Mohr pipette (graduated pipette) is a type of graduated laboratory pipette used to measure and transfer variable volumes of liquid accurately. It is a glass (or sometimes plastic) straight tube with graduation marks along the length. It is different from volumetric pipette because it can deliver different volumes as per the required marking.
It is named after Karl Friedrich Mohr. The main identifying feature is that the graduation marks do not continue up to the tip. The graduations stop before the tapered delivery tip starts, and the lower tip portion remains ungraduated.
Mohr pipette is a drain-out type pipette calibrated as “to deliver” (TD). The liquid is allowed to run out by gravity till the required mark is reached. The small amount of liquid left in the ungraduated tip is not counted in the delivered volume, so it should be left as it is and not blown out.
Parts of Mohr Pipette (graduated pipette)
- Suction mouth (upper end or mouthpiece)
It is the plain fire-polished opening at the top of the pipette. It is used to connect rubber bulb or pipette pump for suction. In more than 5 mL pipette, this end may be slightly constricted to hold cotton plug to stop liquid entering suction device. - Pipette body (main tube)
It is the main cylindrical glass/plastic structure of the pipette. It holds the liquid during measuring and transfer. It gives support and strength to the whole pipette. - Graduated stem (shaft or barrel)
It is the elongated straight-walled central part of the pipette. It contains the volumetric scale and is used for taking readings. It also allows controlled flow while handling. - Graduated scale (graduation marks)
These are marked lines on the stem showing volume measurements. In Mohr pipette, the graduation marks stop before the tapered tip starts. So the reading is taken only within this graduated region. - Calibration marks
These are standardized marks showing that the pipette is calibrated for correct volume delivery (TD). It helps in reliable measurement during lab work. - Tapered tip (nozzle)
It is the narrowed lower end from where liquid comes out. It controls the flow rate during draining. In Mohr pipette, this tip part is ungraduated, and any residual liquid left inside it is not included in measured volume, so it is not blown out. - Color coded band (if present)
It is a colored marking used for quick identification of pipette capacity range. It helps when many pipettes are kept together.
How to Use a Mohr Pipette (graduated pipette)
How to use a Mohr pipette (graduated pipette)
- Cleaning and conditioning
The pipette is first checked if it is clean and dry. A small amount of the same liquid is taken and rotated inside to wet the inner wall (conditioning). This wash is discarded. Pipette bulb or pipette pump is used, mouth pipetting is not done. - Filling the pipette
The tip is dipped into the liquid properly, not touching the bottom of container. Air bubbles should not be allowed to enter. The bulb is fitted on the mouth end and liquid is sucked up about 1 cm above the required starting mark. - Holding the liquid level
The bulb is removed quickly. Index finger is placed tightly on the top opening to hold the liquid column. Thumb is avoided because control is less. - Wiping the outside
Outside surface of the pipette is wiped with tissue paper. Tip opening is not touched while wiping. Any drop sticking outside is removed. - Setting the meniscus at starting mark
The pipette is kept vertical and the graduation mark is kept at eye level. Finger pressure is released slowly so liquid drains down. The bottom of meniscus is adjusted exactly on the starting line. Hanging drop is removed by touching tip to inner side of the beaker. - Delivering the liquid
The pipette is brought to receiving vessel. The pipette is kept vertical and the tip is allowed to touch the inner wall of receiver (slanting receiver is done). Liquid is allowed to flow smoothly by slight release of index finger. - Stopping at final mark
Flow is stopped when the bottom of meniscus reaches the required final graduation mark. Finger is pressed firmly to stop exactly at that line. - Leaving residual liquid
The liquid in the ungraduated tapered tip is not drained out. It is not blown out. This residual liquid is not part of measured volume in Mohr pipette and it is left in the tip and discarded later.
Types of Mohr pipette (graduated pipette)
A. By accuracy and material
- Class A
It is a high precision Mohr pipette used for accurate quantitative work. It is commonly made of borosilicate glass and the error is minimum. - Class AS (accuracy special)
It is a high accuracy pipette with swift delivery type. A waiting time is followed after meniscus reaches the mark (about 5 to 15 seconds) for complete draining. - Class B
It is a general purpose Mohr pipette. The allowable error is about twice of Class A, so used for routine lab work and teaching labs. It may be soda-lime glass or plastic type.
B. By delivery style (ISO 835 typology)
- Type 1 (partial delivery)
Zero mark is at the top. The pipette delivers between graduation marks and the full capacity is at the lowest graduation line. - Type 2 (total delivery, zero at top)
Nominal value is taken from the top marking and total delivery is done up to the required graduation. It is used when full capacity is intended to be delivered. - Type 3 (total delivery, zero at bottom)
The pipette is numbered with zero mark at the bottom side. This type is less common in traditional Mohr design.
C. By tip shape
- Genre 1 (standard taper tip)
It has standard taper nozzle. Flow rate is normal and commonly used type. - Genre 2 (long taper tip)
It has a long taper nozzle for controlled draining. It may give smoother delivery in some work.
Uses of Mohr Pipette (graduated pipette)
- It is used to measure and transfer variable volumes of liquids accurately. It delivers liquid between two graduation marks (point to point).
- It is used in titration work to deliver measured amount of titrant or other reagent into a solution. It gives better precision than measuring cylinder.
- It is used for preparing serial dilutions in laboratory. It is useful when repeated accurate volume transfer is needed in dilution series.
- It is used in microbiology and cell culture work for transferring media and other sterile liquids. It is also used to aliquot antibiotics, growth factors, and similar additives in small measured volumes.
- It is used for sample and reagent preparation like buffer solutions and general lab reagents. Required volume can be taken as per graduation.
- It is used in clinical and diagnostic laboratories for measuring and dispensing different liquid volumes during tests.
- It is used in controlled experiments and calibration work where a specific volume delivery is required many times. It helps in maintaining fixed and controlled volume in general chemistry experiments.
Advantages of Mohr pipette (graduated pipette)
- It is used to measure and deliver different volumes from the same pipette. A single pipette can dispense variable amount as per graduation marks.
- It gives point to point delivery, so fractional volumes can be taken easily (example 3.2 mL, 7.8 mL). It is more accurate than measuring cylinder for such work.
- Graduation marks stop before the tapered tip starts, so the measured liquid stays in the straight cylindrical part. This keeps volume reading more linear and predictable.
- Error due to liquid behaviour in the tip is reduced. The tip fluid dynamics and surface tension effect is not counted because residual liquid in nozzle is not included in measured volume.
- It is a drain-out TD pipette, so blow-out is not required. Final drop is not forced out, which makes dispensing simple and avoids extra error due to blowing.
Limitations of Mohr pipette (graduated pipette)
- It is not suitable for all liquids. Highly viscous liquids and liquids having high surface tension are not measured properly by this pipette.
- Accuracy is lower than volumetric pipette. Volumetric pipette is made for single exact volume, but Mohr pipette is generally less accurate for strict work.
- User delivery error can occur easily. Since graduations stop before the tip, the user must stop the flow exactly at final mark. If the liquid is allowed to drain fully into the tip and out, volume becomes more and reading becomes wrong.
- Parallax error is common. Meniscus should be adjusted at eye level, otherwise wrong reading is taken when seen from above or below.
- It is affected by heat from hand. Holding the pipette for long time can transfer body heat, which can change volume due to expansion of liquid or air column and delivery gets altered.
- Graduation limitation is present. Graduation marks do not go up to the tip like serological pipette, so full flexibility is less and some models have graduations only in upper portion.
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