Direct determination and pretreatment of contaminated water samples
When we analyze water samples for environmental monitoring, we often encounter polluted water samples with high chroma, such as water samples polluted by waste water discharged from the papermaking, tanning, printing and dyeing industries. Not only do they have high chroma, they also have high levels of organic matter, which makes analysis difficult. If the sample is directly tested, it will cause serious interference to the end point of the titration indicator and colorimetric color scale in chemical analysis, and prevent the normal analysis of the instrument, such as blocking the sampling system and contaminating the sensing element. Therefore, it is necessary to give proper pretreatment before analyzing the polluted water samples.
The principle of pretreatment of polluted water samples is: eliminate the influence of interfering substances on the determination without changing the original content of the measured components. Because the composition of the interfering substance is quite complicated, it is difficult to exclude it with a simple processing method, and different processing methods must be adopted for different test components.
In recent years, we have undertaken a large number of environmental water sample testing tasks, a considerable part of which is polluted water samples. In practice, we refer to relevant literature, and take direct measurement methods (potential method, potentiometric titration method) and water sample pretreatment (active carbon decolorization, digestion treatment, distillation, high temperature ashing, etc.) for components in different polluted water samples. Then carry out the analysis method.
First, the direct determination of polluted water samples
1． Determination of acidity and free carbon dioxide
Take 50 ml of the water sample to be tested in a beaker, add a stir bar, place it on a magnetic stirrer, put the two electrodes of the acidity meter adjusted in the beaker, start the stirrer, and use 0.020mol / L hydroxide The sodium solution was dropped to a pH of 8.2 to 8.4, and the amount of sodium hydroxide solution was recorded. When the pH value of the water sample is greater than 4.0, the amount of sodium hydroxide solution consumed is the corresponding total acidity or free carbon dioxide content; if the pH value of the water sample is less than 4.0, another water sample needs to be taken in an electric furnace Boil for 2 to 3 minutes. After cooling, titrate as before. The difference in the sodium hydroxide solution consumed for the titration of two water samples is the corresponding free carbon dioxide content.
The calculation of total acidity, strong acidity, weak acidity, and free carbon dioxide content is the same as the indicator titration method.
2． Determination of total alkalinity (carbonate, bicarbonate, hydroxide)
Take 50 ml of the water sample to be tested in a beaker, add a stir bar, place it on a magnetic stirrer, put the two electrodes of the acidity meter adjusted in the beaker, start the stirrer, and use 0.040mol / L hydrochloric acid solution Drop to a pH of 4.2 to 4.4 and record the amount of hydrochloric acid solution.
When the pH value of the water sample is greater than 8.4, the pH value must be dropped to 8.2, the reading (P) must be recorded, and then the pH value can be continued to 4.2 to 4.4, and the reading (M) must be recorded.
The calculations of total alkalinity, carbonate, bicarbonate, and hydroxide contents are the same as those of the indicator titration method.
In the above two experiments, when inserting the two electrodes of the acidity meter into the beaker, be careful not to start the stirrer, so as not to break the electrode with the stirrer.
Direct determination and pretreatment of contaminated water samples
Pretreatment of polluted water samples
For different detection objects, different pretreatment methods should be used. The main methods are:
1. Determination of chloride
Take 50 ml of the polluted water sample to be tested in a porcelain evaporating dish, add 1.0 ml of a 1.0 mol / L sodium carbonate solution, evaporate to dryness on a sand bath, and transfer to a high-temperature furnace at 600-700 ° C for 5-10 minutes, or The alcohol torch is completely ashed. After cooling, the residue is dissolved in 50 ml of distilled water, 1 drop of phenolphthalein indicator is added, and it is neutralized with 1.0 mol / L sulfuric acid until it is colorless. At the same time, a blank test is performed.
2. Determination of sulfate
Take 50 ml of the contaminated water sample to be tested in a beaker, add 5 liters of 1 + 1 hydrochloric acid, evaporate to dry on a sand bath, add 5 ml of aqua regia, evaporate to dry, rinse the wall with a small amount of water, and re-evaporate (repeat 2 ～ 3 times), take off the cooling, add 50 ml of distilled water, adjust the acidity after the precipitation is dissolved, and then measure the sulfate by the volumetric method or the gravimetric method. At the same time, a blank test is performed.
If the iron content of the water sample is high, the treated solution can be neutralized with 1 + 1 ammonia water until it becomes alkaline. After the iron is completely formed into iron hydroxide, add 3 to 5 drops of ammonia water, and leave it for one hour to filter. Wash the beaker and filter paper three times with distilled water, collect all the filtrate in the beaker, adjust the acidity after boiling out the ammonia, and then measure the sulfate radical according to the above method.
3． Determination of nitrate
Take 25 ml of the polluted water sample to be tested in a beaker, add 2 ml of aluminum hydroxide solution, and stir. After 10 minutes, it was filtered into a porcelain evaporator, and the beaker and filter paper were washed several times with distilled water. The filtrate was added with 2 ml of a 0.1 mol / L sodium carbonate solution, evaporated to dryness on a sand bath, and then the colorimetric method was used to determine nitrate. At the same time, a blank test is performed.
Preparation of aluminum hydroxide solution: dissolve 62 g of secondary aluminum sulfate (AR) or 171 g of potassium aluminum sulfate (AR) in 500 ml of water, and slowly add concentrated ammonia water. After all aluminum ions are formed into aluminum hydroxide, filter by suction. , Wash with water until there are no chloride ions (check with silver nitrate), then remove the aluminum hydroxide colloid and add 300 ml of distilled water to shake and set aside.
4. Determination of Nitrite
Take 25 ml of the polluted water sample to be tested in a beaker, add 2 ml of aluminum hydroxide solution, and stir. After 10 minutes, filter into a 50 ml colorimetric tube, wash the beaker and filter paper several times with distilled water (be careful not to make the total volume exceed 50 ml), and then determine the nitrite by colorimetry. At the same time, a blank test is performed.
The aluminum hydroxide solution was prepared as above.
5. Determination of ammonium salt
Take 250 ml of contaminated water sample to be tested in a distillation flask, adjust to neutrality with 0.1mol / L sulfuric acid or sodium hydroxide (check with precision pH test paper), add a few glass beads to prevent explosion, and use 10 ml 2 A small beaker of% boric acid solution receives the distillate (the condensing tube must be inserted into the boric acid solution). Connect the distillation system so that there is no gas leakage. Then add 10 ml of phosphate buffer solution to the distillation flask and heat the distillation to make the distillate. Up to 100 ml. At the same time, ammonia-free distilled water was used as a blank test. Take a part of the distillate to determine ammonium ion by colorimetry.
Preparation of phosphate buffer solution: 7.15 g of secondary anhydrous potassium dihydrogen phosphate (AR) and 45.08 g of dipotassium hydrogen phosphate (AR) are dissolved in a small amount of non-ammonia distilled water, transferred into a 500 ml volumetric flask, and added with ammonia-free distilled water Set volume to scale.
6. Determination of potassium, sodium, calcium, magnesium, iron, copper, lead, zinc, manganese, cadmium, etc.
According to the needs of each project, take some contaminated water samples into the beaker, add 1 ml of concentrated nitric acid per 100 ml of water sample, evaporate on a sand bath, and then add 5 ml of aqua regia and 100 ml of water sample. 5 ml of perchloric acid, steam until the white smoke is gone, rinse the cup wall with a small amount of distilled water and evaporate to dry. Repeat this for 2 or 3 times, and then add an appropriate amount of distilled water and 0.5% concentrated hydrochloric acid equivalent to the original sample water to dissolve the precipitate. And then dilute it to the original sample volume with distilled water. The content of each element was determined by atomic absorption analysis. At the same time, a blank test is performed.
If the measurement method requires controlling acidity, care must be taken to adjust its acidity.
7. Decolorization of activated carbon for determination of calcium, magnesium, sodium, potassium, sulfate, chloride (lightly polluted colored water samples)
Take 200 ml of the polluted water sample to be tested in a beaker, add about 3 g of activated carbon powder, and stir. After 1 hour, the solution was filtered, and the filtrate was collected in a 200-liter volumetric flask. The sodium and potassium elements were measured by a flame photometer method, and the remaining components were measured by a volume method.