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Analytical chemistry experiment specifications and precautions Experimental water

2021-03-19
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Analytical chemistry experiments should use pure water, generally distilled water or deionized water. Some experiments require the use of double distilled water or higher specifications of pure water (for example, experiments in electroanalytical chemistry, liquid chromatography, etc.). Pure water is not absolutely free of impurities, but the content of impurities is very small.

Note: It is difficult to determine the true pH value in water of primary and secondary purity. Therefore, the range of its pH value is not regulated; in primary water, it is difficult to determine its oxidizable substances and evaporation residues, so it is not regulated.

1 distilled water

Pure water obtained by removing non-volatile impurities from water by distillation is called distilled water. The same is the pure water obtained by distillation, and the type and content of impurities contained in it are also different. The water distilled with a glass distiller contains Na+ and SiO2- ions; while the pure water prepared with a copper distiller may contain Cu+ ions.

2 Deionized water

The pure water obtained by using ion exchangers to remove cation and anion impurities in water is called ion exchange water or “deionized water”. Untreated deionized water may contain microorganisms and organic impurities, so care should be taken when using it.

3 Inspection of pure water quality

There are many quality inspection indicators for pure water. The analytical chemistry laboratory mainly tests the resistivity, pH, calcium and magnesium ions, and chloride ions of the experimental water.

1) Resistivity:

Select a conductivity meter suitable for the determination of pure water (the minimum range is 0.02mS×cm-1) for measurement (see the above table)

2) pH:

The pH value is required to be 6~7, and the inspection method is as follows:

①Simplified method:

Take 2 test tubes and add 10 mL of the water sample to be tested, one of which should add 2 drops of methyl red indicator and it should not show red; the other test tube add 5 drops of 0.1% bromothymol blue (bromothymol blue) Not showing blue is satisfactory.

②Instrument method:

Use a pH meter to measure the pH value of the pure water in equilibrium with the atmosphere, and the pH value is between 6 and 7 as qualified.

3) Calcium and magnesium ions:

Take 50mL of the water sample to be tested, add 1mL of pH=10 ammonia-ammonium chloride buffer and a little chrome black T (EBT) indicator, it will not show red (it should show pure blue).

4) Chloride ion:

Take 10 mL of the water sample to be tested, acidify it with 2 drops of 1mol×L-1, HNO3, then add 2 drops of 10g×L-1, AgNO3 solution, and shake it up without turbidity.

In the chemical analysis method, except that deionized water must be used for complexometric titration, distilled water can be used for other methods. The pure water used in the analysis experiment must be kept pure and avoid pollution. A container made of polyethylene is usually used to hold pure water for experiments.

Common reagents and their use and storage

The quality of reagents used in analytical chemistry experiments directly affects the accuracy of the analysis results. Therefore, it should be based on the specific conditions of the test, such as the sensitivity and selectivity of the analysis method, the content of the analysis object and the accuracy requirements of the analysis results, etc. , Reasonable selection of the reagents of the corresponding level can not only ensure the normal operation of the experiment, but also avoid unnecessary waste. In addition, the reagents should be stored reasonably to avoid contamination and deterioration.

1 category

There are thousands of chemical reagent products, and with the development of science and technology and production, new types of reagents will continue to emerge. There is no unified classification standard. Here we only briefly introduce standard reagents, general reagents, and high-purity reagents. Reagents and special reagents.

1) Standard reagents

Standard reagents are standard substances used to measure the chemical quantity of other substances (to be measured). They are customarily called reference reagents. They are characterized by high main body content and reliable use. my country stipulates that the main content of the first benchmark for titration analysis and the working benchmark for titration analysis are 100±0.02% and 100±0.05%, respectively.

2) General reagents

General reagents are the most commonly used reagents in laboratories, and their specifications are divided by the amount of impurities contained in them, including general-purpose first, second, third, and fourth grade reagents and biochemical reagents.

3) High purity reagent

The biggest feature of high-purity reagents is that their impurity content is lower than that of superior or standard reagents. They are used for the decomposition of samples and the preparation of test solutions in trace or trace analysis, which can minimize the interference caused by the blank value. Improve the reliability of the measurement results. At the same time, in the technical indicators of high-purity reagents, the main components are equivalent to those of superior or benchmark reagents, but the items that indicate the impurity content are 1 to 2 times more.

4) Special reagents

As the name suggests, special reagents refer to reagents for special purposes. For example, chromatographic pure reagents used in chromatographic analysis methods, chromatographic analysis special carriers, fillers, fixed liquids and thin-layer analysis reagents, spectral pure reagents used in optical analysis methods and special reagents in other analytical methods. In addition to meeting the requirements of high-purity reagents, special reagents are more importantly under the limit that the interfering impurity components do not cause obvious interference in specific applications.

2 matters needing attention

a. After opening the bottle cap (plug) and taking out the reagent, the bottle (plug) should be covered immediately to prevent the reagent from absorbing moisture, staining and deterioration.

b. The bottle cap (plug) is not allowed to be placed randomly, so as not to be contaminated by other substances and affect the quality of the original bottle of reagent.

c. Reagents should be taken directly from the original reagent bottle, and more reagents are not allowed to be poured back into the original reagent bottle.

d. The solid reagent should be taken with a clean and dry spoon. The small spoon after taking the strong alkaline reagent should be washed immediately to avoid corrosion.

e. When using a pipette to take liquid reagents, never use the same pipette to take two kinds of reagents at the same time.

f. The bottle containing the reagent should be affixed with a label indicating the name, specification and date of manufacture of the reagent. Reagents without a label or with illegible label and handwriting cannot be used casually until the ingredients have been determined.

3 Save

Improper placement of reagents may cause changes in quality and composition. Therefore, it is very important to preserve the reagents correctly. General chemical reagents should be stored in a well-ventilated and clean house to avoid contamination by moisture, dust and other substances, and corresponding storage methods and measures should be taken according to the nature of the reagent.

a. Reagents that easily corrode glass and affect the purity of the reagents should be stored in plastic or paraffin-coated glass bottles, such as hydrofluoric acid, fluoride (sodium fluoride, potassium fluoride, ammonium fluoride), caustic alkali (hydrogen Potassium oxide, sodium hydroxide) etc.

b. Reagents that are easy to decompose when exposed to light, and are easily oxidized and volatile in air should be stored in a brown bottle and placed in a cool and dark place. Such as hydrogen peroxide (hydrogen peroxide), silver nitrate, pyrogallic acid, potassium permanganate, oxalic acid, sodium bismuth, etc. are easily decomposed by light; stannous chloride, ferrous sulfate, sodium sulfite, etc. are easily decomposed by air Oxidizing substances; bromine, ammonia and most organic solvents are volatile substances.

c. Reagents with strong water absorption should be kept tightly sealed, such as anhydrous sodium carbonate, caustic soda, peroxide, etc.

d. Reagents that are easy to interact, flammable, and explosive should be stored separately in a cool and ventilated place. For example, acid and ammonia, oxidant and reducing agent are easily interacting substances; organic solvents are flammable reagents; chloric acid, peroxide Hydrogen oxide and nitro compounds are explosive reagents.

e. The highly toxic reagents should be kept in special storage, and the procedures for taking them should be strictly avoided to avoid poisoning accidents. For example, cyanide (potassium cyanide, sodium cyanide), hydrofluoric acid, mercury dichloride, arsenic trioxide (arsenic), etc. are highly toxic Reagents.

Analytical balance and its weighing method

Analytical balance is one of the most used basic equipment in quantitative analysis experiments. It is a precise and expensive instrument. It is usually required to be able to accurately weigh to 0.0001g, and its maximum load is generally 100-200g. In order to get accurate weighing results, it is necessary to understand its structure, performance and usage methods before starting the quantitative analysis experiment.

General analytical balances are weighed according to the principle of leverage. Two balance pans are placed on both sides of the equal-arm balance beam. One pan is placed on the weighing object and the other pan is placed with weights. When the torques on both sides are equal, the balance reaches equilibrium. . At this time, the mass of the object to be weighed is equal to the mass of the weight. This is the basic principle of balance weighing.

1 species

Air damping balance

Electro-optical balance

Single disc balance

Electronic balance

2 Weighing method

Glass or ceramic vessels usually used in analysis and testing work, its surface will absorb moisture in the atmosphere, powder solid samples or reagents in particular will absorb more moisture, this adsorbed moisture content will change with the atmospheric temperature In order to avoid the influence of moisture content on the measurement results, when weighing the mass of the utensils or samples used, they must be kept in a completely dry state. For this reason, when weighing an object or weighing a sample, it is necessary to adopt appropriate balances and utensils and pay attention to standard operations.

Methods of weighing with analytical balances include: direct weighing method, designated accurate mass weighing method (also called weighted weighing method), designated weighing method of a certain mass range (also called weight reduction method or decrement method) three Kind. The following are introduced separately:

(1) Direct weighing method

To know the mass of an object of unknown mass, the object can be weighed directly on the balance to obtain the accurate mass of the object, which is called the direct weighing method.

(2) Specify accurate mass weighing method

In analytical experiments, sometimes it is required to weigh a sample or reference material of a certain quality, and when these reagents are powdery substances with little hygroscopicity, this weighing method can be used to weigh.

The basic operation method is:

Use a dry container (small beaker, watch glass) or a piece of weighing paper (fold it into a spatula) to put it on the balance pan and weigh its mass, and then use a medicine spoon to add a little less than the required mass. In this way, operate according to different models of balances until the mass of the added sample is equal to the specified mass value.

(3) Specify the weighing method of a certain mass range

Basic operation method: Use a strip of paper to cover the weighing bottle (containing the required sample) and take it out of the desiccator, put it on the balance pan and directly weigh its mass, and record it as m total. Use the same method to take out the weighing bottle and move it to the top of the sample receiver. Use a piece of paper to clamp the handle of the bottle cap and gently tap the outer edge of the bottle mouth to make the sample slowly drop into the receiver in small quantities. When the poured sample is close to the weight to be weighed, gently tap the edge of the bottle mouth while slowly turning the bottle body upright, so that the reagent sticking to the bottle mouth falls back into the weighing bottle. Close the cap of the weighing bottle, and weigh the mass of the weighing bottle after pouring out the sample. If the amount of pouring is far from the quality of the required sample, repeat the above operation until the poured sample is close to the required amount, accurately weigh out the mass of the weighing bottle and record it as m1.

m total-m1 = m style 1.

With the same operation, the second, third, and fourth samples can be continuously weighed. Therefore, when multiple samples within a certain mass range need to be weighed, and the samples are easy to absorb moisture, easy to oxidize or volatilize, this weighing method—decreasing method can be used for weighing.

Basic operation of titration analysis

A variety of glass measuring instruments are often used in titration analysis. Among them, burettes, pipettes, and volumetric flasks are used to accurately measure the volume. The measuring tools for titration analysis experiments are usually referred to as these three glass instruments.

1 Pipette

a. Single-label pipette

It is used to accurately pipette a fixed volume of solution (see Figure 1.4(k)). There are various specifications, such as 50mL, 25mL, 20mL, 15mL, 10mL, 5mL, 2mL, 1mL, etc., which can be carried out according to the experiment. Optional. This kind of pipette is generally used more, and it is used to call it a single-label pipette.

b. The operation of the pipette

①Washing

It must be washed with detergent solution or chromic acid lotion before use. Use the ear wash ball to suck in the detergent to half of the pipette’s swelling part, place it flat and rotate it for a few weeks so that the inner glass walls are in contact with it, and then release the detergent (if chromic acid lotion is used, it should be put back In the original lotion bottle), rinse with tap water for several times and then rinse with distilled water (three times).

② Pipette solution

Before pipetting the solution, pipette a small amount of the solution to rinse the pipette. The rinsing method is the same as the washing method. Then insert the solution into the solution, draw the solution above the scale with the ear wash ball, immediately press the pipette port with your index finger and then take it out, slightly reduce the pressure of the index finger and turn the pipette to make the solution flow out slowly. At the same time, observe the liquid level. When the surface reaches tangent to the scale, immediately press your index finger and wipe off the test solution stuck to the lower end of the pipette with a filter paper sheet (note: the filter paper sheet should not be attached to the pipette nozzle to avoid sucking off the test solution). Insert it vertically into the receiver, make the lower end of the pipette touch the inner wall of the receiver and tilt the receiver slightly, fully release the index finger to let the solution flow down freely, after the solution has completely flowed out, wait 15 seconds before taking out the pipette .

How to operate the pipette

③After using the pipette, wash it with tap water and distilled water, and put it back on the instrument rack.

c. Pipette

A pipette with a scale is called a pipette, and it is generally used to pipette a volume of less than 10 mL. You can pipette any volume on the pipette scale as needed. The commonly used pipette specifications are 0.1-10mL. The basic operation of the pipette is the same as that of the pipette.

2 Burette

The burette is a glass tube with a uniform scale. It is used to hold the operating liquid for titration in the titration analysis method. The scale is engraved at equal distances during manufacturing. Since the diameter of the glass tube cannot be absolutely uniform, there will be errors in the scale of the same value, so it needs to be corrected.

a. Classification

There is a piston at the lower end of the burette to control the titration speed. According to the different properties of the solution, there are acid burettes with glass pistons for acid liquids, and alkali burettes with glass beads and rubber tubes for lye.

b. Operation

☞Pre-treatment

Acid burette:

When the burette is filled with solution, there should be no dripping or seepage. If dripping or seepage is found, it is generally caused by the piston is not matched or the piston is not properly oiled. If the piston is not matched, it is a product quality problem and cannot be dealt with, just use a qualified one. If the burette product is qualified, the cause of dripping or seepage is generally caused by improper oiling of the piston.

Alkaline burette:

In the same way, after the basic burette is filled with solution, there should be no dripping or seepage. If dripping or seepage is found, it may be because the hose is aging and inelastic, just change a hose; or it may be the size of the glass beads. The hose is not matched, you can replace it with a suitable glass bead.

☞Titration

The operation gestures for titration are shown in the figure. The operation methods are as follows: the two hands are properly coordinated, the operation is free, continuous dripping, only adding one drop (even if the solution is suspended) and only adding half a drop. In this process, pay attention to the following point:

  1. When shaking the conical flask, rotate it in the same direction, so that it is even and does not splash.

  2. The burette should not leave the mouth of the bottle too high or touch the mouth of the bottle, that is, when the titration is not started, the conical flask can be easily removed. When the titration operation, the burette nozzle extends into the conical flask but does not exceed the bottleneck.

  3. During the titration, the left hand cannot leave the piston and allow the operating fluid to flow by itself.

  4. Half-drop operation: Carefully discharge (acid burette) or squeeze (basic burette) operating liquid half drop, lift the conical flask, make its inner wall lightly touch the burette nozzle, make the half-drop operation hanging on the burette nozzle The liquid sticks to the inner wall of the conical flask, and then wash it off with a washing bottle.

  5. Pay attention to the change in the color of the solution near the drop point. At the beginning of the titration, the speed can be slightly faster, but it should be dripped instead of flowing into a “water line”. When approaching the end point, drop a drop, shake it a few times, and observe the color change; continue adding one drop or half a drop until the solution is complete The color just changes from one color to another and does not change within 1 to 2 minutes, which is the end point.

3 volumetric flask

a. Purpose and specifications

It is used to prepare standard solution or reference solution, and it is also used for multiple dilution of the solution. There are various volumetric flasks (5, 10, 25, 50, 100, 500,…, 2000mL). Generally, the bottleneck scale refers to the TC volume, some instruments have two scales, and the upper scale refers to the TD volume.

b. Operation

Before using the volumetric flask, check whether the stopper matches the bottle.

Fill the volumetric flask with water and stop it, press the cork tightly with your left hand, and hold the bottom of the flask with your right hand to make the flask stand upside down. The stopper should be tied to the neck of the bottle with a string. It should not be placed casually to avoid contamination or confusion. After opening the stopper, operate according to the diagram.

When preparing the solution, first dissolve the accurately weighed substance in a small beaker, and then operate according to the figure.

Pour the solution into the volumetric flask along the glass rod. After the solution is transferred, lift the beaker slightly up the glass rod. At the same time, make the beaker stand upright to prevent the droplets on the mouth of the cup from flowing out of the cup, and then put the glass rod back into the beaker. Then, rinse the inner wall of the beaker and the glass rod with a washing bottle, transfer all the washing water into the volumetric flask, and repeat this operation four or five times to ensure complete transfer. The above process is called “quantitative transfer” operation.

Figure 4 Operation of volumetric flask

After quantitative transfer, add diluent (for example, water). When adding water to about half of the bottle, shake the bottle (not upside down) to make the solution initially uniform, and then continue to add to about 0.5cm away from the graduation line with a small dropper Add distilled water drop by drop until the liquid level is tangent to the marking line, close the stopper, press the stopper with your index finger, hold the neck with the remaining four fingers, hold the volumetric flask with the other hand (five fingers) and invert it repeatedly, shaking The solution is completely uniform, this operation is “constant volume”.

Precautions for the use of titration analysis instruments

  1. It must be washed clean. Unclean instruments will have water drops on the glass wall to make the measurement volume inaccurate; for titration analysis measuring tools (burettes, pipettes and volumetric flasks), it is required to wash them until there are no water drops;

  2. The volumetric instrument cannot be heated or rapidly cooled, and cannot be dried;

  3. Observe the liquid level according to the lowest point at the bottom of the meniscus;

  4. When observing the liquid level scale, the line of sight must be at the same level as the scale, otherwise errors will be introduced.

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