Weekly Topic for the Analysis Section (October 17–October 23)

Overview of the analysis process The task of quantitative analysis is to determine the content of relevant components in a substance. To complete a quantitative analysis task, it usually involves the following steps. Sample collection and preparation → Sample drying → Sample decomposition → Elimination of interferences → Measurement → Calculation of results ① Sample collection and preparation Different sampling methods are used depending on whether the analyte is a gas, liquid, or solid. During the sampling process, it is crucial to obtain samples that represent the average composition of the material being tested. If the composition of the sampled material is not representative, accurate analysis is useless; it may even lead to incorrect conclusions, causing significant losses in production or research. Typically, the amount of sample taken for analysis is very small—sometimes less than 1 gram. How can the component content in a sample weighing less than 1 gram be representative of the content in thousands of tons of material? The common method for obtaining representative samples is to select multiple sampling points from different locations and depths within a large quantity of material; the numerous samples obtained are then crushed, screened, mixed, and reduced in volume to produce a small number of analysis samples. Taking ore as an example, the sampling and preparation methods are briefly introduced here.     First, select appropriate sampling points and sampling volumes based on the stacking condition of the ore and the size of its particles. Based on the empirical formula mQ≥kd2, the minimum mass mQ of the sample required (in kg) can be determined. In this formula, k is the reduction constant, which is an empirical value. The worse the uniformity of the sample, the larger the k value; k usually ranges from 0.05 kg·mm-2 to 1.0 kg·mm-2. d is the maximum particle size of the sample (diameter, in mm).     The collected samples must undergo multiple processes of crushing, sieving, mixing, and quartering before they can meet the requirements for analysis.     Crushing is divided into coarse crushing, medium crushing, fine crushing, and even grinding, so that the particle size of the sample becomes small enough to pass through the required sieve pores. The relationship between the sieve numbers and pore diameters of standard sieves is shown in Table 7.1. To ensure the representativeness of the samples, after each crushing and sieving process, the coarse particles that do not pass through the sieve holes should be crushed further until all of them pass through; these coarse particles must not be discarded, as their chemical composition may differ from that of the fine particles.     Table 1 Relationship between sieve numbers and pore sizes of standard sieves. Sieve number (mesh size): 3, 6, 10, 20, 40, 60, 80, 100, 120, 140, 200. Pore diameter in mm: 6.72, 3.36, 2.00, 0.83, 0.42, 0.25, 0.177, 0.149, 0.125, 0.105, 0.074. The purpose of reduction is to gradually decrease the amount of the ground sample; generally, the quartering method is used – the sampled material after screening is mixed together, piled into a conical shape and then compressed into a disc-like form, after which it is divided into four equal parts, with the two diagonal parts being discarded. To determine whether to further reduce the size of the two retained samples, the above formula can be used for calculation based on the relationship between particle size and sample volume.     For example, if there is a sample of 10 kg that, after crushing, passes through all sieve holes No. 10 (with a maximum particle diameter of 2 mm), and the value of k is 0.3 kg·mm-2, then the amount of sample that should be retained is:
mQ ≥ 0.3 kg·mm-2 × (2 mm)² / 10 kg = 1.2 kg.
Since 10 kg × (1/2)³ = 10 kg × 1/8 = 1.25 kg, in order to retain more than 1.2 kg from the 10 kg sample after reduction, the sample can be reduced in size only up to 3 times.     The required particle size for analysis samples is related to factors such as the ease of decomposing the sample; ore samples are generally required to pass through sieves numbered 100–200. ②Drying of the sample: Crushed samples have a large surface area, which makes them prone to absorbing moisture from the air. This absorbed water is known as free water. In order to accurately determine the content of the components of interest in the sample, it is necessary to remove this free water by drying the sample at different temperatures, depending on its properties, before weighing it. For samples that are prone to decomposition during drying or that absorb moisture more readily from the air after drying, the “air-drying” method can be used. Some substances are prone to exploding when heated, so the moisture must be removed at room temperature in a desiccator. ③Sample decomposition In quantitative analysis, it is generally necessary to first decompose the sample. During this process, it is important to prevent the volatilization and loss of the components being analyzed, while also avoiding the introduction of impurities that could interfere with the measurement. An appropriate decomposition method should be selected based on the properties of the sample and the testing method used. For the determination of inorganic samples, wet analysis is commonly used; that is, the sample is decomposed and transferred into a solution, which is then analyzed. The commonly used decomposition methods are dissolution and melting ； For the decomposition of organic samples, dry ashing and wet digestion methods are commonly used.     1. Dissolution method: Depending on the properties of the sample, this method involves using acids, bases, or solvents to dissolve the sample; it is also the most commonly used method. In the dissolution of actual samples, besides water—the commonly used solvent—there are also several commonly used acid or alkali solutions. These are summarized as follows: HCl: Metals and alloys whose reactivity rank above hydrogen in the activity series, as well as basic oxides and weak acid salts, can all be dissolved in HCl. Taking advantage of the reducibility and coordination ability of Cl‑, pyrolusite (MnO2) and hematite (Fe2O3) can also be dissolved.     HNO3 is oxidizing; with the exception of platinum, gold, and certain rare metals, most metals can be dissolved in nitric acid. However, metals that can be passivated by HNO3 (such as aluminum, chromium, and iron), as well as metals that react with HNO3 to form insoluble acids (such as antimony, tin, and tungsten), cannot be dissolved by HNO3.     Concentrated and hot H2SO4 has strong oxidizing and dehydrating properties; it can dissolve various alloys and ores. It is also commonly used to decompose organic substances. It has a high boiling point (338°C); by heating it and dissolving it until H2SO4 begins to emit white smoke (SO3), HCl, HF, and HNO3 in the solution can be removed.     H3PO4 turns into pyrophosphoric acid when heated; it possesses strong chelating capabilities and is commonly used to dissolve alloy steels and insoluble minerals.     Hot HClO4 has strong oxidizing and dehydrating properties. Heating until white fumes of HClO4 appear (203°C) can remove low-boiling-point acids. However, hot HClO4 can explode when in contact with organic substances; therefore, HNO3 should be used first to oxidize the organic substances and reducing agents, before adding HClO4.     HF has weak acidity, but F– has a strong coordinating ability. HF is often used in combination with H2SO4 or HNO3 to decompose silicates; however, since HF reacts with Si to form the volatile compound SiF4, it must be used to decompose samples in platinum or polytetrafluoroethylene containers.     NaOH is mainly used to decompose certain amphoteric metals (such as aluminum) or oxides (such as Al2O3).     Aqua regia is obtained by mixing concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1. It has extremely strong oxidizing and decomposing properties; it can be used to decompose certain poorly soluble precious metals, alloys, and sulfides.     2. Melting method: In this method, the sample is mixed with a solid solvent and placed in a crucible made of a specific material. It is then melted at high temperatures to decompose the sample; subsequently, water or a solution is used for leaching, thereby transferring the components into the solution. Based on the acidity or alkalinity of the solvent used, they can be divided into acid fusion methods and alkali fusion methods.     Acid melting: Common acidic solvents include K2Cr2O7 and KHSO4. SO3 produced by decomposition at high temperatures can react with basic oxides, thereby enabling the decomposition of oxide ores such as those of iron, aluminum, titanium, zirconium, and niobium; quartz or platinum crucibles can be used for melting.     Alkali fusion: Commonly used alkaline solvents include Na2CO3, NaOH, Na2O2, etc., which are used to decompose most acidic minerals. Due to the strong corrosiveness of Na2O2, only iron, silver, or corundum crucibles can be used during melting.     3. Dry ashing method The dry ashing method involves placing the organic sample in a muffle furnace and heating it to high temperatures (400–700°C) to decompose it; the resulting inorganic residue is then extracted with an acid to produce an analytical solution. Since this method does not use a flux to decompose the sample, it results in low blank values, which is of great significance for the analysis of trace elements.     Volatile elements can be determined using a low-temperature ashing apparatus. By passing an oxygen stream excited by high-frequency electricity through the sample at a temperature of only 150°C, the sample can be decomposed, which is used to determine elements such as As, Se, and Hg in biological samples.     Additionally, the oxygen bottle combustion method is also a commonly used dry ashing technique. In this method, the sample is wrapped in quantitative filter paper and secured with a platinum wire, then placed in a sealed flask filled with oxygen for combustion. The halogens, sulfur, phosphorus, and metal elements in the sample are converted into halide ions, sulfate ions, phosphate ions, and metal oxides, which are dissolved in the absorbent solution allowing for separate determination. This method has advantages such as complete decomposition of the sample, simple and fast operation, and suitability for analyzing small amounts of sample.     4. Wet digestion method: The wet digestion method uses a mixture of nitric acid and sulfuric acid as a solvent, which is heated together with the sample to facilitate decomposition. For samples that contain compounds prone to forming volatile substances such as nitrogen, arsenic, mercury, etc., distillation is generally used for decomposition. This method is simple and fast, but care must be taken regarding the purity of the solvent used for decomposition; otherwise, impurities may be introduced due to the impure solvent. For example, the Kjeldahl method uses wet digestion for decomposition. First, the organic nitrogen in the sample is decomposed into inorganic ammonium salts. Then, NaOH is added to convert these ammonium salts into ammonia gas, which is allowed to volatilize. The emitted ammonia gas is absorbed by an excess amount of hydrochloric acid. Subsequently, the remaining hydrochloric acid is titrated with a standard NaOH solution. Based on the amount of NaOH consumed during the titration and the total amount of hydrochloric acid present, the original nitrogen content in the sample can be calculated. ④Elimination of interference Complex samples often contain multiple components, and when measuring one of these components, the other components present can cause interference, which must be eliminated. Using a masking agent to eliminate interference is a relatively simple and effective method. But sometimes, masking agents alone are not sufficient to completely eliminate interference; especially when the concentration of the interfering substances is high or no suitable masking agent is available, it is necessary to separate and remove the component of interest from the interfering components in advance. Common separation methods include precipitation separation, extraction, ion exchange, and chromatographic separation. The methods used to eliminate interference vary among different analysis techniques, and they should be treated differently. For example, when using the EDTA complexometric titration method to analyze Zn2+ in a mixed solution containing Al3+ and Zn2+, Al3+ acts as a serious interference; F- can be added to form stable complexes with Al3+, thereby masking Al3+ and allowing for the direct titration and analysis of Zn2+. ⑤ Calculation of measurement and analysis results: An appropriate analysis method is selected for measurement based on the properties and content of the component being analyzed, as well as the requirements regarding the accuracy of the analysis results. During the measurement, the experiment is conducted in accordance with the procedures of the standard test method, and the measured experimental data are recorded. To minimize the random error in the measurements, multiple parallel tests should be conducted (usually 3 times) and the average value taken.     Based on the mass of the sample, the measured data, and the stoichiometric relationships of the relevant reactions during the analysis, the content of the components in the sample can be calculated. This content can be expressed as the mass, mass fraction, or molar concentration of the analyte. Furthermore, the principles of statistical data analysis can be employed to evaluate the accuracy and precision of the analytical results.               

In actual analytical work, apart from dry analysis, it is usually necessary to first decompose the sample and quantitatively transfer the component to be determined into a solution before carrying out the measurement. When decomposing the sample, the following principles should be followed: ① The decomposition of the sample must be complete ； ②During the decomposition of the sample, there must be no loss of the component to be determined ； ③The component to be tested and interfering substances must not be introduced. Depending on the properties of the sample and the measurement methods used, common decomposition methods include dissolution, fusion, and dry ashing. 　　1. Dissolution method: The method of using an appropriate solvent to dissolve the sample and produce a solution is called the dissolution method. Common solvents include water, acids, and bases. 　　(1) For soluble inorganic salts, the water dissolution method can be used to prepare a solution by simply dissolving them in distilled water. 　　(2) Acid dissolution method: Various inorganic acids and mixed acids are commonly used as solvents for dissolving samples. By utilizing the acidity, oxidizing power, and chelating ability of these acids, the component to be analyzed is transferred into the solution. The commonly used acids are as follows. 　　①Hydrochloric acid (HCl): Most chlorides are soluble in water. Metals with a potential order lower than that of hydrogen, as well as most metal oxides and carbonates, can dissolve in hydrochloric acid. In addition, the Cl– ion possesses certain reducing properties, and it can also form complex ions with many metal ions, thereby facilitating the dissolution of the sample. It is commonly used to dissolve samples such as hematite (Fe2O3), stibnite (Sb2S3), carbonates, and pyrolusite (MnO2). 　　②Nitric acid (HNO3) has strong oxidizing properties. Almost all nitrates are soluble in water. Except for platinum, gold, and certain rare metals, concentrated nitric acid can dissolve almost all metals and their alloys. Iron, aluminum, chromium, etc., are passivated by nitric acid; when dissolving them, adding a non-oxidizing acid such as hydrochloric acid can remove the oxide film, allowing for proper dissolution. Almost all sulfides can also be dissolved by nitric acid, but hydrochloric acid should be added first to cause sulfur to volatilize as H2S, so as to prevent elemental sulfur from coating the sample and interfering with the decomposition process. 　　③Sulfuric acid (H2SO4) dissolves the sulfates of all metals except calcium, strontium, barium, and lead. Hot concentrated sulfuric acid has strong oxidizing and dehydrating properties, and is commonly used to decompose metals such as iron, cobalt, and nickel, as well as metal alloys like aluminum, beryllium, antimony, manganese, thorium, uranium, and titanium. It is also used to break down organic substances in samples such as soil. Sulfuric acid has a relatively high boiling point (338°C). When anions of low-boiling-point acids such as nitric acid, hydrochloric acid, and hydrofluoric acid interfere with the determination, sulfuric acid is often added and the mixture is evaporated until white fumes (SO3) appear, thereby removing these interfering substances. 　　④Phosphoric acid (H3PO4) and its phosphate ions possess strong chelating capabilities; as a result, almost 90% of ores can be dissolved in phosphoric acid. It includes many other acid-insoluble minerals such as chromite, ilmenite, niobite, rutile, etc., and can also effectively dissolve alloys containing high levels of carbon, chromium, and tungsten. When using phosphoric acid for dissolution alone, the temperature should generally be controlled between 500 and 600°C, for a duration of no more than 5 minutes. If the temperature is too high and the time is too long, insoluble pyrophosphate precipitates will form, and polysilicic acid will be generated, adhering to the bottom of the vessel; this also corrodes the glass. 　　⑤Hot, concentrated hydrogen perchloric acid possesses strong oxidizing properties and can rapidly dissolve steel and various aluminum alloys. It can oxidize elements such as Cr, V, and S to their highest oxidation states. The boiling point of perchloric acid is 203°C; when evaporated until it starts to smoke, low-boiling-point acids can be removed, and the residue is readily soluble in water. Perchloric acid is also commonly used as a dehydrating agent in gravimetric methods for determining SiO2. When using HClO4, contact with organic substances should be avoided to prevent explosions. 　　⑥Hydrofluoric acid (HF) has weak acidity, but the F— ion possesses strong coordination ability; it can form complex ions with ions such as Fe(III), Al(III), Ti(IV), Zr(IV), W(V), Nb(V), Ta(V), and U(VI), allowing these compounds to dissolve in water. It can also react with silicon to form SiF4, which then escapes from the solution. 　　(3) Mixed acid dissolution method ① Nitric acid HNO3 and hydrochloric acid HCl are mixed in a volume ratio of 1:3. Due to the oxidizing property of nitric acid and the coordinating property of hydrochloric acid, it possesses better solubility. It can dissolve metals such as Pb, Pt, Au, Mo, W, and alloys such as Bi, Ni, Cu, Ga, In, U, V. It is also commonly used to dissolve sulfides of Fe, Co, Ni, Bi, Cu, Pb, Sb, Hg, As, Mo, and ores of Se, Sb, etc. 　　②Backward aqua regia is prepared by mixing HNO3 and HCl in a 3:1 volume ratio. It can decompose sulfides of metals such as Ag, Hg, Mo, etc., as well as Fe, Mn, and Ge. A mixture of concentrated HCl, concentrated HNO3, and concentrated H2SO4, known as royal sulfuric acid, can dissolve ores with a high silicon content and aluminum alloys, respectively. 　　③ HF + H2SO4 + HClO4 can decompose metals such as Cr, Mo, W, Zr, Nb, Tl and their alloys, as well as samples such as silicates, ilmenite, fly ash, and soil. 　　④ HF+HNO3 are commonly used to decompose silicides, oxides, borides, and nitrides, etc. 　　⑤ H2SO4 + H2O2 + H2O are mixed in a volume ratio of 2:1:3 for H2SO4:H2O2:H2O. It can be used for the digestion of samples such as oils, grains, and plants. Adding a small amount of CuSO4, K2SO4, and selenium powder as catalysts can make the digestion faster and more complete. 　　⑥ HNO3 + H2SO4 + HClO4 (in small amounts) are commonly used to decompose chromium ore and various biological samples, such as animal and plant tissues, urine, feces, and hair. 　　⑦ HCl+SnCl2 is mainly used to decompose limonite, hematite, magnetite, etc. 　　(4) Alkali dissolution method: The main solvents used in this method are NaOH and KOH, or small amounts of Na2O2 and K2O2. It is commonly used to dissolve amphoteric metals such as aluminum, zinc and their alloys, as well as their hydroxides or oxides; it can also be used to dissolve acidic oxides such as MoO3 and WO3.

Aqueous dissolution method, acid dissolution method, alkali fusion method

Depending on the reagents used for decomposing the sample, the decomposition methods can be divided into wet and dry methods. The wet method uses solutions of acids, bases, or salts to decompose the sample, while the dry method uses solid salts or bases to melt or sinter the sample for decomposition. I. Acidic decomposition: Since acids are relatively easy to purify, excess acids, with the exception of phosphoric acid, can also be easily removed. During decomposition, no cations other than hydrogen ions are introduced; this method has advantages such as simplicity of operation, low operating temperature, and minimal corrosion to containers, which contributes to its widespread use. The disadvantage of the acid decomposition method is its poor ability to decompose certain minerals, and some elements may be lost through volatilization. 1. Hydrochloric acid: The boiling point of concentrated hydrochloric acid is 108°C; therefore, the dissolution temperature should be kept below 80°C. Otherwise, due to rapid evaporation of the hydrochloric acid, the sample will not decompose completely. ① Elements or compounds that are soluble in hydrochloric acid are: Fe, Co, Ni, Cr, Zn ； Ordinary steel, high-chromium iron, most metal oxides (such as MnO2, 2PbO•PbO2, Fe2O3, etc.), peroxides, hydroxides, sulfides, carbonates, phosphates, borates, etc. ②Substances insoluble in hydrochloric acid include the oxides of Al, Be, Cr, Fe, Ti, Zr, and Th that have been burned, SnO2, Sb2O5, Nb2O5, Ta2O5, zirconium phosphate, monazite, xenotime, the sulfates of strontium, barium, and lead, spinel, and pyrite ； Sulfides of mercury and certain metals, chromite, niobium and tantalum ores, and various thorium and uranium ores. ③ As (Ⅲ), Sb (Ⅲ), Ge (IV) and Se (IV), as well as Hg (II), Sn (IV) and Re (VIII), tend to volatilize and be lost from hydrochloric acid solutions (especially when heated). When the solution is heated, other volatile acids present in the sample, such as HBr, HI, HNO3, H3BO3, and SO3, will of course also be lost. 2. Nitric acid: ① Elements and compounds that are readily soluble in nitric acid include all metals except gold and the platinum group metals as well as those that are easily passivated by nitric acid; crystalline uranium ore (UO2) and thorite (ThO2); lead ores; almost all native minerals of uranium along with their carbonates, phosphates, vanadates, and sulfates. ②Nitric acid is not suitable for decomposing oxides as well as the elements Se, Te, As. Many metals form an insoluble oxide protective layer when immersed in nitric acid, and therefore do not dissolve; these metals include Al, Be, Cr, Ga, In, Nb, Ta, Th, Ti, Zr, and Hf. Ca, Mg, and Fe can dissolve in dilute nitric acid. 3. Sulfuric acid: ① Concentrated sulfuric acid can decompose sulfides, arsenides, fluorides, phosphates, antimony minerals, uranium minerals, monazite, fluorite, etc. It is also widely used in alloys of oxidized metals such as Sb, As, Sn, and Pb, as well as in various metallurgical products, but lead precipitates as PbSO4. After complete dissolution, some of the remaining acid can be easily removed by heating until smoking occurs, but this will result in the loss of some arsenic. Sulfuric acid is also often used to dissolve oxides, hydroxides, and carbonates. Due to the low solubility of calcium sulfate, sulfuric acid is not suitable for dissolving substances whose main component is calcium. ②An important application of sulfuric acid is the removal of volatile acids, but Hg(II), Se(IV), and Re( VII) may be lost to some extent. Phosphoric acid and boric acid can also be lost. 4. Phosphoric acid can be used to decompose many silicate minerals, most sulfide minerals, natural rare earth phosphate minerals, and mixed oxides of divalent uranium and hexavalent uranium. The most important analytical application of phosphoric acid is the determination of divalent iron in chromite, ferrites, and various silicates insoluble in hydrofluoric acid. Although phosphoric acid has a strong decomposing ability, it is usually used only for certain individual tests, rather than for systematic analysis. Phosphoric acid can form insoluble salts with many metals, even in strongly acidic solutions, which poses many difficulties for analysis. 5. Perchloric acid: Dilute aqueous solutions of perchloric acid, whether warm or cold, do not possess oxidizing properties. Thicker acids (60%–72%) have no oxidizing power when cold, but are strong oxidants when heated. Pure perchloric acid is an extremely dangerous oxidizer; it will explode if stored, and therefore must never be used. Extreme care must be taken when handling mixtures of perchloric acid, water, and dehydrating agents such as acetic anhydride or concentrated sulfuric acid. It is also a strict rule to be extremely cautious whenever perchloric acid is mixed with compounds of unknown properties. Hot concentrated perchloric acid reacts with almost all metals (except gold and some platinum group metals), oxidizing them to their highest oxidation states; only lead and manganese remain in lower oxidation states, namely Pb(II) and Mn(II). However, under these conditions, Cr is not completely oxidized to Cr (VI). Adding chlorides to the solution ensures that all iridium is in the +4 valence state. Perchloric acid can also dissolve sulfide minerals, chromite, apatite, chromium trioxide, as well as carbonides embedded in steel. 6. Hydrofluoric acid: The decomposition using hydrofluoric acid is widely applied in the analysis of silicates derived from natural sources or industrial production; it is also suitable for many other substances, such as oxides of Nb, Ta, Ti, and Zr, as well as ores of Nb and Ta or ores with low silicon content. In addition, minerals such as tungsten-niobium steel, silicon steel, rare earths, and uranium can also be easily decomposed by hydrofluoric acid. Many minerals, including quartz, beryl, zircon, chromite, topaz, cassiterite, corundum, pyrite, kyanite, stibnite, pyrrhotite, andradite, spinel, graphite, rutile, sillimanite, and certain tourmalines, present difficulties in being decomposed using hydrofluoric acid. 7. Mixed acids: Mixed acids can often complement each other’s strengths, and sometimes yield new, stronger dissolving capabilities. Aqua regia (HNO3∶HCl = 1∶3): It can dissolve precious metals as well as various sulfide minerals such as cinnabar, cadmium, mercury, and calcium. It can also break down natural oxides of uranium, uraninite, and many other derivatives containing rare earth elements, thorium, and zirconium, as well as certain silicates, alum minerals, wulfenite, molybdenite, and most natural sulfate minerals. Phosphoric acid – Nitric acid: Can decompose sulfides and oxides of copper and zinc. Phosphoric acid – sulfuric acid: Can decompose many oxidized minerals, such as iron ore and some silicates that are stable to other inorganic acids. Perchloric acid – sulfuric acid: Suitable for decomposing very stable minerals such as chromospinel. Perchloric acid — hydrochloric acid — sulfuric acid: can decompose iron ore, nickel ore, and manganese ore. Hydrofluoric acid – nitric acid: Can decompose ferrosilicon, silicates, and samples containing tungsten, niobium, titanium, etc. II. Melting decomposition method: For samples that cannot be completely decomposed using acids or other fluxes, melting can be employed for decomposition. This method involves mixing a flux with the sample, and then, at high temperatures, converting the sample into compounds that are soluble in water or acid. The melting method requires high-temperature equipment, as well as a large amount of solvents, cations, and crucible materials, which is disadvantageous for some types of analyses. 1. Classification of fluxes: ① Alkaline fluxes: such as alkali metal carbonates and their mixtures, borates, hydroxides, etc. ②Acidic fluxes: include acid sulfates, pyrosulfates, hydrofluorides, boron anhydrides, etc. ③Oxidizing fluxes: such as sodium peroxide, mixtures of alkali metal carbonates with oxidizers, etc. ④Reducing fluxes: such as mixtures of lead oxide and carbonaceous substances, mixtures of alkali metals and sulfur, mixtures of alkali metal sulfides and sulfur, etc. 2. Basic principles for selecting fluxes: Generally, acidic samples require alkaline fluxes, while alkaline samples need acidic fluxes. Oxidizing samples are best treated with reducing fluxes, and reducing samples with oxidizing fluxes; however, there are exceptions to these rules. 3. Introduction to Common Solvents ① Carbonates. Na2CO3 or KNaCO3 is commonly used as a flux to decompose ore samples, such as albite, barite, niobium-tantalum ores, iron ores, manganese ores, etc. The melting temperature is generally between 900 and 1000°C, with a duration of 10 to 30 minutes. The ratio of flux to sample varies significantly depending on the type of sample: it is 1:1 for iron ores or manganese ores, around 5:1 for silicates, and 10 to 20:1 for some refractory materials such as zirconium silicate, glazes, and refractory materials. Platinum crucibles are typically used for this purpose. The disadvantage of the carbonate melting method is that some elements are lost through volatilization; mercury and thallium volatilize completely, while Se, As, and iodine are lost to a large extent. Fluorine, chlorine, and bromine suffer less loss. ②Sodium peroxide. Sodium peroxide fusion is often used to dissolve metals and alloys that are extremely difficult to dissolve, chromite ore, and other minerals that are hard to decompose, such as ilmenite, niobium-tantalum ores, beryl, zircon, and tourmaline. The disadvantage of this method is that sodium peroxide is impure and cannot be further purified. Some crucible materials often end up mixed into the sample solution; to overcome this drawback, Na2CO3 or NaOH can be added. Below 500°C, platinum crucibles can be used; below 600°C, zirconium and nickel crucibles can be used. Other materials that may be used include iron, silver, and corundum. ③Sodium (potassium) hydroxide. Alkali metal hydroxides have a low melting point (328°C), and their melting can occur at temperatures much lower than those of carbonates. It is highly effective for silicates (such as kaolin, refractory clay, ash, slag, glass, etc.), especially for the melting of aluminosilicates. Furthermore, it can also be used to decompose lead, vanadium, Nb, Ta, as well as boron minerals and many hydrides, phosphates, and fluorides. For the melting of hydroxides, Ni crucibles (600°C) and silver crucibles (700°C) outperform other crucibles. The ratio of flux amount to sample amount is 8–10∶1; the disadvantage of this method is that the flux tends to absorb moisture. Therefore, spattering is likely to occur during melting. The advantage is speed, and the solidified melt readily dissolves F–, Cl–, Br–, As, B, etc., with no loss. ④Potassium (sodium) pyrosulfate. Potassium persulfate can be obtained using the K2S2O7 product, or by dehydrating KHSO4. The temperature during melting should not be too high, nor should the duration be too long. If the sample is difficult to decompose, it is advisable to cool the melt from time to time and add a few drops of concentrated sulfuric acid, although this is not very convenient. For BeO, FeO, Cr2O3, Mo2O3, Tb2O3, TiO2, ZrO2, Nb2O5, Ta2O5, and rare earth oxides, as well as the nonsilicate minerals of these elements, such as ilmenite, magnetite, chromite, niobite, etc., pyrosulfate fusion is particularly effective. Platinum and fused quartz are commonly used crucible materials for such melting; the former is slightly corroded, while the latter performs better. The ratio of flux to sample amount is 15:1. Pyrosulfate melting is not suitable for many silicates; in addition, cassiterite, zircon, and zirconium phosphate are also difficult to decompose. The applicability of pyrosulfate fusion is limited by the volatilization losses of many elements. III. Sources of error in the dissolution and decomposition processes 1. Losses due to droplets and volatilization: When dissolution is accompanied by the release of gases or occurs at the boiling point, a small amount of solution is lost – bubbles form and carry this solution away in the form of droplets. Covering the surface with a watch glass can help **reduce these losses**. Another source of error is the upward creep of salts along the walls of the crucible during melting or solution evaporation; heating the crucible as evenly as possible, preferably in an oil bath or sand bath, or sometimes using crucibles made of different materials, can help avoid this phenomenon. When inorganic substances dissolve, in addition to volatile acids and anhydrides such as hydrogen halides and sulfur dioxide, many other compounds may also be lost. Elements that form volatile compounds include As, Sb, Sn, Se, Hg, Ge, B, Os, Ru, and C, P, Si, as well as Cr, which form hydrides. The losses caused by volatilization can be prevented in many ways. In some cases, carrying out the reaction in a flask equipped with a reflux condenser is sufficient to achieve the goal. When the sample melts and decomposes, the possibility of volatilization losses increases significantly due to the high reaction temperature; however, this kind of loss can be **reduced by covering the crucible. 2. Losses due to adsorption: In the vast majority of cases, the relative amount of solute loss increases as the concentration decreases. In all adsorption processes, the properties of the adsorption surface play a decisive role. Different containers exhibit significantly varying adsorption properties, and the order of adsorption varies depending on the substances. Thoroughly cleaning the container can significantly reduce adsorption. Removing the grease from the glass surface significantly reduces surface adsorption. In many cases, acidifying the solution is sufficient to prevent inorganic cations from adsorbing onto glass or quartz. Generally, the degree of anion adsorption is low; therefore, for those ions that are strongly adsorbed, ligands can be added to convert them into anions, thereby reducing adsorption. 3. Elimination of foam: When evaporating liquids or carrying out wet oxidation to decompose samples, especially biological samples, foam formation can sometimes be an issue. To solve this problem, the sample can be left to stand in concentrated nitric acid overnight; sometimes, pre-ashing the organic material at 300–400°C before wet chemical decomposition is very effective in eliminating foam. A more common method to prevent foaming is to add chemical additives such as aliphatic alcohols; silicone oils can also be used sometimes. 4. Blank values: When using solvents and fluxes, it is necessary to take into account the significant blank values that may occur. Although high-purity reagents are now available, their usage amount is relatively large compared to the sample. Sintering technology is also used as a means to reduce the amount of reagents required, thereby lowering the blank value. Unclean utensils are often the main source of errors. For example, the crucible retains residues from previous measurements that have been melted or alloyed, and during subsequent analysis, these latter residues may be released. Furthermore, the reaction between the sample and the container can also alter the blank value. For example, silicates, phosphates, and oxides tend to react with the glaze of porcelain boats and crucibles; for this reason, quartz crucibles are preferable, as quartz only reacts with oxides at high temperatures. For residues of oxides or silicates, platinum crucibles may be the best choice. In most cases, careful selection of the container material can still eliminate blank values

This post was last edited and replied to by *ngguanglueguo on 2011-10-17 at 16:48. Reply 1#: *ngguanglueguo The decomposition of inorganic substances falls under the category of sample preparation in analytical chemistry; its purpose is to convert solid inorganic samples into liquid solutions that are easier to analyze. I. Acid dissolution at room temperature. The sample is dissolved using an appropriate acid such as hydrochloric acid, sulfuric acid, phosphoric acid, or a mixture of these acids, thereby transferring the component of interest for determination from the solid phase to the liquid phase. II. High-temperature and high-pressure digestion method. Special equipment is used to decompose samples that cannot be broken down under normal conditions under high pressure and high temperature conditions, in order to enable measurement. III. High-temperature gasification method. This method is used when the volatiles obtained from the high-temperature decomposition of a solid sample can serve as the target components. Such as the determination of S in mineral powder, tube furnace method, etc. There’s nothing to copy; it was created by me, so please feel free to give feedback if it’s not good.

Reply 1# *ngguanglueguo 1. Dissolution method: The method of converting a sample into a solution using an appropriate solvent is called the dissolution method. Common primary solvents include water, acids, bases, etc. 2. Melting method: This involves mixing the sample with an acid or base, and utilizing the multiphase reaction that occurs between the sample and the solvent at high temperatures to convert the components of the sample into compounds that are soluble in water or acids and bases. 3. Dry ashing method: The sample is heated in a muffle furnace at a certain temperature to cause its decomposition and ashing, after which the residue is dissolved using an appropriate solvent. 4. Pressure dissolution method: The sample reacts with the solvent under high temperature and pressure to dissolve. 5. Microwave sample dissolution method: Uses microwave energy to heat and dissolve the sample.

The main methods for preparing samples include four steps: crushing, sieving, mixing, and sampling

Reply 1# *ngguanglueguo 1. Dissolution method: The method of using an appropriate solvent to dissolve the sample and convert it into a solution is called the dissolution method. Common solvents include water, acids, and bases. 　2. Melting method The melting method involves mixing the sample with an acidic or basic flux, and utilizing the multiphase reactions that occur between the sample and the flux at high temperatures to convert the components of the sample into compounds that are soluble in water or acids. 　3. Dry ashing method: The sample is heated in a muffle furnace at a certain temperature to cause it to decompose and be ashed, after which the remaining residue is dissolved using an appropriate solvent. 4. Microwave sample dissolution method: This involves using microwave energy to heat and dissolve the sample along with the solvent in a sealed, pressure-resistant, and high-temperature resistant polytetrafluoroethylene container.

Reply 1# *ngguanglueguo: dissolution method, melting method, and dry ashing method

1. Precipitation separation method 2. Solvent extraction separation method 3. Chromatographic separation method 4. Ion exchange method