What is Sodium Potassium Tartrate?
This article deals with Sodium potassium tartrate, also known as Rochelle salt, which is a double salt of tartaric acid. It was first prepared in the year 1672 by an apothecary, Pierre Seignette, of La Rochelle, France. Rochelle salt is a white crystalline powder and a slightly bitter taste. It has a pH range of 6.5 – 8.5. It is obtained by the reaction of Sodium Hydroxide on the crude Cream of Tartar which is a by-product of the Wine Industry. It is almost insoluble in alcohol and ether.
Properties of Sodium Potassium Tartrate
Sodium Potassium Tartrate Uses
Used in Laboratory reagent, one of the ingredients in Biuret reagent to measure the concentration of protein.
It is used in the electroplating process (Increases cathode efficiency.), in electronics, and piezoelectricity.
It is used in cigarette paper. (Delays the consumption of cigarette paper.)
It helps in maintaining alkaline pH.
It is used in the silvering of mirrors. (Act as a reducing agent)
It is also used in the Pharmaceutical Industry.
It is used in gas purification.
It is an ingredient in Fehling's solution test (reagent used in reducing sugars) and determination of uric acid.
Sodium Potassium Tartrate Structure
Sodium Potassium tartrate/ Rochelle salt contains ten oxygen atoms, twelve hydrogen atoms, four carbon atoms, one potassium atom, and one sodium atom.
Preparation of Sodium Potassium Tartrate
Tartar with a minimum tartaric acid level of 68 percent is the primary ingredient. This is initially dissolved in water or a previous batch's mother liquor. After that, it's pH-ed with a hot saturated sodium hydroxide solution, decolored with activated charcoal, and chemically purified before being filtered.
The filtrate is evaporated to 42° Bé at 100 °C and then fed through granulators, where Seignette's salt crystallises over time. Centrifugation is used to extract the salt from the mother liquor, which is followed by granule washing. The salt is then dried in a rotating furnace and sieved before being packaged. Grain sizes offered commercially range from 2000 m to 250 m. (powder).
Reactions Involved in the Preparation of Rochelle Salt/Potassium Tartrate
The first step involves the Conversion of sodium bicarbonate to sodium carbonate
\[2NaHCO_{3} \rightarrow + CO_{2} + H_{2}O\]
Potassium bitartrate reacts with sodium carbonate to generate Sodium Potassium Tartrate / Rochelle salt:
\[KHC_{4}H_{4}O_{6} + Na_{2}CO_{3} \rightarrow C_{4}H_{4}O_{6}KNa.4H_{2}O\]
Preparation of Sodium Potassium Tartrate
The following is a thorough technique for making sodium potassium tartrate.
Ingredients and apparatus required:
Sodium bicarbonate (500 g)
Potassium bitartrate (200 g)
Container made of Pyrex
pyrex measuring cup, 500 mL
2 mL measuring spoon
Filter for coffee
Jar with a cap
To stir in the oven, use a spoon.
Water Saucepan Filter paper
Step-1:
Follow the procedures below to convert sodium bicarbonate to sodium carbonate:
Add 500 g of sodium bicarbonate to a pyrex jar (baking soda).
Preheat the oven to 65°C and bake it for an hour.
Set the temperature to 120 degrees Celsius and leave it there for another hour.
For 175°C and 230°C, repeat the preceding procedure.
Allow the container to cool to room temperature after removing it from the oven.
Fill a covered jar halfway with washing soda (sodium carbonate).
Step-2:
Sodium potassium tartrate (Rochelle salt) is formed when potassium bitartrate (cream of tartar) interacts with sodium carbonate:
Take a 500mL beaker and fill it halfway with water. Fill it with 250 mL of water. In a water-filled beaker, dissolve 200 g potassium bitartrate.
Place the beaker in a pot with water and heat it.
Heat until the outside water begins to simmer.
In the beaker, pour 2.5 ml of the washing soda from the first experiment. The solution will bubble if you stir it thoroughly.
Continue to add sodium carbonate until there are no more bubbles in the solution.
Filter the solution using filter paper once no bubbles have formed.
You may also filter the hot solution using a coffee filter.
By boiling and evaporating the solution, it may be concentrated to 400 mL.
Allow the filtrate to cool before storing it.
Collect the resultant crystals after several days of storage using the decantation method.
To dry the crystals, use filter paper.
Result: This yields around 210 g of sodium potassium tartrate (Rochelle salt).
Did you know?
Rochelle Salt must be stored in a dry place with airtight packing and, away from humidity and in normal temperature conditions and its IUPAC name is sodium-potassium-2,3-dihydroxy butane-1,4-dioate.
FAQs on Sodium Potassium Tartrate
1. What is Sodium Potassium Tartrate?
Sodium Potassium Tartrate is a double salt of tartaric acid with the chemical formula KNaC₄H₄O₆·4H₂O. It is a white, crystalline solid that is best known by its common name, Rochelle Salt. It is widely recognised for its piezoelectric properties and its crucial role in certain chemical reagents.
2. What is the chemical formula and structure of Sodium Potassium Tartrate?
The chemical formula for Sodium Potassium Tartrate tetrahydrate is KNaC₄H₄O₆·4H₂O. Structurally, it consists of a tartrate anion (⁻OOC-CH(OH)-CH(OH)-COO⁻) where the two carboxylic acid groups are deprotonated. These negative charges are balanced by one sodium (Na⁺) ion and one potassium (K⁺) ion. The compound crystallises with four molecules of water of hydration.
3. Why is Sodium Potassium Tartrate commonly known as Rochelle Salt?
The compound gets its common name, Rochelle Salt, from the location of its discovery. It was first prepared around 1675 by an apothecary named Pierre Seignette, who lived in La Rochelle, France. For this reason, it is also sometimes referred to as Seignette's Salt.
4. What are the key physical properties of Sodium Potassium Tartrate?
Sodium Potassium Tartrate exhibits several distinct physical properties. Key among them are:
- Appearance: It is a white or colourless crystalline powder.
- Solubility: It is highly soluble in water but only slightly soluble in alcohol.
- Taste: It has a characteristic cooling, saline taste.
- Melting Point: It melts at a relatively low temperature, around 75°C (167°F).
- Hygroscopy: It is a deliquescent substance, meaning it readily absorbs moisture from the air.
- Specific Gravity: It has a specific gravity of approximately 1.79.
5. How is Sodium Potassium Tartrate (Rochelle Salt) prepared in a laboratory?
The preparation of Sodium Potassium Tartrate is a straightforward neutralisation reaction. It is typically prepared by reacting potassium bitartrate (cream of tartar, KHC₄H₄O₆) with a calculated amount of sodium carbonate (Na₂CO₃). The reaction is heated in an aqueous solution until effervescence (release of CO₂) ceases. The resulting solution is then filtered and cooled to allow crystals of sodium potassium tartrate tetrahydrate to form.
6. What are the major uses and applications of Sodium Potassium Tartrate?
Sodium Potassium Tartrate has a diverse range of applications across different fields:
- Chemical Analysis: It is a key component of Fehling's solution and Biuret reagent, used for testing reducing sugars and proteins, respectively.
- Food Industry: It is used as a food additive with the E number E337, acting as an emulsifier, sequestrant, and acidity regulator.
- Electroplating: It is used in electroplating baths to help form a more uniform and adherent metal coating.
- Electronics: Due to its strong piezoelectric effect, it was historically used in transducers for microphones, phonograph pickups, and earpieces.
- Other Uses: It is also used in the process of silvering mirrors, as a mild laxative in medicine, and as a component in some cigarette papers.
7. What is the specific function of Sodium Potassium Tartrate in Fehling's solution?
In Fehling's solution, Sodium Potassium Tartrate acts as a chelating agent. Fehling's solution is alkaline, and in the absence of a chelating agent, the copper(II) ions (Cu²⁺) from copper sulfate would precipitate as insoluble copper(II) hydroxide (Cu(OH)₂). The tartrate ions from Rochelle salt bind to the Cu²⁺ ions, forming a stable, soluble, deep-blue complex called bis(tartrato)cuprate(II). This complex keeps the Cu²⁺ ions dissolved and available to act as an oxidising agent for detecting aldehydes.
8. What is the piezoelectric effect, and why is Rochelle Salt significant for it?
The piezoelectric effect is the ability of certain materials to generate an electric voltage in response to applied mechanical stress or pressure. Conversely, these materials deform when an electric field is applied. Rochelle Salt is historically significant because it was one of the first materials discovered to exhibit this property to a very strong degree. This discovery led to its widespread use in early electro-acoustic transducers, such as crystal microphones and phonograph cartridges, before it was largely superseded by more stable synthetic materials like quartz and barium titanate.
9. Is it safe to eat Sodium Potassium Tartrate?
Yes, in small quantities, Sodium Potassium Tartrate is considered safe for consumption and is approved as a food additive (E337) in many countries. It is used in food products as an emulsifier and to regulate acidity. However, when consumed in larger doses, it acts as a saline laxative. Therefore, while it is an ingredient in some foods, it should not be consumed in large amounts.
10. How does Sodium Potassium Tartrate differ from Cream of Tartar?
The primary difference lies in their chemical composition. Cream of Tartar is potassium bitartrate (KHC₄H₄O₆), which is an acidic single salt of tartaric acid containing only potassium ions. In contrast, Sodium Potassium Tartrate (Rochelle Salt) is a neutral double salt of tartaric acid, containing both sodium (Na⁺) and potassium (K⁺) ions. This difference in composition makes Rochelle Salt neutral and gives it different properties, such as higher solubility and its specific function as a chelating agent in Fehling's solution.
