In the previous content, we explained how to use spectrophotometry to detect the lead content in water, but no matter which part of the operation, as long as you are not careful, it will affect the final detection result. Therefore, if you want to obtain accurate detection data, you must understand the matters needing attention when detecting lead content in water by spectrophotometry. Avoid these errors in actual operation.
Matters needing attention in detection reagents and laboratory utensils
1. If the dithizone reagent is not pure, we can filter out the insoluble matter. The filtrate is placed in a separatory funnel, and each time is extracted with (1+100) ammonia water 20mL, a total of five extractions, at this time the dithizone enters the water phase. After merging the water phase, it can be neutralized with 6mol/L hydrochloric acid, and then extracted twice with 250mL of dichloromethane to combine into the chloroform phase. Put this dithizone chloroform solution in a brown bottle and store it in 5 Keep in the refrigerator at ℃.
2. If the chloroform is placed for a long time, it is easy to be affected by light and air, and the dithizone in the medicament will be oxidized. Therefore, the quality of the chloroform should be checked in advance when it is used, and if it is unqualified, it should be re-distilled and purified immediately .
3. When adjusting the pH value of the water sample, 0.1% methyl thymol blue can be used as an indicator. When pH=1.2-2.8, the tender of methyl thymol blue will change from red to yellow, and when pH=8.0-9.6, it will change from yellow to blue.
4. In fact, the most important thing for this type of method is to ensure that the utensils and reagents used in the test do not contain trace amounts of lead. Therefore, before starting the test, the experimental utensils must be soaked or shaken with dilute nitric acid, and then use laboratory ultra-pure Use after washing with water several times.
Methods of checking and eliminating interference during detection
1. Elimination of excessive interferences in water samples
Bismuth, tin and thallium dithizone salts and lead dithizone have different maximum absorption wavelengths. By measuring the absorbance of the sample at 510nm and 465nm, it is possible to check whether such interference exists. From the absorbance of the sample at each wavelength position, subtract the absorbance of the blank test at the same wavelength position to obtain the correction value of the sample absorbance, and calculate the ratio of the absorbance correction value at 510nm to the absorbance correction value at 465nm. This ratio is 2.08 for the lead salt of dithizone and 1.07 for the bismuth dithizone salt. If the ratio is significantly less than 2.08, it indicates that there is interference,
To eliminate the interference, we can take another 100mL sample and add 5mL of 5% sodium sulfite solution to reduce the residual iodine (the iodine solution added during sampling is to avoid the loss of volatile organic lead compounds in the water sample digestion process). If necessary, use 20% nitric acid solution or ammonia solution (1+9) on the pH meter to adjust the pH of the sample to 2.5, transfer the sample to a 250mL separatory funnel, and use 0.1% dithizone each time Extract with 10mL of special solution, at least three times, or extract until the chloroform layer remains green, and then extract with 20mL of chloroform each time to remove dithizone (the green disappears). The water phase is prepared for measurement.
2. Interfering substances that need attention when pH=8-9
At pH=8-9, the main elements that interfere with the determination of lead extraction are bismuth (Ⅲ), stannous and thallium, but generally water samples contain very little thallium, so you don't need to consider it, and bismuth (especially tin) often exists , We should pay special attention. Generally, when pH=2-3, first use dithizone-chloroform extraction to remove, and at the same time the interference ions removed by extraction include copper, mercury, and silver ions.
Then, in the reducing solution of citrate-potassium cyanide-hydroxylamine hydrochloride with pH=8.5-9.5, lead was extracted with dithizone-trichloromethane. The purpose of adding hydroxylamine hydrochloride is to reduce some oxidizing substances, such as ferric iron and other oxidizing substances that may exist, to prevent dithizone from being oxidized.
Potassium cyanide can mask a variety of metals such as copper, zinc, nickel, and cobalt. Citrate is a tribasic carboxylate, a masking agent with strong complexing ability in a wide pH range. Its main function is to complex cations such as calcium, magnesium, aluminum, chromium, and iron to prevent the The hydroxides of these metals are formed during precipitation.
If the measurement range is 0-75ug of lead content, the following ions in the water sample will not affect the final test results. They are silver, mercury, bismuth, copper, arsenic, antimony, tin, iron, chromium, and nickel. , Cobalt, manganese, zinc, calcium, strontium, barium, magnesium, etc.
