Titration is a means of quantitative analysis and a chemical experimental operation. It determines the content of a certain solute by the quantitative reaction of two solutions. It indicates the titration end point according to the color change of the indicator, and then visually observes the consumption volume of the standard solution and calculates the analysis result.
Principle
The titration process requires a quantitative reaction, the reaction must be completely carried out, and the rate must be fast, that is, the equilibrium constant and the rate constant must be large. Moreover, the reaction must not have by-products that interfere with the measurement, and side reactions are not allowed.
In the titration of two solutions, the solution of known concentration is placed in the burette, and the solution of unknown concentration is placed in the conical flask below. A solution of known concentration is usually called a standard solution, and its concentration is measured by titration with a solid reference reagent that is not susceptible to deterioration.
When the reaction stops, read the volume of the solution in the used burette, and then use the formula to calculate the concentration.
Depending on the type of reaction, titrations are divided into the following categories:
Acid-base neutralization titration (using neutralization reaction) (indicators commonly used methyl orange and phenolphthalein)
Redox titration (using redox reactions)
Precipitation titration (using a reaction that produces a precipitate)
Complexometric titration (using complexation reactions)
Equipment
Burette
1. The burette is a glass instrument that can accurately measure the consumption volume of the titrant during titration. It is a slender glass tube with a precise scale and a uniform inner diameter. It can continuously release different volumes of liquid as needed, and can accurately read the liquid. volume.
2. The volume of the burette for constant analysis is 25mL and 50mL, the minimum scale is 0.1mL, and the reading can be estimated to 0.01mL.
3. There are two types of burettes: with plugs and without plugs, which are commonly referred to as acid burettes and basic burettes. The acid burette is also called a burette with a stopper. Its lower end has a glass stopcock switch, which is used to hold acidic, neutral and oxidizing solutions, but not alkaline solutions such as NaOH.
Alkaline burette, also known as stopperless burette, has a rubber tube at the lower end and a glass bead in the middle to control the flow rate of the solution. It is used to hold alkaline solution and non-oxidative solution.
4. Use of burette:
(1) Preparation before use
①Washing: tap water → lotion → tap water → distilled water
② Apply Vaseline: the surface of the large end of the piston and the inner wall of the small end of the piston groove
③Leak detection: fill the burette with water to the highest mark, clamp it on the burette clamp and place it for 2 minutes
Use filter paper for the acid burette to check whether there is water seepage at both ends of the piston and the tube clamp, then rotate the piston 180° and check again.
Alkaline burette, put it for 2 minutes, if there is water leakage, replace the rubber tube or glass beads of suitable size.
④Rinse: In order to ensure that the standard solution in the burette is not diluted, the burette should be washed three times with the standard solution, 5~10mL each time.
⑤ Filling the solution: Hold the burette in the left hand, tilt the burette, take the reagent bottle in the right hand and pour the solution into the burette until it is filled above the zero mark.
⑥ Exhaust air bubbles: When there are air bubbles at the tip of the acid burette, hold the unmarked upper part of the burette with your right hand, and open the piston with your left hand, so that the solution can quickly wash away the air bubbles; when there are air bubbles in the basic burette, bend the rubber stopper upward and squeeze with two fingers Press the glass beads to eject the solution from the tip of the tube to remove air bubbles.
⑦ Zero point: adjust the liquid level to be level with the zero scale line, and the initial reading is “0.00mL”
⑧Reading:
a. The burette should be placed vertically when reading;
b. When injecting or releasing the solution, it should be left for 1~2 minutes before reading;
c. The initial reading is preferably 0.00mL;
d. The lowest point of the meniscus is read from a colorless or light-colored solution, and the line of sight should be tangent to the level of the meniscus;
e. The dark solution should read the highest point on the upper edge of the liquid surface;
f. It is necessary to estimate one bit when reading.
(2) Titration operation: clamp the burette on the right
①Acid burette: The piston handle is to the right, the left hand extends from the back of the burette to the right, the thumb is in front of the burette, the index finger and middle finger are behind the pipe, and three fingers are parallel to gently hold the piston handle. Note: Do not force outwards to avoid pushing out the piston
②Alkaline burette: The thumb of the left hand is in front and the index finger is behind, pinch the top of the glass beads in the rubber tube to form a gap between them and the glass beads, and the solution can flow out.
Note: Do not pinch the rubber tube under the glass beads, and do not move the glass beads up and down, otherwise air will enter and form air bubbles.
③ Shake the flask while dripping: The titration operation can be carried out in a conical flask or a beaker. Carry out titration in the conical flask, hold the conical flask with the thumb, index finger and middle finger of the right hand, and the other two fingers are on the lower side, so that the bottom of the flask is about 2~3cm high from the titration table, and the lower end of the burette is about 1cm deep into the mouth of the bottle. Control the titration speed with the left hand, add the solution dropwise, shake the conical flask with the right hand, and shake while dripping.
(3) Precautions for titration operation:
①When titrating, it is best to start from 0.00 mL each time.
②When titrating, the left hand cannot leave the cock, and the solution cannot be allowed to flow by itself.
③ When shaking the bottle, the wrist joint should be rotated to make the solution rotate in the same direction (left or right rotation can be used). Do not vibrate back and forth to avoid splashing the solution. Shaking must have a certain speed, and the solution must be rotated to form a vortex, and the shaking should not be too slow to affect the progress of the chemical reaction.
④When titrating, pay attention to observe the color change around the dripping point, do not look at the scale change on the burette.
⑤ Titration speed control
Continuous dripping: It can be slightly faster at the beginning, and it is “seeing the dripping line”. At this time, it is 10 mL/min, that is, about 3 to 4 drops per second. Be careful not to drip into a “waterline”, as the titration speed is too fast.
Interval dropwise addition: When approaching the end point, it should be added drop by drop, that is, add a drop and shake it a few times, and then shake it again. Half-drop dropwise addition: Finally, for each half-drop added, shake the conical flask a few times until the solution has a clear color, so that a drop does not fall, flow into the bottle along the wall of the vessel, rinse the inner wall of the neck with distilled water, and shake well.
⑥ Half-drop control and purging:
When using an acid tube, gently turn the cock so that the solution hangs on the outlet nozzle to form a half drop, dip it off with the inner wall of the conical flask, and then rinse it with a washing bottle.
For the alkali tube, when adding half a drop of solution, first release the thumb and index finger, stick the suspended half drop of solution on the inner wall of the conical flask, and then release the ring finger and little finger, so as to avoid bubbles at the tip of the outlet tube.
When half a drop of solution is dripped, the method of tilting the conical flask can also be used to rinse the solution attached to the wall into the bottle, which can avoid too many purging times and cause excessive dilution of the dripping material.
Volumetric flask
The volumetric flask is a commonly used measuring glass measuring device for measuring the volume of liquids that can be contained. The main purpose is to prepare standard solutions of accurate concentration or quantitatively dilute solutions. Common specifications are 10, 25, 50, 100, 250, 500, 1000mL
(1) Check whether the cork is leaking;
(2) Check whether the position of the scale mark is too close to the bottle mouth;
(3) Fixed bottle stopper;
(4) Washing: soak in detergent water, rinse with tap water, rinse with distilled water;
(5) Preparation of solutions;
(6) It is not suitable to store the reagent solution for a long time;
(7) Rinse with water immediately after use.
Indicator
Titration reactions require sensitive indicators to indicate completion of the reaction. The indicator quickly changes to another color when the reaction is complete. This allows the experimenter to determine the termination of the reaction based on the discoloration of the indicator.
The indicator of neutralization titration is organic weak acid or weak base, and their discoloration range is near the isoelectric point. Such as the discoloration of weak acid (represented by HIn):
HIn (acid form)=H++In- (base form)
Indicators generally have two forms, and the two forms show different colors. The indicator exhibits a transitional color within the discoloration range. Some indicators come in three different color forms.
Due to the “sudden jump” phenomenon in the discoloration range, the color will become very fast. Only one drop of solution can make the indicator change color completely. Therefore, when selecting the indicator, just let the pH value at the completion of the reaction fall within the sudden change. It is enough to jump within the range, and it is not necessary to be exact.
Indicators for other types of titrations are generally substances that react sensitively with a certain reactant. When the reactants are consumed, the indicator will change color. Even some reactants can act as indicators, such as potassium permanganate.
Operate
For titration, the burette should be clamped vertically on the burette holder.
If an acid tube is used, the ring finger and little finger of the left hand are bent towards the palm of the hand, gently stick to the outlet tube, and use the remaining three fingers to control the rotation of the piston. However, care should be taken not to pull the piston outwards to avoid water leakage caused by pushing out the piston; also do not push it too far in order to avoid difficulty in the rotation of the piston and inability to operate freely.
If an alkali tube is used, the ring finger and little finger of the left hand clamp the outlet tube, and the thumb and index finger pinch the latex tube to the side (both left and right) where the glass beads are located, so that the solution flows out from the gap beside the glass beads. Note: ① Do not squeeze the glass beads hard, and do not make the glass beads move up and down; ② Do not pinch the latex tube under the glass beads; ③ When stopping the titration, first loosen the thumb and index finger, and finally loosen the ring finger and little finger.
No matter which burette is used, the following three methods of adding liquid must be mastered: ① Continuous dropwise addition; ② Add only one drop; ③ Add half a drop so that the drop does not fall.
The following points should be noted in the titration operation:
(1) When shaking the bottle, the solution should be moved in a circular motion in the same direction (left and right rotations can be used), but do not let the bottle mouth contact the burette, and the solution should not be splashed.
(2) When titrating, the left hand cannot leave the piston and let it flow.
(3) Pay attention to the change of the color of the solution around the drop point of the solution.
⑷ At the beginning, it should be shaken while dripping. The titration speed can be slightly faster, but it cannot flow into a “waterline”. When approaching the end point, you should instead add a drop and shake a few times. Finally, shake the Erlenmeyer flask after each half-drop of solution is added until a noticeable color change occurs in the solution. The method of adding half a drop of solution is as follows: slightly rotate the piston to make the solution hang on the outlet nozzle to form a half drop, dip it off with the inner wall of the conical flask, and then use a washing bottle to rinse the bottle wall with a small amount of distilled water. When adding half a drop of solution with an alkali tube, first loosen the thumb and index finger, stick the suspended half drop of the solution on the inner wall of the conical flask, and then release the ring finger and little finger. This avoids air bubbles at the tip of the outlet tube that could cause errors in the readings.
⑸ It is best to start each titration from 0.00 (or from a fixed scale line near zero), which can reduce the error.
(6) After the titration is completed, the remaining solution in the burette should be discarded, and it should not be poured back into the original bottle to avoid contaminating the entire bottle of operating solution. Immediately wash the burette, and fill the whole tube with distilled water for use.
Reading
After filling or releasing the solution, you must wait 1 to 2 minutes for the solution attached to the inner wall to flow down, and then read. If the rate of solution release is slow (for example, in the final stage of the titration, when only half a drop of solution is added at a time), wait 0.5 to 1 minute before reading. Before each reading, check whether there are water droplets on the tube wall and whether there are air bubbles at the tip of the tube.
It must be read to the second decimal place, that is, it is required to estimate to 0.01mL. Note that the width of the tick marks themselves should be taken into account when estimating the reading.
Before taking the initial reading, the solution hanging from the tip of the tube should be removed. When the end point of the titration is reached, the piston should be closed immediately, and care should be taken not to let the solution in the burette flow out slightly, otherwise the final reading will include the half-drop of liquid flowing out. Therefore, before taking the final reading, care should be taken to check whether the solution is suspended at the tip of the outlet tube. If so, this reading cannot be taken.
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Titrations