Utente:Paul Gascoigne/sandbox: differenze tra le versioni

Versione delle 18:32, 26 gen 2007

Versione delle 16:33, 3 feb 2007

Riga 93:

*15 - Quelli del P.U.Z.Z.A. (''Bender Should Not Be Allowed on TV'')

*16 - Musica dal profondo (''The Devil's Hands are Idle Playthings'')

Il '''Total organic carbon''' - in italiano '''Carbonio organico totale''', spesso abbreviato in ('''TOC''') - è una misura della quantità di [[carbonio]] legato in un [[composto organico]] ed è spesso utilizzato come indicatore non-specifico della qualità delle acque.

Nelle tipiche misure di TOC vengono misurati sia il carbonio organico totale presente, sia il carbonio inorganico. Per ottenere un valore esatto di TOC occorrerà quindi sottrarre il valore di carbonio inorganico al risultato ottenuto. Una diffusa variante nell'analisi di TOC consiste nel rimuovere la porzione di carbonio inorganico prima dell'analisi, solitamente mediante acidificazione del campione (ad esempio mediante l'aggiunta di HCl) che leghi al suo interno il carbonio inorganico (H+ + CO2 --->HCO3).

==Analyzers==

Virtually all TOC analyzers measure the [[CO2|CO₂]] formed when organic carbon is oxidized and/or when inorganic carbon is acidified. [[Oxidation]] is performed either through [[Platinum_group#Applications|Pt-catalyzed]] combustion or with a [[UV]]/[[Sodium_persulfate|persulfate]] reactor. Once the CO₂ is formed, it is measured by a detector: either a conductivity cell (if the CO₂ is aqueous) or a non-dispersive infrared cell (after purging the aqueous CO₂ into the gaseous phase). Conductivity detection is only desirable in the lower TOC ranges in deionized waters, whereas NDIR detection excels in the higher TOC ranges. A variation described as [[Membrane Conductivity Detection]] can allow for measurement of TOC across a wide analytical range in both deionized and non-deionized water samples. Modern high-performance TOC instruments are capable of detecting carbon concentrations well below 1 µg/L (1 part per billion or ppb).

A total organic carbon analyzer determines the amount of carbon in a water sample. By acidifying the sample and flushing with nitrogen or helium the sample removes inorganic carbon, leaving only organic carbon sources for measurement. There are two types of analyzers. One uses combustion and the other wet oxidation. This is used as a water purity test, as the presence of bacteria introduces organic carbon.

Combustion:

In a combustion analyzer, half the sample is injected into a chamber where it is acidified, usually with phoshphoric acid, to turn all of the inorganic carbon into carbon dioxide as per the following reaction:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3-2

This is then sent to a detector for measurement. The other half of the sample is injected into a combustion chamber which is raised to between 600-700°C, some even up to 1200°C. Here, all the carbon reacts with oxygen, forming carbon dioxide. It's then flushed into a cooling chamber, and finally into the detector. Usually, the detecor used is a non-dispersive infrared spectrophotometer. By finding the total inorganic carbon and subtracting it from the total carbon content, the amount of organic carbon is determined.

Wet Oxidation

Wet oxidation analyzers inject the sample into a chamber with ammonium peroxydisulfate and phosphoric acid. This is separated into two portions, one that goes into a delay coil which takes the inorganic carbon for measurement, using the same method as the combustion analyzers. The other half of the sample is injected into an oxidation chamber, where it is bombarded with UV light from a mercury vapor lamp. Here, free radicals form from the peroxydisulfate and react with any carbon available to form carbon dioxide. The carbon from both chambers are run through membranes which measure the conductivity changes that result from the presence of varying amounts of carbon dioxide. Same as the combustion analyzer, the total carbon formed minus the inorganic carbon gives a good estimate of the total organic carbon in the sample.

==Applications==

TOC is the first chemical analysis to be carried out on potential petroleum source rock in [[oil exploration]]. It is very important in detecting contaminants in drinking water, cooling water, water used in semiconductor manufacturing, and water for pharmaceutical use. Analysis may be made either as an online continuous measurement or a lab-based measurement.

TOC detection is an important measurement because of the effects it may have on the environment, human health, and manufacturing processes. TOC is a highly sensitive, non-specific measurement of all organics present in a sample. It, therefore, can be used to regulate the organic chemical discharge to the environment in a manufacturing plant. In addition, low TOC can confirm the absence of potentially harmful organic chemicals in water used to manufacture pharmaceutical products. TOC is also of interest in the field of potable [[water purification#Tertiary treatment|water purification]] due to disinfection of byproducts. Inorganic carbon poses little to no threat.

[[en:Total organic carbon]]