Spectrophotometric Detection of Heavy Metals

　　At present, there is no strict unified definition of heavy metals. Generally, it refers to metals with a specific gravity greater than 5, about 45 kinds, such as copper, lead, zinc, iron, cobalt, nickel, manganese, cadmium, mercury, tungsten, molybdenum, gold, silver, etc. Although heavy metals such as manganese, copper, and zinc are trace elements required for life activities, most heavy metals such as mercury, lead, and cadmium are not necessary for life activities, and all heavy metals are toxic to the human body when they exceed a certain concentration. Heavy metals in environmental pollution mainly refer to heavy metal elements with significant biological toxicity such as mercury, cadmium, lead, chromium, and metalloid arsenic. If heavy metal elements are directly discharged into rivers, lakes, or oceans without treatment, or enter the soil, these rivers, lakes, oceans, and soil will be polluted because they cannot be biodegraded. Under the biological magnification of the food chain, they are enriched thousands of times and finally enter the human body. If the heavy metals accumulated in fish or shellfish are eaten by humans, or if the heavy metals are absorbed by crops such as rice and wheat and then eaten by humans, the heavy metals will enter the human body and cause heavy metal poisoning, which may cause strange diseases (such as Minamata disease and osteomalacia) or even death. Secondly, heavy metals can interact strongly with proteins and enzymes in the human body, making them inactive, and can also accumulate in certain organs of the human body, causing chronic poisoning. There are countless poisoning incidents caused by excessive heavy metals.

　　In summary, the characteristics of heavy metal pollution are: (1) It can cause environmental pollution even at very low levels. (2) Heavy metals cannot be decomposed by microorganisms. On the contrary, some heavy metals may be converted into more toxic metal-organic compounds under the action of microorganisms. (3) Heavy metals in surface water can be enriched through the food chain of organisms and reach a very high concentration. (4) After entering the human body, heavy metals can interact strongly with physiological macromolecules (such as proteins, enzymes, etc.) to make them inactive. They may also accumulate in certain organs of the human body, causing chronic cumulative poisoning. As a result, online measurement and emergency measurement of heavy metals have gradually received full attention from relevant departments and have now become a market hotspot. At present, the heavy metals that are monitored by environmental protection are arsenic, lead, chromium, cadmium, and mercury. In addition, the specific heavy metals in the electronics industry with relatively large pollution areas include copper and nickel.

　　There are many methods for determining heavy metals, such as atomic fluorescence, atomic absorption, plasma emission spectroscopy, mass spectrometry, etc. in atomic spectroscopy; visible light spectrophotometry, ultraviolet spectrophotometry, fluorescence spectrophotometry, etc. in molecular spectroscopy; selective electrodes, anodic dissolution, etc. in electrochemistry; and other methods such as neutron activation and ion chromatography. For portable on-site monitoring, spectrophotometry and anodic dissolution are mainly used.

　　In addition, when the anodic dissolution method is used for on-site measurement, the following points must be noted to ensure the accuracy of the data.

　　1. Cr, Hg, and As have multiple segmented concentration curves within the measuring range. This means that these three parameters need to be measured more than once in actual use.

　　2. When measuring, the measuring electrodes need to be carefully polished before the test. If the electrodes are not polished well, the test accuracy will be affected. Then, the four parameters of Cu, Pb, Zn, and Cd need to be plated with mercury film on the polished electrodes.

　　3. The equipment has three electrodes, one of which is the reference electrode, but drift still occurs during the test process. For example, for hexavalent Cr, it may be necessary to test the standard sample again after 3-4 tests to examine the degree of drift. For Cu, Pb and the like, it is necessary to retest the standard sample after 6 or 7 tests.

　　In this light, it is questionable whether the anodic dissolution method is really faster than the colorimetric test.

　　In addition, in general, the accuracy and anti-interference ability of spectrophotometry are better than those of electrochemical detection . Although the anodic dissolution method does not require the turbidity and color of the water sample, some ions and organic matter will interfere with the results. As for the spectrophotometry method, as long as the sample pretreatment is done well, the anti-interference ability is generally better than that of the anodic dissolution method.