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Preferred IUPAC name
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3D model (JSmol) | |||
| ChemSpider |
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| ECHA InfoCard | 100.011.609 | ||
| E number | E336(antioxidants, ...) | ||
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| Properties | |||
| KC4H5O6 | |||
| Molar mass | 188.177 | ||
| Appearance | White crystalline powder | ||
| Density | 1.05 g/cm3 (solid) | ||
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| Solubility | Soluble in acid, alkali Insoluble inacetic acid,alcohol | ||
Refractive index (nD) | 1.511 | ||
| Pharmacology | |||
| A12BA03 (WHO) | |||
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| Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 22 g/kg (oral, rat) | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Potassium bitartrate, also known aspotassium hydrogen tartrate, with formulaKC4H5O6, is thepotassiumacid salt oftartaric acid (acarboxylic acid)—specifically, l-( + )-tartaric acid.[1] Especially in cooking, it is also known ascream of tartar. Tartaric acid and potassium naturally occur in grapes, its crudeprecipitate formtartar orwine stone are byproducts ofwinemaking found deposited on top ofmust barrels which can be purified.[2]
Approved by the FDA as a direct food substance, cream of tartar is used as an additive, stabilizer, pH control agent, antimicrobial agent, processing aid, and thickener in various food products. It is used as a component ofbaking powders and baking mixes, and is valued for its role in stabilizing egg whites, which enhances the volume and texture ofmeringues andsoufflés. Its acidic properties prevent sugar syrups from crystallizing, aiding in the production of smooth confections such as candies and frostings. When combined withsodium bicarbonate, it acts as aleavening agent, producingcarbon dioxide gas that helps baked goods rise. It will also stabilizewhipped cream, allowing it to retain its shape for longer periods.
Potassium bitartrate further serves asmordant in textile dyeing, as reducer ofchromium trioxide in mordants for wool, as a metal processing agent that prevents oxidation, as an intermediate for other potassiumtartrates, as a cleaning agent when mixed with a weak acid such asvinegar, and as reference standardpH buffer. It has a long history of medical and veterinary use as a laxative administered as a rectal suppository, and is used also as acathartic and as adiuretic.[3] It is an approved third-classOTC drug in Japan[citation needed] and was one of active ingredients in Phexxi, a non-hormonal contraceptive agent that was approved by the FDA in May 2020.[4]
Potassium bitartrate was first characterized by Swedish chemistCarl Wilhelm Scheele (1742–1786).[5] This was a result of Scheele's work studyingfluorite andhydrofluoric acid.[6]
Scheele may have been the first scientist to publish work on potassium bitartrate, but use of potassium bitartrate has been reported to date back 7000 years to an ancient village in northern Iran.[7] Modern applications of cream of tartar started in 1768 after it gained popularity when the French started using it regularly in their cuisine.[7]
In 2021, a connection between potassium bitartrate and canine and feline toxicity of grapes was first proposed.[8] Since then, it has been deemed likely as the source of grape and raisin toxicity to pets.[9]
Potassium bitartrate is naturally formed in grapes from the acid dissociation of tartaric acid into bitartrate and tartrate ions.[10]
Potassium bitartrate has a low solubility in water. Itcrystallizes in wine casks during thefermentation ofgrapejuice, and can precipitate out of wine in bottles. The rate of potassium bitartrate precipitation depends on the rates of nuclei formation andcrystal growth, which varies based on a wine's alcohol, sugar, and extract content.[11] The crystals (wine diamonds) will often form on the underside of a cork in wine-filled bottles that have been stored at temperatures below 10 °C (50 °F), and will seldom, if ever, dissolve naturally into the wine. Over time, crystal formation is less likely to occur due to the decreasingsupersaturation of potassium bitartrate, with the greatest amount of precipitation occurring in the initial few days of cooling.[11]
Historically, it was known asbeeswing for its resemblance to the sheen of bees' wings. It was collected and purified to produce the white, odorless,acidic powder used for many culinary and other household purposes.
These crystals also precipitate out of fresh grape juice that has been chilled or allowed to stand for some time.[12] To prevent crystals from forming in homemade grapejam or jelly, the prerequisite fresh grape juice should be chilled overnight to promote crystallization. The potassium bitartrate crystals are removed by filtering through two layers ofcheesecloth. The filtered juice may then be made into jam or jelly.[13] In some cases they adhere to the side of the chilled container, making filtering unnecessary.
The presence of crystals is less prevalent in red wines than in white wines. This is because red wines have a higher amount oftannin and colouring matter present as well as a higher sugar and extract content than white wines.[11] Various methods such as promoting crystallization and filtering, removing the active species required for potassium bitartrate precipitation, and adding additives have been implemented to reduce the presence of potassium bitartrate crystals in wine.[10]
In food, potassium bitartrate is used for:
Additionally, it is used as a component of:
A similar acid salt,sodium acid pyrophosphate, can be confused with cream of tartar because of its common function as a component of baking powder.
Adding cream of tartar to egg whites gives volume to cakes, and makes them more tender.[19] As cream of tartar is added, the pH decreases to around the isoelectric point of the foaming proteins in egg whites. Foaming properties of egg whites are optimal at this pH due to increased protein-protein interactions.[20] The low pH also results in a whiter crumb in cakes due to flour pigments that respond to these pH changes.[19] However, adding too much cream of tartar (>2.4% weight of egg white) can affect the texture and taste of cakes.[19] The optimal cream of tartar concentration to increase volume and the whiteness of interior crumbs without making the cake too tender, is about 1/4 tsp per egg white.[19]
As an acid, cream of tartar with heat reduces sugar crystallization ininvert syrups by helping to break downsucrose into its monomer components -fructose andglucose in equal parts.[21] Preventing the formation of sugar crystals makes the syrup have a non-grainy texture, shinier and less prone to break and dry. However, a downside of relying on cream of tartar to thin out crystalline sugar confections (like fudge) is that it can be hard to add the right amount of acid to get the desired consistency.
Cream of tartar is used as a type ofacid salt that is crucial inbaking powder.[21] Upon dissolving in batter or dough, thetartaric acid that is released reacts withbaking soda to formcarbon dioxide that is used forleavening. Since cream of tartar is fast-acting, it releases over 70 percent of carbon dioxide gas during mixing.
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Potassium bitartrate can be mixed with an acidic liquid, such as lemon juice or white vinegar, to make a paste-like cleaning agent for metals, such asbrass,aluminium, orcopper, or with water for other cleaning applications, such as removing light stains fromporcelain.[22]This mixture is sometimes mistakenly made with vinegar andsodium bicarbonate (baking soda), which actuallyreact toneutralize each other, creatingcarbon dioxide and asodium acetate solution.
Cream of tartar was often used in traditionaldyeing where thecomplexing action of the tartrate ions was used to adjust the solubility andhydrolysis ofmordant salts such astin chloride andalum.
Cream of tartar, when mixed into a paste withhydrogen peroxide, can be used to cleanrust from somehand tools, notably handfiles. The paste is applied, left to set for a few hours, and then washed off with a baking soda/water solution. After another rinse with water and thorough drying, a thin application of oil will protect the file from further rusting.
Slowing the set time ofplaster of Paris products (most widely used ingypsum plaster wall work and artwork casting) is typically achieved by the simple introduction of almost any acid diluted into the mixing water. A commercial retardant premix additive sold by USG to trade interior plasterers includes at least 40% potassium bitartrate. The remaining ingredients are the same plaster of Paris andquartz-silicaaggregate already prominent in the main product. This means that the only active ingredient is the cream of tartar.[23]
For dyeing hair, potassium bitartrate can be mixed withhenna as the mild acid needed to activate the henna.
Cream of tartar is often used in traditional dyeing prcesses, where its complexing action helps fix mordant salts liketin chloride andalum to fabric. It is especially useful in brighteningcochineal dye.[24][25]
Cream of tartar has been used internally as apurgative, but this is dangerous because an excess of potassium, orhyperkalemia, may occur.[26][27]
Potassium bitartrate is also used as a laxative. The carbon dioxide it releases causes mechanical distension against the intestinal wall, inducing bowel movements. Combined with sodium bicarbonate in a poly(ethylene glycol)-based suppository, it is a useful and safe treatment for patients at risk of electrolyte, renal, or cardiovascular disorders.[28] It is a third-class over-the-counter drug in Japan.[1]
Potassium bitartrate is the United States'National Institute of Standards and Technology's primary reference standard for apH buffer. Using an excess of the salt in water, asaturated solution is created with apH of 3.557 at 25 °C (77 °F). Upon dissolution in water, potassium bitartrate willdissociate into acid tartrate, tartrate, and potassiumions. Thus, a saturated solution creates a buffer with standard pH. Before use as a standard, it is recommended that the solution be filtered ordecanted between 22 °C (72 °F) and 28 °C (82 °F).[29]
Potassium carbonate can be made by burning cream of tartar, which produces "potash" This process is now obsolete but produced a higher quality (reasonable purity) than "potash" extracted from wood or other plant ashes.
Potassium bitartrate is often produced as a by-product ofwinemaking. Lees (sediment) are collected following the fermentation process and dissolved in hot water, filtered, and cooled.[30] Occasionally, excesstartaric acid is added to the lees to increase the saturation of potassium bitartrate in solution.[31] Since the solubility of potassium bitartrate is highly dependent on temperature,[1] the pure salt begins to crystallise out of solution, and can be collected via filtering once again.[30] This solubility can also be exploited to produce potassium from Seignette's Salt (also known as Rochelle's Salt orpotassium sodium tartrate). Acidification to pH 3.5 with small amounts of hydrochloric acid will precipitate potassium bitartrate.[24]
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