Zinc needs to be in ionic form before it can be extracted, and the concentration of zinc in seawater is very low. It is difficult to directly extract it with general extraction agents. You can first evaporate and concentrate a certain amount of seawater, and you may be able to extract it. I will do it. Extraction of zinc is difficult below a certain concentration. And the purpose of your back-extraction is to detect the concentration of zinc. The extraction method is generally inaccurate, especially when the zinc concentration is very low. I have used this method before. I remember it was extracted with carbon tetrachloride. This It's not allowed. Why don't you use atomic absorption spectrometry to measure it? This method is more accurate for testing low-concentration zinc content. However, this method is also prone to deviations because zinc is relatively sensitive and is not skilled or standardized.