Micro-chemical test for sulfate: Difference between revisions
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===SO<sub>4</sub><sup>2-</sup> test using calcium chloride solution === | ===SO<sub>4</sub><sup>2-</sup> test using calcium chloride solution === | ||
Add a drop of CaCl<sub>2</sub> solution (1% aqueous solution) to the test drop. Heat the drop gently until the edge of the drop starts to dry (do not dry completely!). Observe the crystals forming at the fringe under the microscope | Add a drop of CaCl<sub>2</sub> solution (1% aqueous solution) to the test drop. Heat the drop gently until the edge of the drop starts to dry (do not dry completely!). Observe the crystals forming at the fringe of the drop under the microscope with a magnification of about 100x. | ||
When the solution contains sulfate, gypsum crystals form that can easily be recognized by their short and twinned shape (image 1). | When the solution contains sulfate, gypsum crystals form that can easily be recognized by their short and twinned shape (image 1). | ||
[[file:HJS CaSO4 092503-1.jpg|thumb|300px|right|Image 1: Short, typically twinned gypsum crystals at the border of a drying test drop.]] | |||
===Samples containing both calcium and sulfate === | ===Samples containing both calcium and sulfate === | ||
In solutions containing both Ca<sup>2+</sup> and SO<sub>4</sub><sup>2-</sup>, gypsum crystals can be observed at the fringe of a test drop while drying. These have very distinct shapes if the drop was previously acidified with 2M hydrochloric acid or 2 M nitric acid respectively. | In solutions containing both Ca<sup>2+</sup> and SO<sub>4</sub><sup>2-</sup>, gypsum crystals can be observed at the fringe of a test drop while drying. These have very distinct shapes if the drop was previously acidified with 2M hydrochloric acid, or 2 M nitric acid, respectively. | ||
When comparing the amount of gypsum formed in drops with or without addition of calcium chloride solution, it can be estimated whether the salt solution contains the same amount of calcium and sulfate or if one of the two ions is predominant. For this purpose, the test has to be repeated with known salt mixtures. | When comparing the amount of gypsum formed in drops with or without addition of calcium chloride solution, it can be estimated whether the salt solution contains the same amount of calcium and sulfate or if one of the two ions is predominant. For this purpose, the test has to be repeated with known salt mixtures. | ||
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===SO<sub>4</sub><sup>2-</sup> test using barium chloride solution=== | ===SO<sub>4</sub><sup>2-</sup> test using barium chloride solution=== | ||
The drop to be tested is acidified with a | The drop to be tested is acidified with a small drop of dilute (2M) hydrochloric acid and then a drop of barium chloride solution is added. | ||
If sulfate is present, finely crystalline white barium sulfate (BaSO<sub>4</sub>) precipitates out. | If sulfate is present, finely crystalline white barium sulfate (BaSO<sub>4</sub>) precipitates out. | ||
== Literature == | == Literature == | ||
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<biblist/> | <biblist/> | ||
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[[category:Bläuer,Christine]][[category: | [[category:Bläuer,Christine]][[category:approved]][[category: microchemistry]] | ||
Latest revision as of 18:16, 26 November 2017
Author: Christine Bläuer
back to Micro-chemical testing
Micro-chemical test for sulfate (SO42-)
For more information on the required equipment and materials see the Micro-chemistry article.
Procedure
SO42- test using calcium chloride solution
Add a drop of CaCl2 solution (1% aqueous solution) to the test drop. Heat the drop gently until the edge of the drop starts to dry (do not dry completely!). Observe the crystals forming at the fringe of the drop under the microscope with a magnification of about 100x. When the solution contains sulfate, gypsum crystals form that can easily be recognized by their short and twinned shape (image 1).
Samples containing both calcium and sulfate
In solutions containing both Ca2+ and SO42-, gypsum crystals can be observed at the fringe of a test drop while drying. These have very distinct shapes if the drop was previously acidified with 2M hydrochloric acid, or 2 M nitric acid, respectively. When comparing the amount of gypsum formed in drops with or without addition of calcium chloride solution, it can be estimated whether the salt solution contains the same amount of calcium and sulfate or if one of the two ions is predominant. For this purpose, the test has to be repeated with known salt mixtures.
SO42- test using silver nitrate solution
The drop to be tested is acidified with a very small drop of dilute (2M) nitric acid and then a drop of silver nitrate solution is added. If sulfate is present typical silver sulfate crystals are formed.
SO42- test using barium chloride solution
The drop to be tested is acidified with a small drop of dilute (2M) hydrochloric acid and then a drop of barium chloride solution is added. If sulfate is present, finely crystalline white barium sulfate (BaSO4) precipitates out.