This post is devoted to briefly explain how to use the refractometer for the estimation of the concentration in ethanol-water mixtures. In this case, an old model is used for these measurements.
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| Fig. 01 Table top refractometer. This is an old specimen but it still can do the job. |
The refractometer presented in Fig. 01 is for analog measurement so that some calibration may be required as well as good visual skills.
2 Placing the sample in the table top refractometer
1 Parts of the refractometer
Some components of this device are no longer in used but shall be listed here anyway,
- monocular lens for reading including an integrated knob for adjustment if poor sight of the sample is found,
- 2 inlets and 2 outlets for tubing connections. These can still be used to pass a fluid coming from a thermal bath so that the sample to be measured is kept at constant temperature,
- thermowell to monitor the tru temperature of the fluid entering the refractometer,
- a knob for refractive index (nD) determination with a scale inside the apparatus,
- a knob for achromation so that the coloured zone can be well difined,
- a knob working as a lock to access the location of the prism where the sample is to be placed
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| Fig. 02 Left side view of the table top refractometer and its components. |
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| Fig. 03 Right side view of the table top refractometer and its components. |
2 Placing the sample in the table top refractometer
The following steps/comments would help to load the liquid sample in the refractometer. If the sample is not liquid the procedure is different.
- turn the knob to unlock the movable part and expose the lens where the sample is to be placed,
- poor 3-4 droplets on the lens. More liquid would cause a spill. Only a small quantity is enough to form a film over the lens once this is closed again,
- turn the movable part to its initial position slowly. The film shall be formed,
- if several measurements are to be done it is not necessary to lock the compartiment.
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| Fig. 03 A sketch showing how to load the sample droplets into the table top refractometer. |
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| Fig. 04 Sample compartiment. |
Once the sample has been placed on the refractometer, the reading of the refractive index can be done.
3 How to read the refractive index
The measurement of the refractive index is done in the visual field through the monocular of the refractometer. Once the sample has been placed, follow these steps/comments,
- use knob for refractive index measurement to place the sample in the visual field,
- the sample image should appear as dark or coloured shadow in the circle,
- turn this knob to set the image to cover, as exactly as possible, the upper half of the circle until the refractive index line,
- next, use the achromation knob to get the sample image as dark as possible,
- once this is done, you can read the refractive index in the horizontal scaling.
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| Fig. 05 A schematic of the visual field inside the table top refractometer. |
Since this procedure demands a little practice and good eye-sight, you will improve with the time.
4 The ethanol-water mixtures and its refractive index (nD)
If you have read previous sections, you must be familiarized with the use of a table top refractometer. Let us now consider a table of mixtures of ethanol-water and its refractive index (nD) (see below). You can do two things with this,
- if you know the concentration of ethanol in the mixture then you can use the table to easily find its refractive index and
- you can estimate the concentration of the mixture by measuring its refractive index and searching for it in the table.
| Ehtanol concentration % v/v |
Refraction index (nD) |
|---|---|
| 0.00 | 1.3330 |
| 2.00 | 1.3342 |
| 4.00 | 1.3354 |
| 6.00 | 1.3367 |
| 8.00 | 1.3381 |
| 10.00 | 1.3395 |
| 12.00 | 1.3410 |
| 14.00 | 1.3425 |
| 16.00 | 1.3440 |
| 18.00 | 1.3455 |
| 20.00 | 1.3469 |
| 22.00 | 1.3484 |
| 24.00 | 1.3498 |
| 26.00 | 1.3511 |
| 28.00 | 1.3524 |
| 30.00 | 1.3535 |
| 32.00 | 1.3546 |
| 34.00 | 1.3557 |
| 36.00 | 1.3566 |
| 38.00 | 1.3575 |
| 40.00 | 1.3583 |
| 42.00 | 1.3590 |
| 44.00 | 1.3598 |
| 46.00 | 1.3604 |
| 48.00 | 1.3610 |
| 50.00 | 1.3616 |
| 52.00 | 1.3621 |
| 54.00 | 1.3626 |
| 56.00 | 1.3630 |
| 58.00 | 1.3634 |
| 60.00 | 1.3638 |
| 62.00 | 1.3641 |
| 64.00 | 1.3644 |
| 66.00 | 1.3647 |
| 68.00 | 1.3650 |
| 70.00 | 1.3652 |
| 72.00 | 1.3654 |
| 74.00 | 1.3655 |
| 76.00 | 1.3657 |
| 78.00 | 1.3657 |
| 80.00 | 1.3658 |
| 82.00 | 1.3657 |
| 84.00 | 1.3656 |
| 86.00 | 1.3655 |
| 88.00 | 1.3653 |
| 90.00 | 1.3650 |
| 92.00 | 1.3646 |
| 94.00 | 1.3642 |
| 96.00 | 1.3636 |
| 98.00 | 1.3630 |
| 100.00 | 1.3614 |
This table was found on the internet and no detail is known about the refractometer used for its determiantion. However, you will see that your table top refractometer may not be as precised as the data in the table.
This is the end of the post. I hope you find it useful.
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- LE 02 - Start and stop push button installation 24V DC
- LE 03 - Turn on/off an 24V DC pilot light with a push button
- LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
- LE 05 - Emergency stop button installation
- About PID controllers
- Ways to control a process
- About pilot lights
- Solving the Colebrook equation
- Example #01: single stage chemical evaporator
- Example #02: single stage process plant evaporator
- Example #03: single stage chemical evaporator
- Example #04: triple effect chemical evaporator
- Gas absorption - General comments
- Equilibrium diagrams from gas component solubility data
- Distillation - General comments
- LE- Distillation column comments
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Ildebrando.
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