Name: mass loss in gravimetric analysis
Uploaded: 2024-04-04T10:51:25.000Z
Duration: 1 min 10 s
Channel: JoVE

תנודתיות מפרידה מרכיב רצוי, המכונה אנליט, מהתרכובות האחרות בדגימה על ידי המרתו למין נדיף באמצעות תהליכים כמו חימום מבוקר או תגובות כימיות.
הסרת המינים הנדיפים משנה את משקל הדגימה.
לכן, מדידת ההבדל במשקל המדגם לפני ואחרי תנודתיות נותנת את כמות האנליט במדגם המקורי, בתנאי שאף מרכיב אחר אינו תנודתי בתהליך.
לחלופין, ניתן לטפל באנליט באמצעות מגיב כימי בכלי סגור כדי להפוך אותו למין נדיף שונה מבחינה כימית.
לאחר מכן החומר הנדיף מוסר לחלוטין מתערובת התגובה על ידי חימום התמיסה או בעבוע גז דרך התערובת. לאחר מכן, הוא נספג באופן סלקטיבי במלכודת שנשקלה מראש המכילה חומר סופג מתאים.
הפרש המשקל בין המלכודת לפני ואחרי התנודתיות מצביע על כמות החומר הנדיף המשתחרר מהדגימה, הקשור סטויכיומטרית לכמות האנליט בדגימה.

תנודתיות

Volatilization

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גרבימטריית הנידוף היא טכניקה אנליטית המודדת את המסה שאבדה עקב הנידוף של החומר. טכניקה זו משמשת להערכת כמות החומר הנדיף בדגימה. כדי לבצע שיטה זו, חממו כמות ידועה מהדגימה לטמפרטורה גבוהה בכור היתוך או כלי מתאים אחר. החומר הנדיף בדגימה מתאדה, והאדים נפלטים לחלוטין מהכור היתוך או על ידי חימום הדגימה או בעבוע זרם של גז אינרטי דרך הכלי. שאר הרכיבים הלא נדיפים נשארים בכור ההיתוך, ומסתם נמדדת בקנה מידה. ההבדל במסה לפני ואחרי הנידוף נותן את המסה של החומר הנדיף, אשר לאחר מכן ניתן להשתמש בו כדי לחשב את ריכוזו בדגימה המקורית. גישה זו מסייעת לקבוע את מי ההתגבשות הקיימים בתרכובות הלחות.

לחלופין, ניתן לטפל בדגימה עם מגיב כימי בכלי סגור ליצירת מין נדיף שונה מבחינה כימית עם הרכב ידוע. לאחר מכן החומר הנדיף נספג באופן סלקטיבי במלכודת שקולה מראש מלאה בחומר ספיגה מתאים. לדוגמה, הפחמן הדו-חמצני המשתחרר מתרכובות המכילות קרבונט יכול להיספג באופן סלקטיבי בסיד-ליים, ו-NaOH יכול להיספג באופן סלקטיבי במלכודת עם סיליקט לא סיבי. לאחר מכן ניתן לשקול את המלכודות כדי לקבוע את כמות החומר הנספג.
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תנודתיות מפרידה מרכיב רצוי, המכונה אנליטי, מהתרכובות האחרות במדגם על ידי המרתו למין נדיף באמצעות ...

Education

JoVE Core

Analytical Chemistry

כימיה אנליטית

Unit 1

יחידה 1

Introduction to Separation Methods

מבוא לשיטות הפרדה

KEY TERMS AND CONCEPTS

מונחי מפתח ומושגים

mass loss in gravimetric analysis

Volatilization gravimetry is a way to measure mass loss in a sample by driving off a volatile substance. It helps estimate how much volatile material is present. In this method, a known amount of sample is heated to a high temperature in a crucible or another suitable vessel.

As the sample heats, the volatile substance evaporates. The vapor is then completely removed from the crucible, either by continued heating or by bubbling an inert gas through the vessel. The non-volatile part stays behind, and its mass is measured on a balance. The difference between the starting mass and the final mass gives the mass of the volatile substance. That value can be used to calculate its concentration in the original sample.

This approach is useful for finding the water of crystallization in hydrated compounds. It can also be done with a chemical reagent in a closed vessel. In that version, the sample forms a chemically different volatile species with a known composition.

The volatilized substance can then be trapped in a pre-weighed absorbent. For example, carbon dioxide released from carbonate-containing compounds can be absorbed in soda-lime. NaOH can be absorbed in a trap filled with non-fibrous silicate. The trap is weighed again to determine how much material was captured.

Mass Loss in Gravimetric Analysis

states of matter and phase changes

Heating and Cooling During Phase Changes

Heating and cooling can change a substance from one state of matter to another. These phase changes depend on internal energy, which is the total kinetic energy of particles and the potential energy from their interactions. In the condensed phases, stronger intermolecular forces and the pressure on the substance affect that internal energy. The internal energy is highest in the gaseous state, lowest in the solid state, and in between for the liquid state. Phase transitions happen when...

distillation: vapor&ndash;liquid equilibria

Distillation and Boiling Point Changes

Distillation separates a miscible liquid mixture by using differences in boiling points. It works best when the two liquids have a significant boiling point gap, usually at least 20°C. In this process, the mixture is heated until the more volatile component reaches its bubble point and begins to vaporize. The vapor then rises into a condenser. A condenser is a glass tube with separate inner and outer sections. Cold water flows through the outer section and cools the vapor in the inner section,...

filtration

Filtration Setup and Filtrate Collection

Filtration separates a solid from a liquid by passing a suspension through a porous medium. The solid stays on the filter, and the liquid, called the filtrate, passes through. This makes filtration a physical separation process, not a chemical change. The filtering medium is chosen for the purpose of the separation, the amount of material, and the nature of the precipitate. A good filter medium should be inert, mechanically strong, nonabsorbent toward dissolved materials, and able to let...

centrifugation

Centrifugation for Separating Particles

Centrifugation separates particles in a liquid by using differences in size and density. The sample is placed in centrifugation tubes and spun at high angular velocity. This creates centrifugal force, which makes the different components move at different rates. After spinning, the supernatant, or liquid above the settled material, is decanted. In some cases, the pellet is kept for further purification. In other cases, the supernatant is kept and may be spun again for additional processing. <br...

sublimation

Sublimation in Separation and Purification

Sublimation is the direct change of a solid into a gas. It skips the liquid state. A common example is solid carbon dioxide, or dry ice, which turns into gaseous carbon dioxide at standard pressure and room temperature. The phase diagram shows when sublimation can happen. This process occurs at the solid-gas boundary. It is not seen above the triple point of a substance. The reverse process is deposition, which is when a gas changes directly into a solid. Sublimation and deposition can help...

recrystallization: solid&ndash;solution equilibria

recrystallization-solidsolution-equilibria

Recrystallization for Purifying Solid Compounds

Recrystallization is a way to purify solid compounds by using differences in solubility. It does not involve a chemical reaction. Instead, it relies on physical properties of the desired compound and the impurities. The process is based on solid-solution equilibrium, also called solubility equilibrium. In this equilibrium, a component is split between the solid phase and the solution phase. At equilibrium, the chemical potential of that component is the same in both phases. To begin...

crystal growth: principles of crystallization

crystal-growth-principles-of-crystallization

Crystal Formation Through Supersaturation

Crystal formation through supersaturation begins when a solution contains more dissolved solute than it can normally hold. In crystallization, atoms or molecules line up into a rigid crystal lattice so the system can lower its energy. To start the process, the solute concentration and the solution temperature must be adjusted so the supersaturation ratio, S, is greater than one. Supersaturation can be increased in two main ways. One method is to concentrate the solution by evaporating the...

precipitation and co-precipitation

Selective Ion Separation by Precipitation

Selective ion separation by precipitation is used to isolate ions from a solution. In qualitative inorganic analysis, ions that form sparingly soluble precipitates with the same reagent can be separated by using differences in their solubility products. A solubility product is the limit at which an ionic solid begins to form from dissolved ions. One example is the separation of Cu(II) and Fe(II) ions as insoluble sulfides. Copper(II) sulfide is first precipitated by adding acidic H2S. The acid...

coagulation

Wastewater Cleanup by Coagulation

Coagulation helps remove colloidal particles from wastewater. Colloids are tiny solid particles suspended in a liquid. They are usually negatively charged, so they attract a compact first layer of positive ions. More counterions gather around them and form an electrical double layer. This charge barrier keeps the particles apart and stabilizes the colloid. These suspended solids can be a problem because they may contain toxins that are hard to remove. Coagulation is used to make the particles...

electrodeposition

Electrodeposition for Metal Ion Separation

Electrodeposition separates metal ions from a solution by using an electric current and an electrolytic cell. The cell has an anode and a cathode. When current flows, oxidation happens at the anode. At the cathode, metal ions gain electrons, undergo reduction, and deposit on the large metal surface. This method works well when metals have very different reduction potentials. Copper and nickel in the same solution can be separated this way. First, the solution is made acidic. Then current is...

extraction: partition and distribution coefficients

extraction-partition-and-distribution-coefficients

Solvent Partitioning in Chemical Extraction

Solvent partitioning in chemical extraction describes how a solute spreads between two immiscible solvents. Immiscible solvents do not mix, such as an aqueous phase and an organic phase. The distribution law, also called Nernst's distribution law or the partition law, explains how that solute is shared at a constant temperature and equilibrium. At equilibrium, the ratio of solute concentrations in the two solvents stays constant. For extraction from water into an organic solvent, this...

extraction: effects of ph

pH Control in Liquid Extraction

pH control in liquid extraction changes how amines and acids move between organic and aqueous phases. The pH of the aqueous phase affects the charge on acidic and basic solutes. The charged form is usually more soluble in the aqueous phase. For a weak base such as an amine, the neutral form is called B. Its partition coefficient, K, describes how the neutral molecule distributes between the two liquids. If the conjugate acid of the amine is soluble only in the aqueous phase, while the base...

extraction: advanced methods

Chelate Extraction for Metal Ion Separation

Chelate extraction is a way to separate metal ions by using an organic ligand, or chelating agent, to form an uncharged metal complex. The chelating agent needs hydrophobic groups, and it must act as a weak acid so it can lose a proton and bind to the metal. Most organic ligands used for this method are not stable in water or are insoluble in the aqueous phase. For that reason, the chelating agent is first placed in the organic phase. It then moves into the aqueous phase, where the...

chromatography: introduction

Chromatography: Separation by Polarity

Chromatography separates compounds by how they divide between a stationary phase and a mobile phase. The stationary phase is the material that holds the compounds for a time. The mobile phase is the solvent that carries the solutes to be analyzed. In column chromatography, a mixture flows through the stationary phase. The compounds separate based on their structure and polarity. Solutes that interact weakly with the stationary phase move faster through the column and exit first. Solutes that...

dialysis

Dialysis Separation by Diffusion

Dialysis separates small molecules from larger ones by diffusion through a semipermeable membrane. It is a purification method used to remove unwanted ions or small organic molecules from a sample while keeping larger analytes in place. The membrane is usually made of cellulose acetate or cellulose nitrate. Its pores are typically about 1 to 5 nm wide. Molecules such as chloride, sodium, and glucose can pass through these pores. Larger molecules, including proteins, hormones, and enzymes, are...

volatilization

Volatilization gravimetry is a way to measure mass loss in a sample by driving off a volatile substance. It helps estimate how much volatile material is present. In this method, a known amount of sample is heated to a high temperature in a crucible or another suitable vessel. As the sample heats, the volatile substance evaporates. The vapor is then completely removed from the crucible, either by continued heating or by bubbling an inert gas through the vessel. The non-volatile part stays...

ion exchange

Ion Exchange Resins and Charged Molecules

Ion exchange chromatography separates charged molecules from a solution by swapping them with mobile, or active, ions on a stationary phase. It is used to separate ions, soften and deionize water, and purify solutions. The method depends on a solid column made of high-molecular-weight, chemically stable polymers that are crosslinked to make them porous and essentially insoluble. These polymers are functionalized with either acidic or basic groups. When acidic groups are present, the resin is a...

key terms and concepts

Learn how volatilization gravimetry measures mass loss after heating a sample, and how trapped vapors like carbon dioxide are weighed in analysis.

Mass Loss in Gravimetric Analysis | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

Mass Loss in Gravimetric Analysis

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