(1) Basic requirements for water analysis
There are two types of pure water used in general chemical analysis: distilled water and deionized water (ie ion-exchanged water). Both types of analytical water must meet the following requirements:
①The appearance is clear and transparent, colorless and odorless, the pH value is about 5.5-7.5, and there is no sediment.
②The silver nitrate test is qualified. The test method is to add 1% silver nitrate solution to the water, if no white turbidity occurs, it indicates that the C1-ion content in the water is low, which meets the requirements of analytical water.
③Ming Hei T test is qualified. The test method is to adjust the pH value of the water to 10. After adding the chrome black T indicator, it turns blue, indicating that the content of metal impurities is low and meets the requirements of water for analysis.
(2) Preparation method of pure analytical water
In daily chemical analysis, people tend to ignore the influence of the purity of analytical water on the titration endpoint and analytical results, which is very wrong. In fact, there are often some interfering ions in the two analytical waters, which will affect the accuracy of the titration endpoint and the analytical results. So how can we get purer analytical water? It can be prepared by the following methods:
①When the tap water used for Weisiyan distilled water contains a lot of mechanical impurities, it should be clarified in advance, and it must first pass through a suitable type of filter before entering the Yandistiller.
②When the hardness of tap water is high, there are often more water fields in the electric distilled water device, which not only affects the yield of distilled water, but also reduces the quality of distilled water. Therefore, the scale in the still should be cleaned regularly. The distillation pot should be cleaned every 7-10 days, and the condensation part should be cleaned every 6 months.
③ Commercially available distilled water is generally easy to introduce cut from the distiller, and the indicator is closed when titrating calcium and magnesium. Adding a small amount of copper reagent (sodium diethylamino dithiopotassium, referred to as DDTC) in the alkaline medium, and Cu 2+ to form a brown color precipitate, which can eliminate the interference of Cue' and obtain an obvious titration end point. In the primary distillation, impurities such as calcium and magnesium are often contained, which can be removed by ion exchange.
④In ion-exchanged water, it is easy to contain silicic acid. Because there are few silicate ions from the water, it is not easy to be completely exchanged by the anion exchange resin, and a part remains in the water. When measuring silicon by the potassium silicate volumetric method, a positive error is introduced. For this reason, it is recommended to use The pure water is distilled first and then exchanged, which can basically remove all anionic and cationic impurities.
