Sodium Potassium Tartrate

Dhristi JEE 2022-24

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 formula / Rochelle salt

\[C_{4}H_{4}O_{6}KNa.4H_{2}O\]

Molar Mass

282.1 g/mol

Density

1.79 g/cm³

Melting Point

75⁰C

Boiling Point

220⁰C

 

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. Write Some Applications of Rochelle Salt.

It is utilised in the food business, wall plaster, pharmaceutical industry, metal treatment, electroplating (increased anodic solubility, greater surface uniformity, increased cathode efficiency. ), mirror silvering, and the Piezoelectric effect, among other things. Because Rochelle salt is a deliquescent substance, any transducers made from it degraded while stored in moist circumstances.


It has been used as a laxative in medicine. It has also been employed in the silvering of mirrors. It's a component in Fehling's remedy (reagent for reducing sugars).

2. Is Rochelle Salt Soluble in Alcohol?

It is mildly soluble in alcohol and soluble in water, melts at 75°C, has a specific gravity of 1.79, and displays double refraction. It's commonly seen in the form of Seidlitz powders in medicine as a moderate purgative. It's a component in Fehling's remedy. It's used to make silvered mirrors. Rochelle salt crystals are commonly employed in piezoelectric devices such as crystal microphones and phonograph pickup cartridges because they are easy to develop.

3. What is the Function of Rochelle Salt in Fehling Solution?

Fehling’s solution is used to identify the presence of aldehydes or groups that contain any aldehyde functional group -CHO and also used to differentiate a ketone group and water-soluble carbohydrates.


This solution is always prepared freshly in the laboratory. It is made initially in two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulphate pentahydrate crystals, and Fehling's B is a clear solution of aqueous potassium sodium tartrate (also known as Rochelle salt) mixed with a strong alkali (commonly known as sodium hydroxide).


Fehling's Solution A and B are kept separate because if we combine the two, the Copper Tartrate complex formed will be quickly degraded, and will not be effective in detecting “reducing sugars” (sugars containing aldehyde groups).

4. How is Rochelle Salt made?

Rochelle salt is simple to manufacture using two ordinary household components. Rochelle salt is potassium sodium tetrahydrate (KNaC4H4O6H2O) or sodium potassium tartrate tetrahydrate. Large piezoelectric crystals are produced by Rochelle salt, which is employed in scientific experiments and as transducers in microphones and phonograph pickups. The substance is a food ingredient with a cooling, salty taste. It's also used to make chemical reagents like Biuret reagent and Fehling's solution.

5. What are the uses of Rochelle Salt?

Below are some of the basic uses of Rochelle salt.

  • As a laxative, sodium potassium tartrate is employed.

  • It's used to make silvered mirrors.

  • Fehling's solution has it as one of the components.

  • In the electroplating process, it is employed.

  • Cigarette paper contains it.

  • It's a substance that's used to break apart emulsions.

  • It's one of the constituents of the Biuret reagent, which is used to assess protein content.

  • It aids in the maintenance of an alkaline pH.

  • In protein crystallography, it's a frequent precipitant.

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