A Precipitation Method
Precipitation done in such a way as to facilitate the separation process, for example: Ag was deposited with AgCl. Aspects that are important and need to be considered in that method is sediment has a very small solubility and can be separated by filtration. At a given temperature, the solubility of substances in a particular solvent is defined as the amount, if dissolved in a solvent of known weight and substance to reach equilibrium with the solvent. This depends on the size of the particle. Supersaturated solution is a solution with greater solute concentration than the state of equilibrium at a certain temperature. Examples of precipitation reaction with AgNO3 reagent:NaCl (aq) + AgNO3 → AgCl (s) + NaNO3 (Utami, 2008).Precipitation is the most widely used technique. The most important thing in the deposition of an analyte is its purity and ease of screening that would have been done in the deposition technique.Analysis of gravimetric precipitation will run fine if the requirements include:
· Perfection sedimentAnalat analyzed has changed completely into sediment or precipitate solubility in other words made as small as possible.• Purity sedimentThe precipitate is pure, clean sediment does not contain other molecules, called also by impurities or contaminants. The precipitate that dirty will contain a greater weight than it should so it will lead to greater error. Contamination by other substances is very easy to happen because of sediment arising from a solution containing a variety of substances.• The composition of sedimentThe precipitate that formed had a specific composition and constant or sediment that is formed can be converted into substances that particular composition.Gravimetry depositional stages, including:• Dissolve analat• Set state solution such as pH and temperature• Establishing sediment• Growing the crystals precipitate (digestion or aging)• Filtering and washing sediment• Heat or anneal to obtain the dry sediment with a particular composition. This is also done to remove the filter paper• Cooling and weigh precipitate
2 Purity deposition coprecipitation
If a precipitate separates from a solution, the precipitate was not always perfectly pure, may contain various amounts of impurities, depending on the nature of sediment and deposition conditions. Sediment contamination by substances that are normally dissolved in the mother liquor called coprecipitation. We must distinguish two types of coprecipitation are important. The first is related to the adsorption on the surface of the affected particle solution, and the second is that in relation to the occlusion of foreign substances during the process of crystal growth of primary particles.Regarding the surface adsorption (adsorption is a process that occurs when a fluid, liquid or gas, bonded to a solid or a liquid (adsorbent) and finally form a thin layer (adsorbate) on the surface), will generally be greatest at the sediment are similar to gelatin and at least in the sediment with macro-crystalline properties that stand out. Deposition with ionic lattice of adsorption seems to follow the rules of Paneth-Fajans-Hahn, which states that ions that form the least soluble salts. So on a slightly soluble sulfate, calcium ions more than the main adsorbed magnesium ions, because calcium sulfate is less soluble than magnesium sulfate. Also ionida mengardsorpsi silver acetate silver is much stronger than silver nitrate on conditions comparable, because the solubility of silver acetate is lower. Deformability (easily changed shape) ions are adsorbed and electrolyte dissociation of compounds that diardsorpsi also have an enormous influence, the smaller the greater the dissociation senyawaa teradsorpsinya. So hydrogen sulfide, a weak electrolyte, strong adsorbed by metal-metal sulfide.Coprecipitation of the second type occurs when sediment is built from particle-particle primary. Primary particles will go through adsorption surface to a certain extent and when these particles joined each other, impurities will be lost in part if formed single crystals are large and the process is slow, or if it is fast possible to join each other crystals produced large composed of small crystals is not tightly bound, and some impurities may carry over into the opposite wall of large crystals. If the substance is isomorphous impurities or form-solid solution with precipitation, the amount of coprecipitation may be very much, because there would be no tendency to eliminate impurities during the ripening process.
Pascapresipitasi (postpresipitasi) is the precipitation that occurs on the surface of the first sediment after it was formed. This happened on a few substances soluble, which form the supersaturated solution, these agents generally have an ion in common with one of the primary ion deposition (first precipitate). So the precipitation of calcium as oxalate in the presence of magnesium, magnesium oxalate gradually separated from the solution and settle on calcium oxalate, the longer the sediment is left in contact with a solution of it, the greater the aberration caused by this cause. Similar effects can be observed in the precipitation of copper (II) sulfide in 0.3 M hydrochloric acid in the presence of zinc ions, zinc sulfide with berpascapresipitasi slowly.Pascapresipitasi different from coprecipitation in terms of:a. Kontiminasi increased with increasing duration of sediment is left in contact with the mother liquor on pascapresipitasi, but usually reduced the coprecipitation.b. In pascapresipitasi, contamination will increase with the more rapid solution is stirred, either by mechanical means or thermal. In general the situation is reverse coprecipitation.c. Number of contamination on pascapresipitasi can be much greater than the coprecipitation.
3 Things To Optimum Precipitation
To obtain optimum conditions should follow the following rules:a. Precipitation should be done in a dilute solution, which aims to minimize errors due to koresipitasi.b. Peraksi mixed slowly and regularly with permanent stirring.c. Precipitation performed on hot solution formed when the sediment is stable at high temperatures.d. Precipitated crystals are usually formed in a long time by using the steam heater to avoid any koprespitasi.e. The precipitate should be washed with a dilute solution.f. To avoid postpresipitasi precipitation or coprecipitation should be re-Minimizing coprecipitation:1. The second method of reagent addition. If it is known that both the sample and the sediment contains an ion that pollutes, a solution containing these ions can be added to another solution. In this way, the concentration of impurities was kept as low as possible during the early stages of precipitation. In the case of hidrous oxide, mjatan carried by the primary particles can be controlled.2. Laundering. Impurity-adsorbed impurities can be removed by washing unless they are surrounded. With sediments that are similar to buttermilk and gelatin, a person must have an electrolyte in a solution of washing to avoid peptisasi.3. Digestion. This technique is very useful for deposition of crystalline, quite useful for sludge-like curd, but not used for the sediment that is gelatin.4. Pengendapa back. If the substance can be dissolved again (case of salts of weak acids in strong acid), it can be filtered, diluted again and again precipitated. Impurity ions will be in a low concentration during the second deposition, and therefore a smaller amount will dikopresipitasi.5. Separation. Impurities can be separated or chemical properties changed with a certain reaction before precipitation formed.
4 Homogeneous Precipitation from solution
In this method, the reagents generated by the slow homogeneous chemical reaction in solution. High density and sediment can be filtered, kopretisipasi reduced to its minimum value. Some examples of precipitation from homogeneous solution is:a) SulfateDimetilsulfat produce sulfate radical by the reaction:
(CH2) 2SO4 + 2H2O → 2CH3OH + 2H + + SO42-
b) Hydroxidecontrolled pH slowly. NH3 is produced from urea by the following reaction:CO (NH2) 2 + H2O → 2NH3 + CO2 at 90-1000Meanwhile, aluminum (Al) precipitated by urea as Al (OH) 3 in succinic acid medium, or Ba as BaCrO4 in ammonium acetate or Ni as glioksim or Al as oksinat.
c) OxalicCalcium is precipitated as CaC2O4.Thiorium also precipitated as Th (C2O4) 2 in the presence of urea.For example: CO (NH2) 2 + 2HC2O4 + H2O → CO2 + 2NH3 + 2C2O42-
(C2H5) 2 C2O4 + 2H2O → 2C2H5OH + 2H + + C2O42-
d) PhosphatePhosphate low berkelarutan can be precipitated by making derivatives of trimethyl phosphate or trietil gradually with hydrolysis. Precipitated as Zr3 Zr (PO4) 4 in (CH3) 3Po4 in media containing sulfuric acid.
5 Wash precipitate
For determining the quantity of analyte in gravimetric methods achieve results that approach the true value, must meet two criteria:§ The process of separation or precipitation of the analyte from other components is complete§ analyte resulting precipitate precisely known composition and having a high purity level, not mixed with impurities.For the second requirement, can be met through the washing of sediment. The purpose of the sediment washing is to remove impurities adsorbed on the surface of the sediment and be carried away mechanically, so that the sediment is obtained pure. Deposition is the deposition of pure analyte that is clean, meaning it does not contain other molecules (other substances that are usually called impurities or contaminants).Washing solution was divided into 3 groups, namely:1. Which prevent the formation of colloidal solution which can lead through filter paper, for example: the use of ammonium nitrate to wash sediment ferihidroksida2. Solution that reduces the solubility of the precipitate, for example: alcohol.3. Solution that can prevent the hydrolysis of salts of weak acid or weak base.
Repeated washing is more effective than washing once with the same total volume.
n
Where:Xo = concentration of impurities before washingN = number of washingXn = concentration of impurities after washingV = volume of washing solution
= Volume of fluid remaining in the sediment
The following are the selection criteria washing solution:- To dissolve impurities well but almost no precipitate dissolves- Does not contain salt that can not evaporate when dipijarkan- To prevent the laundering peptisasi on time deposits- Used washing solution containing ions namesake when it is possible to dissolve precipitate.- The hot washing solution can be used if the solubility of precipitation possible
6 Combustion Deposition
The precipitate that is clean must be dried before experiencing pemijaran.• If sediment still attached to the filter paper, then it must diabukan filter paper before using the fire as small as possible.· Then regularly elevated temperature until it reaches the desired temperature. Care should be taken for the paper to diabukan perfectly, it is necessary to observe the color of sediment, if the white paper terabukan means perfect, otherwise the black stain showed that there was still some paper terabukan not perfect.· Pemijaran at high temperatures allows diuapkannya water attached to the sediment. Stronger water absorption at the higher temperature deposition pemijarannya.· Example: reply-shaped sediment pemijaran gels require a fairly high temperature, while the crystalline precipitate pemijaran not require high temperatures.· Precipitate that had dipijarkan weight can not be directly determined by weighing the object in hot conditions did not produce a fixed price.Must be done cooling to room temperature can only be determined weight.
7 Role of Organic Reagents in Analysis Gravimetry
Organic reagents used in gravimetric analysis are known as organic sediment. Separation of one or more inorganic ions from the mixture made by adding organic reagents for organic compounds that have a large molecular weight, it can be determineda small number of ions with the formation of deposits in large numbers. Organic sediment that both must have specific properties. The precipitate formed by the organic reagent, dried or burned and weighed as oxide. On drying or combustion, the temperature must ditentukn based on their chemical properties. Selectivity (election) is achieved by monitoring the optimum reaction variables such as reagent concentration, pH and the use of protective reagents to reduce the interference of foreign ions.Some inorganic ions can be precipitated by certain organic reagents called settling organic. A number of reagents is useful not only for separation, for the deposition but also can be used for solvent extraction. Organic reagent which is widely used is the chelate-forming reagents. Some organic reagent to form a brown compound with several cations because it contains functional groups that form a base (electron donor). When the polyfunctional ligands can occupy more than two coordination positions of metal ion center, the coordination compounds formed with a ring structure called a chelate. Instructions to predict qualitatively the stability of complexes and equilibrium precipitate an uncharged chelate obtained from analyzes the formation constant of coordination compounds that are characteristic of metal ion and ligand properties.Organic sediment has a special place in the analysis of inorganic deposits formed because usually different from pure inorganic substances, such as between BaS0 and Ni (DMG), which is dimethyl glioksin DMG. Organic compounds are classified as forming chelate complexes, forming the salt and forming lakes. In attempts to form chelate ligands must have H atoms that can be replaced and unpaired electrons for pembnetukan coordination. Organic reagents are widely used because selekitf. Substitution at C can vary. Selectivity means the ability of the organic reagents to join one or two of metal to separate it from other substances. Steric effects (space) to determine the selectivity of chelate-forming reagents, not to precipitate Al.
8 Criteria for the Selection of Organic Reagentsa. Substance must be selective.b. Have specific properties.c. Large molecular weight (Mr greater) so that only small amounts of metal produce a high weight of sediment.d. Some organic reagents have the nature of selectivity that produces deposits with a small amount of cations.e. Having a very small solubility in water so that metal ions can be precipitated quantitatively.
9 Some Important Organic Deposition
Settling organic substance is an ingredient to help the process of separating one or more of a mixture of inorganic ions, organic substances settling regensia also called organic. Settling organic substances used must be ideal, it means settling organic shall be specific, which must give deposition with only one particular sediment.Some settling of organic that has been known widely used in gravimetric analysis such as:1) Dimeti glioksim (DMG), which is useful for the determination of nickel. Excess reagent must be avoided to prevent formation of deposits pereaksinya own. Citric and tartaric are used as protective reagents.2) 8-hidroksiquinolin useful to precipitate some metals and can be used for separation. Added on condition / cool atmosphere and the sediment washed with warm water. The precipitate then dissolved in acid and titrated.3) Acid mandelat used for Zr. The precipitate was burned and the oxide is weighed.4) Cupferon used for separation such as iron and titanium from aluminum. This is useful in acidic conditions, the cold solution, and the sediment was burned then weighed.5) Acid antranilat used in some metal (for Cu). Typically used the sodium salt.6) Acid kuinaldik (for Cu), this method is sensitive by using the reagent complexing agent. In the complex contained only 15% Cu.7) Reagent salisildioksim (for Cu). Tartaric acid is used as a masking agent. The complex is soluble in alcohol but not stable if more than 73 days. Weighed as Cu-salisildioksim.8) 1-nitroso-2-Naphthols (for metal Co.) was used in acidic conditions. The complex was torched and weighed as Co3O4. Pereaksinya made in glacial acetic acid and distilled water