What is burette?
is a measuring device used in titration analysis to accurately measure the volume of the titrant added. It is a measure-out instrument that is used only to measure the volume of the solution it releases. It is a long, thin, uniform, and finely graduated glass tube with a pointed lower end and a cut-off door to control the rate of titration of the solution. Common buret
volume of 50 or 25mL, the minimum scale of 0.1mL, the reading can be estimated to be 0.01mL. There are also semi-micro and micro burets
with a volume of 10mL or less. According to the different configurations of the cut-off doors, there are two types of burets: acid and alkaline.
Like their name, acid burets
can only be used for acidic solutions with strong oxidizing properties, which is related to their construction, and the buret switch below the acid buret is a glass stopper.
The alkaline buret
can only contain alkaline solutions and strongly reducing solutions. The lower end of the alkaline burette has a section of rubber tubing, and the glass ball in the middle of the rubber tubing is the titration switch.
Type of burette
Burette chemistry lab
A. Acid burette / glass stopper burette
The glass piston of an acid burette
is fixed to fit the burette
, so it cannot be replaced at will. It is important to note that the glass piston rotates freely. It is important to note that the glass stopper rotates freely. Usually, the operator needs to remove the piston, wipe dry, in the piston ends along the circumference of a thin layer of petroleum jelly for lubricant (or vacuum piston grease), and then insert the piston, top tight, rotate a few times so that the distribution of petroleum jelly can be uniform (almost transparent). The operator needs to put a rubber band on the end of the piston to keep it in place.
The operator needs to be careful not to apply too much petroleum jelly or the small hole in the piston or the tip of the lower end of the burette
may become clogged. Before use, a leak test should be performed. The general buret
can be used for acid buret
, but because the alkaline buret often makes the glass plug and glass hole bonding, it is difficult to turn, the alkaline buret
should be used for an alkaline buret. However, as long as the alkaline buret is not used for a long time, and the buret is rinsed with water immediately after use, the acid buret can also be used.
B. Alkaline burette
The alkaline burette has a rubber tube attached to the lower end of the tube, and a glass bead is installed inside the tube to control the switch, which is generally used for titration of alkaline titration solution. Its accuracy is not as good as the acid buret, as long as the elasticity of the rubber tube causes changes in the liquid level. Solutions with oxidizing properties or other solutions that can easily interact with the rubber, such as potassium permanganate, iodine, silver nitrate, etc. They cannot use the alkaline burette. Before use, the operator should check whether the rubber tube is broken or aging and whether the size of the glass beads is appropriate. Confirm that there is no leakage before use.
Use of burette
a. Before loading the burette, the buret should be washed and the water should be drained naturally (no water droplets should hang on the inner wall). Wash the burette three times with a small amount of burette solution (about 5-10ml each time) to remove the water remaining in the wall and the tip of the lower end of the tube to prevent the burette from being diluted by water.
b. The burette should be loaded into the burette above the zero of the scale when there will be bubbles at the tip of the burette, which must be excluded, otherwise, it will cause volume error. If the acid buret can turn the piston, so that the rapid flow of solution removes the bubbles; such as the alkaline buret, the rubber tube can be bent upward, and then pinch open the glass beads, the bubbles can be excluded by the solution. Wash the buret and then fill the buret with water to close the knob upright and check for water leakage.
c. Then adjust the liquid level of the solution n to the scale zero, you can carry out the titration.
d. Read the volume of the initial titration solution.
B. Titration burette
a. The burette should be clamped vertically on the burette clamp, and the burette table should be milky white. The buret is about one centimeter from the conical stopper, manipulate the cork valve with the right hand, thumb in front, ring finger behind the middle thumb, right ring finger, and little finger bent in the middle of the buret and cork valve directly below the oblique angle. When rotating the plug valve, the fingers in the palm are empty. In the left hand three fingers hold the short plate, the bottom of the bottle from the table about 2 to 3 cm, the burette below the deep level of the cork about a centimeter, the axis body left elbow joint shaking the conical bottle, while dropping and shaking so that the drops of aqueous solution mixed and homogeneous. The standard method of shaking the conical bottle is: the left hand executes the short plate part of the cone, the wrist sub-strength to make the bottom of the bottle along the clockwise drawing arc, the provisions to make the aqueous solution in the conical bottle proportional rotation, produce vortex, the aqueous solution cannot have churning. The stub and the conical bottle should not touch each other. Alkaline burette operation steps: When burette, hold the burette with the right hand, with the thumb in front and the ring finger behind, and use other fingers to assist in fixing the tip of the immovable tube. Pinch the glass marble belonging to the position with the thumb and ring finger, and squeeze the molding rubber hose forward, so that the glass marble bias palm, the aqueous solution can be discharged from the gap.
b. Liquid water flows from fast to slow. Even drops into a line later drop by drop drops of water fall. The final addition should be added half-drop by half-drop. Half a drop is added in the following way: carefully learn to put down half of the burette liquid hanging in the branch tube, with the conical bottle cavity against the next, followed by washing the bottle down.
c. Terminate the burette by turning off the piston rod as soon as the color developer in the conical bottle marks the endpoint. Wash the bottle to percolate the inner cavity of the conical bottle. Remove the burette, hold the upper part of the tube without liquid with the left hand, make the tube vertical, and read the reading value with the eye flush with the liquid level. When reading the value, estimate the value by one position. When the buret is finished, the remaining aqueous solution in the buret should be discarded, and the buret should be cleaned and clamped on the clip for reservation.
Precaution of burette
a. After filling the burette with the titration solution, the solution on the outer wall of the tube should be dried so as not to cool down the solution inside the tube by running down or evaporating the solution (especially in summer). When holding the buret in hand, avoid holding the wall of the part of the tube with the solution in the palm of your hand, so that the hand temperature is not higher than the room temperature (especially in winter) and the volume of the solution expands (especially when titrating with non-aqueous solutions), causing errors in the reading.
b. When using an acid burette, fix the burette on the burette clamp with the piston handle to the right, extend the left hand from the middle to the right, with the thumb in front of the tube, the index and middle fingers behind the tube, hold the piston handle gently with three fingers parallel to each other, bend the ring and pink the palm of the hand, hold the piston handle end from the bottom to the top with the index and middle fingers, and match the action with the thumb on top. When turning, the middle finger and index finger should not be straight, but slightly bent and gently held to the left. This is easy to operate and prevents the piston from being pushed out.
c. Each titration must start from scale zero so that each measurement offsets the scale error of the burette.
d. After filling the titration solution, the "first reading" of the zero point before titration should be left for 1 to 2 minutes and then read again. If the liquid level reading does not change, still zero, before titration. The titration should not be too fast, 3-4 drops per second are appropriate, and should not become a column of liquid flow down, especially when close to the measurement point, more should be added drop by drop (in the measurement point before the appropriate to speed up some titration). After titration to the endpoint must wait for 1 ~ 2 minutes so that the titrant attached to the inner wall of the flow down after reading, if the speed of the titrant is quite slow, wait half a minute after reading can also be, "final reading" also read at least twice.
e. Buret reading can be clamped vertically on the buret frame or handheld buret upper end to make a free vertical reading scale, reading should also pay attention to the position of the eye and the liquid surface in the same level, otherwise it will cause errors. The reading should be at the lowest point of the lower edge of the curved moon surface, but in case the color of the burette is too dark and the lower edge cannot be observed, the highest point on both sides of the liquid surface can be read, and the "initial reading" and "final reading" should be applied to the Tord.
f. In order to assist in reading, can be lined with a "reading card" behind the buret (coated with a black rectangle of about 4 × 1.5cm white paper) or use a black paper around the buret a circle, pull tight, placed in the liquid surface of the lower scale 1 centimeter (0.1ml) so that the upper edge of the paper before and after a level; at this time, due to the complete disappearance of the reflection, the liquid surface of the curved moon surface is Black, obviously exposed, read the lowest point of the lower edge of this black curved moon surface. When the color of the titrant is deep and the highest point on both sides needs to be read, white paper can be used as the "reading card". If the white back blue line burette is used, its curved moon surface can make the color bar deformation and two meet a point of the tip, you can directly read the scale where the tip is located.
g. There are two kinds of burettes: colorless and brown. Generally, burettes that need to be protected from light (such as silver nitrate titrant, iodine titrant, potassium permanganate titrant, sodium nitrite titrant, bromine titrant, etc.) require a brown burette for comprehensive titration.
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