?
First of all, the purpose of the experiment
Understand the measurement method of dilute solution concentration;
Determine the solubility product of insoluble salt;
3. Consolidate the concepts of activity, activity concentration and correlation coefficient.
Second, the experimental principle
Some ions are in equilibrium at a certain temperature, and the saturated solution of insoluble salt of electrolyte is formed in the solution. The general expression is as follows:
Strictly speaking, the equilibrium constant of the solubility product is called the solubility product, or simply referred to as the solubility product of the ion action constrained by the solution corresponding to the activity product of the ion, but it is considered that the ionic strength of the saturated solution containing almost no electrolyte is very small, so it is unnecessary to warn by the concentration activity.
In silver chloride
From the above formula, if the concentration of insoluble electrolyte ions in saturated solution is measured, the solubility product of solubility product can be calculated. Therefore, the finally measured ion concentration is measured. Design a method to determine the concentration and find the solubility product of the measurement method.
Specific methods for measuring concentration include titration (such as AgCl solubility product), ion exchange method (such as copper sulfate solubility product), conductivity method (such as AgCl solubility product), ion electrode method (such as lead chloride solubility product) and electrode potential method (the relationship between solubility products), that is, spectrophotometry (such as copper hydroiodate solubility product).
?
ⅰ. Determination of solubility product of calcium sulfate (ion exchange method)
?
First of all, the purpose of the experiment
1, ion exchange resin for practice;
Understand the principle and method of determining the solubility and solubility product of calcium sulfate by ion exchange method.
Further practice acid-base titration and filtration in the atmosphere.
Second, the experimental principle
Ion exchange resin is a synthetic solid spherical polymer that can be exchanged with other substances. Ion exchange molecules containing acidic groups contain special reactive groups. Cations are cation exchange resins containing basic groups, which can be exchanged with other substances, and anion exchange resins. Polystyrene sulfonic acid resin, the most commonly used is strongly acidic cation exchange resin, and its structural formula can be expressed as:
In this experiment, strong acid cation exchange resin (R-SO3H) (type 732) was used to exchange Ca2+in saturated solution of calcium sulfate.
2R-SO3H+Ca +→(R SO3)2 Ca+2H+
?
Calcium sulfate is a slightly soluble salt, and the part beyond its solubility increases the balance between ion pairs and simple ions in calcium sulfate saturated solution of Ca2+ and SO42- ions.
Calcium sulfate (AQ)= internal Ca2+SO42-
As the Ca2+ ion exchange equilibrium shifts to the right, the solution flows through the exchange resin, and all the results of calcium sulfate (ag) dissociation are changed from the effluent [H +] to the molar solubility of calcium sulfate calculated by H+. ：
?
[H +] can be measured by an acidity meter, or it can be a titration diagram of standard NaOH solution. Titration is introduced here.
Let [Ca2+] of saturated calcium sulfate solution be c [SO4 2-] = c, and then press [calcium sulfate (AQ)] = y-c.
and
KD, 25℃, ion dissociation constant Kd = 5.2× 10-3.
and
From Equation C, the solubility product Ksp defined by solubility product = [internal Ca2 +] [SO 4 2-] = C2 is obtained as follows.
Third, the experimental steps.
1。 Packed column ion exchange column (instead of alkaline burette) washes a small amount of glass fiber or closes the bottom filled with cotton fat, weighs a certain amount of 732 strong acid cation exchange resin and puts it into a small beaker, and then soaks it with distilled water and removes suspended particles and impurities with water and transfers it to the ion exchange column. The lower end of the knob clamp of the exchange column is opened to let the water flow out slowly until the liquid level is about 1cm higher than the resin, and the clamping screw is clamped. If there are bubbles, it means that the glass rod is clamped. Remove bubbles and add a small amount of glass fiber (or cotton) to the above resin.
2。 After the transition to ensure the complete exchange of Ca2+ into H+ and Na+ resins, the formed H+ must be completely converted. Add 40 ml of 2mol/L hydrochloric acid solution to the exchange column in batches, control the flow rate of 80-85 drops per minute, and let the HCl solution flow through the cross resin for 10 minute. [Note: If a good acid-treated resin is used, it should be directly treated after column loading], and the resin is washed with 50-70ml distilled water until the pH value of the effluent is 6-7 (measured by pH paper).
3 Put the downstream saturated calcium sulfate 1g analytically pure calcium sulfate solid solution into about 70ml, boil it, cool it to distilled water at room temperature, stir it for 10min, and then let it stand for 5min. The filtrate is saturated with calcium sulfate solution with quantitative filter paper (filter paper, funnel and suction bottle should be dry).
4。 Absorb 20.00 ml saturated calcium sulfate solution with foreign exchange, inject it far away from the cross column, control the speed of the effluent from the exchange column for 20-25 drops/minute, and discharge the sewage in the cleaned conical flask. Almost completely saturated solution flows into the resin bed, and after washing the resin with distilled water (about 50 ml of water), the pH of the effluent is 6-7. Please be careful not to lose the waste water in the whole exchange and leaching process.
Hydrogen ion concentration 5 was determined by acid-base titration, and 2 drops of bromine thymolphthalein indicator were added to the sewage. Titrate the solution from yellow to bright blue with standard NaOH solution to reach the titration end point. Accurately record the volume of NaOH solution used, and express the concentration of hydrogen ions in the solution by the following formula.
Data recording and results
Saturated liquid temperature of calcium sulfate
?
?
Saturated solution volume passing through the exchange column (mL)
?
?
NNaOH (mole/liter)
?
?
Vonalo (ml)
?
?
[Hydrogen ion] mole/liter
?
?
Solubility of calcium sulfate?
?
?
Calcium sulfate solubility product Ksp
?
?
Calculate the data with Kd value of about 25°C, and write an experimental report during the calculation.
Error analysis operation error, according to the literature value? Solubility of calcium sulfate, and discuss the reason of error.
Five questions
Why is the liquid flow rate controlled too fast? Why are bubbles not allowed in the resin layer? How to avoid it?
2. What is the experimental result of calculating the solubility product of calcium sulfate?
Why should CO2 distilled water which has been deleted be used to prepare saturated solution of calcium sulfate?
What are the factors that affect the final determination result? What do you think is the most critical step in the whole operation?
5. What is the effect of the following experimental results?
1) transition, the resin can not be completely converted into H+ form.
2) It is not allowed to cool the saturated solution of calcium sulfate to room temperature on the filter.
3) Filter the saturated liquid of calcium sulfate in the funnel and receive the undried liquid in the flask.
4) transformation, the eluent flows out, and when it is lower than neutral, the elution and communication are stopped.
?
Literary value of solubility product of additional calcium sulfate
?
T℃
? 0
? 10
? 20
? 30
? 40
?
Solubility × 102 mol/l
? 1.29
? 1.43
? 1.50
? 1.54
? /
?
The unit is grams per hundred grams (g/ 100g).
? 0. 1759
? 0. 1928
? /
? 0.2090
? 0.2097
?
?
reading material
ion exchange process
The compounds in the ion exchange column of ion exchange resin get the corresponding products due to the exchanged ionic bonds, which is called ion exchange method. This method is widely used in the separation, extraction and purification of elements, decoloration and purification of organic matter, purification of water, and as a reaction catalyst. This is required by the ion exchange method, including the ion exchange resin of the corresponding ion exchange column.
Ion exchange resins include natural and synthetic, and the most important one is synthetic organic resin, which is mainly used as cross-linked styrene and divinylbenzene as polymers of resin matrix structure, and then connected with corresponding upper reaction groups. The synthesized ion exchange resin is an insoluble polymer, which contains reactive groups and has a network structure. There are many ionizable reticular skeletons and some ion exchange active groups in the surrounding solution. The reticular ion exchange resin is extremely insoluble in water or acid or alkali solution, and has no effect on most organic solvents, oxidants, reductants and heat.
A. Classification of ion exchange resins
The role of controversial groups and different ion exchange resins can be divided into different categories. For example, cation exchange resins are used for cation exchange reactions, while ion exchange resins have special functions for anion exchange resins.
1。 Cation exchange resin is a kind of resin with acidic exchange groups, including sulfonic acid group (-SO 3 H), carboxyl group (-COOH) and phenolic hydroxyl group (-OH). In these resins, their cations can be exchanged in solution. According to the strength and pH value of active groups, cation exchange resins are further divided into strong acid cation exchange resins (active group is -SO 3 H), domestic 732 resins (new brand 001-KLOC-0/00), medium acid cation exchange resins (active group -PO3H2) and (# 401-). ) (such as model 724, brand # 10 1-200) and so on.
2。 Anion exchange resin contains basic reactive groups, and the anion of this resin can be anion exchange of solution. Strongly basic anion exchange resin (active group is quaternary ammonium base, such as 7 1 1 #, 7 14 #, etc. ) and weakly basic anion exchange resin are divided into (active groups are primary amine, secondary amine and tertiary amine groups, such as 70 1 # resin, etc. ) )
3。 It has special functional resins, such as chelating resin, amphoteric resin and redox resin (see Table 2-8).
The application should be based on the specific requirements of different types of ion exchange resins in this experiment.
II. Basic principles of ion exchange
? The ion exchange process is that the ions in the solution diffuse through the resin that diffuses into particles, and the exchanged H+ ions are exchanged on the resin (or the active group of Na+ plasma) and have been discharged from the hospital. So it is reversible in the process of ion exchange. The larger the cation exchange resin is, the greater the ion valence exchange potential is, that is, it is combined with the resin.
In Table 2-8, the types of ion exchange resins
type
? Activity group
? kind
? situation
?
cation exchange resin
? strong acidity
? Sulfonic acid group
H-type (R-SO 3 H) Na-type (R- hypocrellin derivative)
? 732, IR- 120 type
?
phosphate
H type (R-PO3H2): Na type (R-PO3N2).
?
?
weak acid
? Carboxylic acid group
Type h (r-co2h): type na (R-CO2Na).
Model 724, IRC-50
?
phenolic group
Hydrogen sodium type
?
?
anion (exchange) resin
? strong basicity
? Quaternary amine group
OH type (R-NR`3OH)
Chlorine type (r-NR "3cl)
? 7 17, IRA-400 type
?
alkalescence
Primary amine group
Type OH (R-NH3OH)
Chlorine type (R-NH3Cl)
70 1, IR-45 type
?
Secondary amino group
Type OH (r-NR "h2oh")
Chlorine type (R-NR "H2Cl)
?
?
Tertiary amine group
OH type (R-NHR`2OH)
Chlorine type (R-NHR "2cl")
?
?
Special functional resin
Chelating resin, amphoteric resin, redox resin
?
Strong cooperation ability:
k+& lt； Sodium ion of hydrogen ion
? Similarly, for the purpose of results, the exchange potential of ion exchange resin increases with the increase of ion valence, such as in strongly basic anionic resin:
AC-F-OH-HCOO-h2po 4-HCO 3-bro 3-CL-& lt； NO3-& lt； NO2- bromide
Generally, the so-called exchange capacity of 1 g dry resin is made, and the exchange capacity is milliequivalent of the corresponding ion exchange number. The exchange capacity of different types of resin is strongly acidic ion exchange resin, generally ≥4.5 mg equivalent/g dry resin, so that the minimum amount of resin can be calculated, which requires specific experiments.
III. Influencing factors of exchange resin
There are many factors affecting resin exchange, mainly including the following aspects:
1。 The properties of resins from different manufacturers are different, and the exchange capacity of different types of resins is also different.
2。 Pretreatment of resin or regeneration quality.
3。 Full of resin, whether the resin in the ion exchange column is full of bubbles.
4。 The ratio of column diameter to outflow velocity caused by ion exchange process is a slow exchange process, and this exchange is a reversible process. The result of outflow velocity exchange has great influence. The outflow speed is too high, too late, and the ion exchange effect from the cross is not good. The column diameter ratio of the outflow speed [the ratio of the height and diameter of the ion exchange column to the ion concentration in the solution, mobile phase and ion exchange (Figure 2-35)] and other factors, such as low ion concentration, can appropriately increase the outflow speed. In the laboratory, the column diameter ratio is 10: 1 or above, and the outflow velocity with larger column diameter can be increased appropriately. In order to obtain better effect, the outflow speed is generally controlled at 20-30 drops/minute.
4. Pretreatment with new resin and regeneration with aged resin.
1。 The purpose of cation exchange resin pretreatment (1) Cleaning and removing some foreign impurities will buy a new resin and soak it in water, so it won't bother you. Discard the pickling solution and change the water continuously until the pickling solution is colorless. ⑵ Corrosiveness Due to the stability requirements, all the new resins purchased are basically sodium type. Caustic treatment may be to convert some non-sodium types into sodium types, which is convenient for the next step. Soak the increased volume in 8% NaOH solution for 30 minutes, and then wash the separated lye with water to neutrality. (3) HCl solution with 7% conversion rate is used for three times, and the volume of each time is 30 minutes. After that, the acid is separated and washed to neutrality (note: distilled water or deionized water should be used for the last time).
2。 Pretreatment of anion exchange resin (1) Add the same amount of 50% ethanol to the newly purchased anion exchange resin, stir, stand overnight, remove the ethanol, and rinse with clear water until the pickling solution is colorless and tasteless. ⑵ Soak in 7% HCl solution for 3 times, each time for 30 minutes, separate the acid, and wash it with water until it is neutral. ⑶ Soak in 8% NaOH solution for 3 times, each time for 30 minutes according to the volume, and rinse with water until the pH value is 8-9.
3。 With the passage of time, discoloration and loss of exchange ability can be the aging treatment of resin to regenerate ion exchange resin. Regeneration of resin's method is similar but different, but the basic steps are the same as pretreatment. Rinse first, and then use H+, Na+ (or OH-, Cl-) ion exchange resin IE browser based on the principle of reversibility of ion exchange process. In the regeneration process, static methods, dynamic methods and other methods can be used. The cation exchange resin is regenerated with 2mol/L hydrochloric acid, for example: (1) static method. After rinsing, add an appropriate amount (2-3 times (by volume) or more) into the resin for 24 hours or more (stirring should always be carried out during standing), discard the acid and wash until it is neutral. (2) The dynamic method is 2-3 times the capacity of 2 mol/L (about 7%) HCl solution (or other acids). Turn on the switch knob of the lower horizontal column, and let it out for the first time. The residual water is discharged from the horizontal column, so that the liquid slowly flows out of the sewage with the measured pH value. When the sewage is strongly acidic at any time, close the knob and let it stand for a period of time, and the result is completed (static victory again).
Note (1) In order to avoid ion exchange in tap water during washing, it is best to wash off most resin acid (or alkali) [pH of effluent is about 2-3( 1 1- 12)] (deionized water) with distilled water. (2) When anion exchange resin can be easily decomposed for more than 40 times, special attention should be paid. ⑶ The resin gradually cracks and breaks during the payment process, but it usually takes 3-4 years or even longer, and it is not easy to pour out. (4) The traded (or regenerated) resin should be used immediately, and cannot be stopped for a long time, because
?
?
Ⅰ cation exchange column
Ⅱ anion exchange column
Ⅲ mixed ion exchange column
?
?
?
?
?
?
Figure 2-35 Figure 2-36 Schematic diagram of ion exchange device with column diameter ratio
?
Its stability is poor. Crossed Na+ type cationic resin is usually more stable than H+ type Cl- and more stable than OH- type anionic resin. 5] The regeneration of resin should be based on the acid (alkali) on the resin. For example, lead nitrate (NO3)2 should be soluble if it is combined with Pb2+.
Five, the specific operation of ion exchange method
1。 The resin should be pretreated or regeneration of resin transformed, and the transformed resin should be placed in distilled water.
2。 The choice of packaging (1) is based on the purpose and situation of the experiment.
If inorganic cation or organic base is adsorbed, cation exchange resin should be used, and inorganic anion or anion exchange resin should be used organically. If amino acids are separated, such as amphoteric substances, cation exchange resins can be used. After fixing sheep, anion exchange resin is used to determine the required type of exchange groups. Weak acid (alkali) and other resins can be used for strong adsorption of ions from cross resistance, and AC resin should be selected for adsorption of weak acid (alkali) resistance. Several ions should be stored in weak adsorption county and re-selected after adsorption by strong exchange resin. Strong acid ion exchange resin (basic peak) should be used as catalyst. (2) In the activation process, the resin packed column is put into the ion exchange column. Column packing, the key point is that gaps or bubbles cannot be resin. The specific method is: exchange deionized water in the column at 1, then put the resin and water into the column, open the lower part of the piston, and the water begins to flow. When the dropping of the resin is completed, the resin is washed with deionized water until the pH of the effluent is neutral. In the process of column packing, special attention should be paid to water and resin layer to avoid bubbles and resin failure. If bubbles are generated unintentionally, stir the branches with a glass rod and mix with the bubbles.
3。 Turn the switch knob away from the lower end of the cross-opening ion exchange column, and put the treated ion exchange column into deionized water (note: the pH value of the effluent is neutral after further detection, if not, continue to wash it with deionized water until it is neutral). Until the deionized water just hiding the resin is added to the ion exchange column of the sample solution to be treated (note: when they don't turn over the resin), the switch knob opens the lower end of the resin column, and the flow rate is controlled at 20-30 drops per minute. When almost all the sample liquid enters the resin, add deionized water (note: there should be no water during the crossing of resin layers to avoid the influence of bubbles and crossing) and continue to cross. ⑷
Calculation of resin regeneration method.