Basic Principle
The pentahydrate of sodium thiosulfate (Na2S2O3●5H2O), commonly known as Haibo, also known as baking soda, is a monoclinic large rhombic crystal with a density of 1.715g/cm3 and is stable in air. It is dissolved in its crystal water at 329K and dehydrated at 373k. The crystal of sodium thiosulfate is soluble in water, and its aqueous solution is weakly alkaline. The oxidation value of sulfur in thiosulfate is +2, and its structural formula is as follows:
In this experiment, sodium thiosulfate was prepared by co-cooking sodium sulfite and sulfur.
Then after filtration, evaporation, concentration and crystallization, Na2S2O3●5H2O crystals can be obtained. Sodium thiosulfate solution forms a supersaturated solution when it is concentrated, and a few crystals (called seeds) are added at this time, and crystals can be precipitated.
The characteristic reaction to identify S2O32- is to add excess AgNO3 solution to the solution containing S2O32-, immediately forming a white precipitate, which quickly turns yellow, brown, and finally black. The reaction formula is:
The determination of thiosulfate is based on the reaction with iodine:
But sulfite can also react with I2-KI solution:
Therefore, before using the standard solution to measure the content of Na2S2O3, Na2SO3 must be removed first. The commonly used method is to add formaldehyde to make Na2SO3 in the solution react with formaldehyde to generate the adduct CH2(Na2SO3)O. This adduct has very weak reducing ability and cannot reduce I2 in I2-KI solution.
Experimental Procedure
1. Preparation of Sodium Thiosulfate (Na2S2O3●5H2O)
(1) Weigh 6.3g of Na2SO3 into a 100mL beaker, add 40mL of distilled water, and cover the beaker with a watch glass. Heat with constant stirring to dissolve, then continue heating to near boiling.
(2) Weigh 2.0g of sulfur powder and put it into a 50mL beaker, add a small amount of 50% ethanol to make the sulfur powder into a paste, add it into the near-boiling Na2SO3 solution in batches under stirring, continue heating and keep boiling for 1~1.5h . During the near-boiling process, it is necessary to constantly stir, and rinse off the sulfur adhering to the wall of the beaker with a small amount of water, and at the same time compensate for the evaporation loss of water.
(3) After the reaction is completed, filter under reduced pressure with a Buchner funnel while it is hot, and discard unreacted sulfur powder.
(4) Transfer the filtrate to an evaporating dish, then place it on an asbestos mesh to heat and evaporate while stirring, concentrate to about 20mL, and then cool to room temperature. If there is no crystallization, add a few grains of Na2S2O3●5H2O, a large amount of crystallization will immediately appear, and stand for 20 minutes.
(5) Use a Buchner funnel to filter under reduced pressure, and wash the evaporating dish with a small amount of absolute ethanol. Put the washing liquid into the Buchner funnel and drain as much as possible.
(6) Transfer the blotted crystals to a watch glass and dry at 40~50℃.
(7) After drying, take out the crystals and weigh them, record the mass of the product, and calculate the yield.
2. Product Identification
(1) Qualitative identification
Take a small amount of the product and add water to dissolve it, then take a few drops of this aqueous solution and add excess 0.1mol/L AgNO3 solution to observe the formation and color change of the precipitate. If the color changes from white→yellow→brown→black, it proves that there is Na2S2O3.
(2) Quantitative determination
Accurately weigh about 0.4g sample (accurate to 0.1mg) in a conical bottle and add deionized water 20mL, which has just been boiled and cooled, to completely dissolve it. Add 10mL neutral 40% formaldehyde solution, 10mL HAc-NaAc buffer solution, add 5 drops of starch solution, titrate with standard iodine solution (0.03mol/L), near the end point, add 1~2mL starch solution, continue titration until the solution is blue, do not disappear within 30 seconds as the key titration, and then do two parts in parallel.
Calculate the content of Na2S2O3●5H2O in the product.
Data Recording and Result Processing
1. Yield calculation
Yield of sodium thiosulfate
| Serial number | 1 | 2 | 3 | Average |
| Consumption of standard iodine solution / mL | ||||
| Na2S2O3●5H2O purity/% |
2. Product analysis
Purity analysis results of sodium thiosulfate product
| Serial number | 1 | 2 | 3 | Average |
| Consumption of standard iodine solution / mL | ||||
| Na2S2O3●5H2O purity / % |
