Starch is packed into semicrystalline granules (containing crystalline and non-crystalline section) called starch granules. These granules are either contained in plant leaves or stored for long term usage in the plant's seeds/roots/fruit. In the leaves, the starch granules are smaller and are located inside chloroplasts. This starch is termed transitory starch and is accessed during the night to provide the plant with energy. The granules contained in the plant's other organs hold starch referred to as storage starch which is reserved for long-term usage. These granules are stored inside special double-envelopped organelles called amyloplasts. Potato tubers contain this type of starch and are used as the potato plant's "battery" when the shoot of the plant has died and thus can not provide the plant with any energy (glucose) via photosynthesis.
The structure of starch granules has been debated and it's not yet clear. Nevertheless, scientists have identified some components. As the two polymers that make up starch are just repeated glucose molecules, starch consists only of glucose. Amylose is polymerized into a coiled chain of glucose molecules (no branching), while amylopectin shows a linear but branched structure. The granules consist of 10-30% amylose (percentage varies depending on source) and 70-90% amylopectin. The branched chains of amylopectin interact together and form double helices while the linear part of amylopectin that is not surrounded by its branches resides together with amylose chains. These amylose chains form the amorphous (non-crystalline) part of the granules while the packed double helices form the crystalline one.
When we boil rice for example, we observe the grains swelling and becoming much more soft. This manifests itself in the process of starch gelatinization.
At high temperatures, the intermolecular bonds of the starch molecules (for example the double helices formed by amylopectin) are broken down (new water-starch hydrogen bonds are formed) and thus the structure of the starch granules is altered. First the amorphous regions are disrupted and then the granule's whole structure gets effected. The granules lose their integrity and burst. Amylose (and a smaller amount of amylopectin) molecules leave the granule and contribute to the increased viscosity of the liquid.
When the temperature drops, recrystallization occurs. This is referred to as starch retrogradation and is responsible for bread staling.
As starch is just a chain of glucose, , molecules (and is broken down by the enzyme amylase found in our saliva) it can provide an organism with its main source of energy through aerobic or anaerobic respitation. This process uses glycose with (aerobic) or without oxygen (anaerobic) to create ATP, the energy "currency" of organisms.
Starch is an organic structure (carbohydrate) composed of two distinct polymers, amylose and amylopectin that are all made up of repeated glucose molecules. It is used as a reserve of energy, providing plants with glucose molecules (and consequently energy) when photosynthesis can not occur (at night or in winter). In humans it's a source of glucose necessary for energy production. Starch is also used in papermaking, glue and laundry.
Ion Selective Electrodes Response of the Glass Electrode Updated 2025-04-18 +Created 2025-01-25
In each interface of the membrane an equilibrium is established:
Ion Selective Electrodes Ion Selective Electrodes (References) Updated 2025-04-18 +Created 2025-01-25
Glass Electrodes make use of the ion-selective properties of glass and are used today in pH measurements.
The indicator electrode consists of a thin glass membrane that is contained inside a thick-walled glass or plastic tube. Inside the tube, there exists a solution of HCl (0.1M) satured with AgCl together with a silver wire. This is an internal Ag/AgCl reference electrode. The indicator electrode is connect with another refenrece electrode (the external one) and thus the cell is complete.
There are many types of ion-selective electrodes (often abbreviated as ISEs) whose design greatly varies. Nevertheless they are all based on the same principle: The membrane potential.
A membrane is any continuous layer made of a semi-permeable material, that separates two solutions. The membrane's characteristics cause the appearance of a membrane potential across the two solutions.
The membrane potential exists, due to the concentration difference (more accurately, activity difference) of the two solutions. Compounds from the denser mixture will diffuse through the membrane to the dilute solution, thus giving rise to a potential difference adjacent to the membrane. The present potential field opposes the movement caused by the diffusion, and a dynamic equilibrium is eventually established.
The membrane potential that is established due to this equilibrium can be calculated via the Nernst equation:, where and refer respectively to the activity of the compound of interest in the sample solution and in a standard/reference solution.
An ion-selective electrode is an electrochemical sensor based on thin films or selective membranes as recognition elements.
They are the most commonly used types of indicator electrodes (electrodes in which the analyte is being studied/measured) in potentiometric measurements due to their accuracy, selectivity and fast response time compared to metallic ones.
Sulfur that occurs as an unwanted compound in natural gas is also removed by hydrogenation, which yields hydrogen sulfide.
Created by the german chemist Carl Friedrich Claus, it makes it possible to convert hydrogen sulfide (from natural gas) to elemental sulfur.
The overall reaction can be descibed as follows:
The overall reaction can be descibed as follows:
The Claus' process served as a better replacement of the Frasch process, which obtained elemental sulfur from naturally found deposits underground.
Hydrogen sulfide (formula: ) is a chemical compound which occurs as a colourless gas with acidic properties. Its characteristic odor, which resembles that of rotten eggs makes the identification of the compound easy.
It is present in the atmosphere (in low concetrations, less than 0.0003ppm) and also works as a signaling molecule for many animals, including humans. Its odor can be identified at concentrations of around 0.1 to 0.15ppm. As the concentration increases, it obtains a sweet odor and becomes deadly.
It's corrosive and flammable.
The works of Zachariasen and Warren (Network Theory and its verification by X-ray diffraction) resulted in big advancements in the study of glass structure. This theory can predict a large number properties of conventional glasses. His theory was later extended by Dietzel.
- Network-modifiers that are placed in order to disrupt the organized structure of the network. As they are cations they place themselves close to the anions (oxygen in for example) that connects the network. They break some bridging covalent bonds and "stick" to the broken ends electrostatically forming ionic bonds of lower energy. This reduces the viscosity so that the glass can be manufactured more efficiently, as it softens in a lower temperature.
- Intermediates which assist in glass formation by forming intermediate bonds to oxygen. They can't form glasses on their own.
In order for glass to form, the network-former should (although there are exceptions) form polyhedral groups in its simplest form. (For example silica, forms tetrahedrals). Each polyhedron should only connect with its neighbouring once via bonding bridges formed by the anions (in this example an Si-O-Si).
The network-former of silicate glass is which is
This type of glass accounts for 90% of commercially used glass. Its uses range from windows to even jars. It is typically made of (w/w) 70% silica (), 10% lime () and 15% soda ().
Silica is the network-former of the glass. Silica, is found in nature as quartz and its pure form is the ideal glass. Nevertheless, soda is added in order to reduce the temperature at which the glass is softened, thus making its production cheaper and better to work with. Since sodium cations are very soluble in water, lime is added in order to decrease the solubility of the glass.
Silica is obtained by sand and mixed with soda ash () and limestone (). These materials are mixed together to get a powder called batch which is later combined with cullet (recycled glass pieces) in order for the glass to once again soften at even lower temperatures. The mix is heated into an oven at temperatures around . Impurities that arise are removed and the melted mass is put into moulds to take its final shape. Annealing is also taking place at the end to ensure glass durability, which is the process of reheated the bottle and then slowly cooling it down in order to further stabilize the structure of the glass.
Soda lime glass is recyclable.
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