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7 The  acid-base titration starts with the dissolution of the solid acid sample in deionized water. Add the end-point indicator, which is phenolphthalein, with two drops to each flask containing the acid sample and deionized water.Properly label the flasks. Be consistent in all of the samples when adding the indicator. Swirl the flasks until the solid acid is completely dissolved. Finally, rinse with deionized water three times around, which is critical to ensure that all solid acid has been removed from the flask walls and dissolved in the solution. All solid particles must be dissolved prior to the titration.
7、酸碱滴定伴随着于固体酸溶解于去离子水中发生。在每支装有酸样品和去离子水的试管中加入两滴终点指示剂——酚酞，并在试管上贴好标签。在加指示剂的时候保持所有样品条件一致。震荡试管直到所有固体酸完全溶解。最后，用去离子水冲洗试管壁周围三次，确保试管壁上的固体酸从试管上脱落并溶解于溶液中。所有的固体酸都必须在滴定前完全溶解。
8 The buret needs to be checked for if it is quantitatively clean, both to avoid contamination and to be sure that titrant volumes are accurately read. Make sure the buret stopcock is clos
ed. Fill the buret with water and then drain it to check the buret, making sure that its walls drain cleanly. Before checking for drainage, wait a minute or two after completely draining the buret. Sometimes droplets appear on the inner walls of the buret after some time, indicating that the buret is not quantitatively clean .In this case ,it is necessary to use standard cleaning procedures to clean the buret. If the buret is droplet free, then it is quandtatively clean and can be used for titration.
8.滴定管需要检查，如果它是定量清洁，既避免了污染又能确保滴定剂卷的准确地阅读。确保关闭滴定管活塞。在滴定管内装满水然后排出检查滴定管，确保其壁上的水排干净。在检查排水时，等待一至两分钟后滴定管完全排干。有时滴定管内壁出现小水滴一段时间，表明滴定管不是定量清洁。在这种情况下，必须用标准的清洁程序清洁滴定管。如果滴定管是液滴自由，那么它是定量清洁和可用于滴定。
10 Titrant can then be filled in the well-rinsed buret. Still use a funnel to add titrant to the buret. Carefully lift the funnel for smooth delivery and to avoid overfilling of the titrant. Similar to the cleaning of the buret, an alternative is to remove the buret from the buret hold
er, and directly pour the titrant from the titrant bottle. Let some titrant run through the buret tip into the waste container and check whether there are any air bubbles in the tip. The bubbles will cause difficulty in obtaining accurate values of volume if they are not removed. The bubbles can be shaken out by opening the stopcock, firmly holding the buret with both hands, and jerking downward a bit. When bubbles are removed, tip off the hanging titrant drop and mount the buret for titration.
10. 然后可以在良好的漂洗滴定管填充滴定剂，可以用漏斗添加滴定液到滴定管中。小心的提起漏斗为了顺利递送，避免滴定液溢出，与清洗滴定管相似，一种代替方法是从试管架上移除滴定管，迅速从滴定剂瓶中倒入滴定剂。让一些滴定剂通过滴定管尖端到废物容器中，检查尖端是否有气泡。如果没有去除气泡会给得到大量精确价值造成困难，但可以通过打开活塞抖出气泡，用双手稳稳地拿着滴定管，然后迅速的向下一点，当气泡移动悬挂在滴定管下端，安装滴定管进行滴定。

U3

weigh翻译
2 Caffeine has the molecular formula C8H10N402 with a molecular weight of 194.19 g/mol. Its chemical name is 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione. Caffeine belongs to a group of compounds called alkaloids, more specifically, a member of the methylxanthines. The caffeine molecule ha base characteristics (alkali-like) and the purine ring system, which is an important framework in living systems.
3 Caffeine is a chemical with a variety of uses. From medicines to beverages to foods, caffeine is one of the most popular natural products used today. It is the most widely used of all the stimulants and acts to stimulate the heart, central nervous system, and the respiratory system. Its usage can increase blood pressure, contraction force, and volume output by increasing heart rate. A small dose of this compound at an amount of 50 to 200 mg increases alertness and reduces drowsiness and fatigue. Caffeine is the main ingredient of many "stay-awake" pills. It is a smooth muscle relaxant and a diuretic. Caffeine is also a food additive. It can be found in popular soft drinks. However, it needs to
be pointed out that caffeine has side effects. Large doses in excess of 200 mg can cause insomnia, restlessness, headaches, and muscle tremors. In addition, continued, heavy use of this chemical may lead to addictiveness. Furthermore, some research connects high caffeine consumption in pregnant women with the malformation of their children.

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