable hydrogens and produce two equivalence points. Triprotic acids have three ionizable hydrogens and produce three equivalence points. Carbonic acid, H2CO3, is a weak diprotic acid. Carbonic acid...

solubility equilibria

Predicting Precipitation with Ksp

Solubility equilibria explain how a dissolved ionic compound and its solid form can exist in balance. In a saturated solution, the solute dissolves and the ions precipitate at equal rates. These ideas matter in real settings such as tooth decay and water purification, where compound solubility affects the outcome. A substance with finite solubility can form a saturated solution when more solute is added than the solvent can dissolve. The result is a heterogeneous mixture of the saturated...

factors affecting solubility

Common Ion Effects on Solubility

The common ion effect lowers the solubility of an ionic compound in water when the solution already contains one of the ions made by that compound. It follows the law of mass action and can be explained with Le Châtelier’s principle. When a common ion is added, the dissolution equilibrium shifts toward the solid, so less of the salt stays dissolved. Silver iodide shows this effect clearly. If silver ions or iodide ions are added to a solution of AgI, the equilibrium shifts left and more AgI...

formation of complex ions

Ligands and Complex Ion Stability

Complex ions form when a central atom, often a transition metal cation, binds to ligands. Ligands are ions or molecules around the metal ion. They can be neutral molecules such as H2O or NH3, or ions such as CN− or OH−. In many cases, the ligands act as Lewis bases by donating a pair of electrons to the central atom. These reactions are part of coordination chemistry, which studies coordination complexes. When a metal ion reacts with one or more ligands to form a complex, the equilibrium...

precipitation of ions

Solubility Limits and Selective Precipitation

Solubility limits and selective precipitation explain when ions form a solid in water. The key idea is the balance between an insoluble compound and its dissolved ions. In aqueous solutions that contain calcium ions and carbonate ions, this equilibrium can exist even if the solution was not made by saturating water with calcium carbonate. When two soluble salts are mixed, such as sodium carbonate and calcium nitrate, the ion concentrations determine whether a precipitate forms. If the reaction...

qualitative analysis

Cation identification by precipitation tests uses selective reactions to find metal ions in a mixture. In qualitative analysis, different reagents are added step by step. Each reaction can produce a characteristic precipitate for a specific group of cations. Other steps, such as changing pH, heating the mixture to dissolve a precipitate again, or adding reagents that form complex ions, help separate ions within the same group. Group IV cations such as Ba2+, Ca2+, and Mg2+ are identified...

henderson-hasselbalch equation

Buffer pH with the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation helps relate buffer pH to the weak acid in the solution. It comes from the ionization-constant expression for a weak acid. By rearranging that expression to solve for hydronium ion concentration, [H3O+], and then taking the negative logarithm of both sides, the equation can be written in terms of pH. In this form, pKa is defined as the negative logarithm of the weak acid ionization constant, or pKa = −log Ka. The equation shows the connection between pH, pKa,...

acid-base titration curves

Tracking pH During Acid-Base Titration

Acid-base titration uses pH changes to track how a solution responds as titrant is added. A titration curve is a graph of a solution property versus the amount of added titrant. In acid-base titrations, pH is the main property to follow because it changes in a predictable way with the solution makeup and helps show the endpoint. This pattern can be seen with a strong acid and strong base, a strong acid and weak base, or a strong base and weak acid. For example, 25.00 mL of 0.100 M HCl titrated...

key terms and concepts

Learn how Ba2+, Ca2+, and Mg2+ are identified with selective precipitation, chromate, oxalate, and charcoal cavity tests in qualitative analysis.

Cation Identification by Precipitation Tests | JoVE K12

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Indicators of Chemical Reactions

Chemical Equations

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are ...

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

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Reaction Yield

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Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

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In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &#x2013; the precipitate. The reaction occurs ...

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Oxidation&ndash;Reduction Reactions

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The Integrated Rate Law: The Dependence of Concentration on Time

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Half-life of a Reaction

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Temperature Dependence on Reaction Rate

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Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. ...

Arrhenius Plots

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Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

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Relating Reaction Mechanisms
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Constant Pressure Calorimetry

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First Law of Thermodynamics

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Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form ...

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Quantifying Heat

Thermal Energy&#160;

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The Equilibrium Constant

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For a reaction that begins with a mixture of reactants only, the product concentration is initially equal to ...

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Homogeneous Equilibria for Gaseous Reactions

Homogeneous Equilibria for Gaseous Reactions
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Enthalpy

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Reaction Quotient

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Relative Strengths of Conjugate Acid-Base Pairs

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Calculating Equilibrium Concentrations

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Strong Acid and Base Solutions

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The Small x Assumption

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Weak Acid Solutions

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Le Chatelier's Principle: Changing Concentration

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Weak Base Solutions

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Le Chatelier's Principle: Changing Volume (Pressure)

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Mixtures of Acids

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Bronsted-Lowry Acids and Bases

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acidbase-strengths-and-dissociation-constants

Acid/Base Strengths and Dissociation Constants

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Polyprotic Acids

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Water: A Bronsted-Lowry Acid and Base

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Acid Strength and Molecular Structure

Binary Acids and Bases
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pH Scale

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Chemical reactions
• Types of reactions
• Kinetics
• Energy
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Content elaboration: In the Physical Science course, coefficients were used to balance simple equations. Other representations, including Lewis structures and three-dimensional models, were also used and manipulated to demonstrate the conservation of matter in chemical reactions. In this course, more complex reactions will be studied, classified and represented with balanced chemical equations and three-dimensional models. Classifying reactions into types can be a helpful organizational tool for recognizing patterns of what may happen when two substances are mixed. Teachers should be aware that the common reaction classifications that are often used in high school chemistry courses may lead to misconceptions because they are not based on the actual chemistry, but on surface features that can be similar from one system to another (e.g., exchanging partners), even though the underlying chemistry is not the same. However, these classifications may be useful in making predictions about what happens when two substances are mixed. Some general types of chemical reactions are oxidation/reduction, synthesis, decomposition, single replacement, double replacement (including precipitation reactions and some acid-base neutralizations) and combustion reactions. Some reactions can fit into more than one category. For example, a single replacement reaction can also be classified as an oxidation/reduction reaction. Identification of reactions involving oxidation and reduction as well as indicating what substance is being oxidized and what is being reduced are appropriate in this course. However, balancing complex oxidation/reduction reactions is reserved for more advanced study. Organic molecules release energy when undergoing combustion reactions and are used to meet the energy needs of society (e.g., oil, gasoline, natural gas) and to provide the energy needs of biological organisms (e.g., cellular respiration). When a reaction between two ionic compounds in aqueous solution results in the formation of a precipitate or molecular compound, the reaction often occurs because the new ionic or covalent bonds are stronger than the original ion-dipole interactions of the ions in solution. Laboratory experiences (3-D or virtual) with different types of chemical reactions should be provided. Reactions occur when reacting particles collide in an appropriate orientation and with sufficient energy. The rate of a chemical reaction is the change in the amount of the reactants or products in a specific period of time. Increasing the probability or effectiveness of the collisions between the particles increases the rate of the reaction. Therefore, changing the concentration of the reactants, changing the temperature or the pressure of gaseous reactants, or using a catalyst, can change the reaction rate. Likewise, the collision theory can be applied to dissolving solids in a liquid solvent and can be used to explain why reactions are more likely to occur between reactants in the aqueous or gaseous state than between solids. The rate at which a substance dissolves should not be confused with the amount of solute that can dissolve in a given amount of solvent (solubility). Mathematical treatment of reaction rates is reserved for more advanced study. Computer simulations can help visualize reactions from the perspective of the kinetic-molecular theory. In middle school, the differences between potential and kinetic energy and the particle nature of thermal energy were introduced. For chemical systems, potential energy is in the form of chemical energy and kinetic energy is in the form of thermal energy. The total amount of chemical energy and/or thermal energy in a system is impossible to measure. However, the energy change of a system can be calculated from measurements (mass and change in temperature) from calorimetry experiments in the laboratory. Conservation of energy is an important component of calorimetry equations. Thermal energy is the energy of a system due to the movement of its particles. The thermal energy of an object depends upon the temperature and chemical composition. Some materials require little energy to change their temperature and other materials require a great deal to change their temperature by the same amount. Specific heat is a measure of how much energy is needed to change the temperature of a specific mass of material a specific amount. Specific heat values can be used to calculate the thermal energy change, the temperature (initial, final or change in) or mass of a material in calorimetry. Water has a particularly high specific heat capacity, which is important in regulating Earth's temperature. As studied in middle school, chemical energy is the potential energy associated with chemical systems. Chemical reactions involve valence electrons forming bonds to yield more stable products with lower energies. Energy is required to break interactions and bonds between the reactant atoms and energy is released when an interaction or bond is formed between the atoms in the products. Molecules with weak bonds (e.g., ATP) are less stable and tend to react to produce more stable products, releasing energy in the process. Generally, energy is transferred out of the system (exothermic) when the products have stronger bonds than the reactants and is transferred into the system (endothermic) when the reactants have stronger bonds than the products. Predictions of the energy requirements (endothermic or exothermic) of a reaction can be made given a table of bond energies. Graphic representations can be drawn and interpreted to represent the energy changes during a reaction. The role of energy in determining the spontaneity of chemical reactions is dealt with conceptually in this course. Entropy and its influence on the spontaneity of reactions are reserved for more advanced study. All reactions are reversible to a degree and many reactions do not proceed completely toward products but appear to stop progressing before the reactants are all used up. At this point, the amounts of the reactants and the products appear to be constant and the reaction can be said to have reached dynamic equilibrium. Dynamic equilibrium means the rate of the reverse reaction is equal to the rate of the forward reaction so there is no apparent change in the reaction. If a chemical system at equilibrium is disturbed by a change in the conditions of the system (e.g., increase or decrease in the temperature, pressure on gaseous equilibrium systems, concentration of a reactant or product), then the equilibrium system will respond by shifting to a new equilibrium state, reducing the effect of the change (Le Chatelier's Principle). If products are removed as they are formed during a reaction, then the equilibrium position of the system is forced to shift to favor the products. In this way, an otherwise unfavorable reaction can be made to occur. Mathematical treatment of equilibrium reactions is reserved for advanced study. Computer simulations can help visualize the progression of a reaction to dynamic equilibrium and the continuation of both the forward and reverse reactions after equilibrium has been attained. Properties of acids and bases and the ranges of the pH scale were introduced in Physical Science. In this course, the structural features of molecules are explored to further understand acids and bases. Acids often result when hydrogen is covalently bonded to an electronegative element and is easily dissociated from the rest of the molecule to bind with water to form a hydronium ion (H3O+). The acidity of an aqueous solution can be expressed as pH, where pH can be calculated from the concentration of the hydronium ion. Bases are likely to dissociate in water to form a hydroxide ion. Acids can react with bases to form a salt and water. Such neutralization reactions can be studied quantitatively by performing titration experiments.

Cation Identification by Precipitation Tests

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