John D. Kudrysch History of Chemistry Help Received: None
Salt: Potassium Dichromate K2Cr2O7
In 1844 potassium dichromate was finally recognized as a poisonous substance by Dr. J.J. Drysdale (1). Chromium was an element discovered in the late 18th century, but the salt potassium dichromide had not become noticed and analyzed until the mid 19th century. It was initially used as a dye for leathers, paints, and it was also used in batteries (1). This salt was used widely in all aspects of life, so its toxicity was not noticed until it was too late in a lot of cases. Potassium dichromate has been closely monitored and taken out of the general public’s reach in casual aspects once it was discovered to be harmful to health. Today it is still used widely in laboratories for organic and analytical analysis and its production is still used for other aspects of common life, just not as readily available for accidental consumption like it was during the 18th and 19th centuries.
Sodium dichromate and potassium chloride as the reagents usually utilized in the synthesis of potassium dichromate. This salt is also soluble in water, and during the dissolving process it ionizes (2).
K2Cr2O7 → 2 K+ + Cr2O72−
Cr2O72− + H2O ⇌ 2 CrO42− + 2 H+
Potassium dichromate is an oxidizing agent for alcohols in organic chemistry. This salt is a strong oxidizing agent, and can be used to either fully oxidize an alcohol or partially oxidize an alcohol. Potassium dichromate can partially oxidize ethanol to the simpler ethanal aldehyde. A simplified version of the reaction between potassium dichromate and ethanol is below (3):
3CH3CH2OH + Cr2O72- + 8H+ → 3CH3CHO + 2Cr3+ + 7H2O
This salt can also be used to fully oxidize alcohols to carboxylic acids. The full equation for potassium dichromate to fully oxidize ethanol to ethanoic acid is below (3):
3CH3CH2OH + 2Cr2O72- + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O
As an oxidizing agent it is still milder than potassium permanganate that is also widely used in organic reactions with alcohols. Because of this more mild aspect potassium dichromate can partially oxidize alcohols into aldehydes whereas more potent oxidizers like potassium permanganate usually produce the product of carboxylic acids (2).
Potassium dichromate has a very bright, red-orange color to it. The reason that this salt is widely used in laboratory settings is because it is not deliquescent, or moisture absorbing from the air, like a lot of the other salts used in industry or lab settings are (2). It has a fairly high melting point at 398 °C, and it’s boiling point is at 500 °C where at that stage it just decomposes (4). It is odorless and has a molecular weight of 294.2 g/mole. This salt is very harmful to the health of organisms, having an LD50 of 25mg/kg (4). It is a very harsh skin irritant and is to be considered extremely hazardous in terms of being corrosive (4). If ingested, even at a non-lethal dose, it can cause serious damage to someone’s blood, kidneys, lungs, liver, respiratory tract, skin, and eyes (4). Usually the best way to handle potassium dichromate it to store it in a glass container that is tightly sealed, and when handling make sure gloves are used at all times, glasses are worn, and it is not left out at any point in time unless being used. It is safe to say that this salt should be considered dangerous to the general public, and the fact that it was used in paints and other pigment modifiers is a troublesome concept for those that were effected before its identification.
This salt ranges at a fair price of about $85.00 per 100g (5). To buy it and have a massive amount is not hard to accomplish, so the compound is found in abundance for lab purposes. Due to its capacity of being very vibrant in color it is known to have a visible aspect to it. UV/Vis-spectrophotometry is one of the most common instrumental techniques utilized for analyzing compounds and elements because many compounds produce their own color wavelength. This salt can dissolve in water, so at certain concentrations and at the appropriate UV wavelength the spectrophotometer could detect the salt’s presence even in a mixture of other colors. This method has probably helped people determine the presence of the salt in paints and other household items that wouldn’t seem fit to keep around the general population.
As stated previously potassium dichromate was initially used for dyes and coloring for the rich reds and oranges that the compound creates. Over the years the use of this compound has changed because of society. Once it was realized that this salt is toxic to organics the usage of potassium dichromate became very limited and monitored. The salt is still used widely in several aspects of day-to-day living other than laboratory oxidation reactions with organic molecules. In cleaning this salt has been used to prepare a cleaning acid, chromic acid, for glassware and etching materials (2). This practice has become more and more rare though because of the toxic usage of hexavalent chromium. In construction potassium dichromate has been used as an ingredient in cement because the salt retards the setting of the mixture and improves its density (2). Because of the health hazard of contracting dermatitis, construction workers usually contract this if they are exposed to it often. Photography also utilized potassium dichromate. The chromium portion of the salt has the property of tanning animal proteins when a strong light source is exposed to them. This translates over to photographic screen printing for images (2). Potassium dichromate is also used in wood treatment, where it is used to stain types of wood by darkening the tannis. Mahogany is effected greatly by this and the brown colors that come from using potassium dichromate cannot be achieved with modern dyes (2).
Aside from organic reactions and day-to-day usage potassium dichromate can also be used in analytical reactions as a reagent. In ethanol determination the concentration of the ethanol can be determined by back titrating with acidified potassium dichromate (2). The dichromate excess is determined by the sodium thiosulfate in the solution. The amount of excess dichromate from the initial amount gives the amount of ethanol present in the solution (2). This reaction was utilized in old police breathalyzers. The alcohol vapors from the mouth would react with the dichromate crystals, turning the orange crystals into green ones. Depending on the concentration in the breath, thus the concentration in the body, determined the amount of green present instead of orange (2).
Another analytical method that this salt is used for is testing for silver. When potassium dichromate is dissolved in a 35% solution of nitric acid it is called Schwerter’s solution (2). This solution is used to detect numerous metals, silver being one of the main benefactors that this solution is utilized for. Depending on the purity of the silver that is being analyzed determines the color of the solution. A pure silver metal will turn the solution really bright red, and as the purity decreases so does the luster of the red. It changes down to a darker red, and eventual brown. The solution will even turn green for 0.500 silver (2).
History has yet again utilized a compound for its usage in non-ideal locations for such a toxic substance. This is similar to the lead paint that had been used for decades until it was found to be dangerous and toxic to the human body as well. The fact that it was detected not until the middle of the 19th century suggests that potassium dichromate is the possible reasoning behind many deaths before it was finally monitored closely. Used in laboratories today as a potent oxidizer, and partial oxidizer, it is utilized in both organic and analytical methods.
(1). http://www.avogel.com/plant-encyclopaedia/kalium_bichromicum.php, accessed 16 Feb, 2015
(2). http://en.wikipedia.org/wiki/Potassium_dichromate, accessed 14 Feb, 2015.
(3). http://www.chemguide.co.uk/organicprops/alcohols/oxidation.html, accessed 14 Feb, 2015
(4). http://www.sciencelab.com/msds.php?msdsId=9927404, accessed 16 Feb, 2015.
(5). http://www.sigmaaldrich.com/catalog/product/sial/207802?lang=en®ion=US, accessed 15 Feb, 2015.