June 3, 2023
automatic polarimeter

What factors are related to the degree of rotation of the automatic polarimeter?

  1. The optical rotation of a matter is associated with temperature. Any temperature increase causes a substance’s optical rotation to increase, such as quartz, etc. However, the optical rotation of certain substances, such as sucrose decreases as the temperature increases. Therefore, the temperature must be carefully noted and monitored. 
  2. The degree of rotation is associated with the wavelength of plane-polarized light. The optical rotation is different for various wavelengths. 
  3. It is related to optically active substances, and different optically active substances have different degrees of optical rotation.
  4. An association exists with the optical path length of the polarized light passing through the material. The optical path determines the degree of the angle that the polarization plane rotates. Therefore, the sample length must be specified to compare the optical rotation of substances. 

What Factors Affect the Measurement Results of the Automatic Temperature Control Polarimeter? 

Temperature error: 

Generally, the polarimeter has a high temperature, and it takes some time for the sample and polarimeter temperature to reach equilibrium. The test method often makes the sample slightly turbid after an extended period, influencing the light transmittance. Large batches of samples result in measurement errors. It should be corrected when the sample temperature deviates from the specified 20 ℃ ± 0.2 ℃.

Temperature variation may also be the reason behind the association or dissociation between the substance molecules to be tested. Consequently, the optical rotation will change. Therefore, the temperature must be maintained constantly to avoid any problems. 

The ambient temperature should be maintained at 20°C while the instrument works. Furthermore, the area should be cool, dry, and weakly lit but well-ventilated. If the temperature rise is too high, various components’ parameters will change, affecting the measurement accuracy. To decrease its heat generation, the power should be shut off when the measurement period exceeds 30 minutes.

Concentration Errors: 

Certain experimental conditions result in a proportional relationship between optical active substances and concentration. In such cases, the specific optical rotation is considered constant. The specific optical rotation is not precisely constant and optical rotation proportional to the length of the optical rotation tube because the relationship between the optical rotation and the solution concentration is not linear. Three different lengths, i.e., 10cm, 20cm, and 22cm, are available in polarimeters; however, the most widely used is 10cm. A 20cm or 22cm length can be used for solutions with weak or dilute optically rotation ability to ensure accuracy. 

The length error of the polarimeter significantly influences the measurement results and should meet the requirements of 0.01–0.2% of the nominal dimensional accuracy. The optical rotation tube is filled with distilled water or blank solvent after the verification for zero-position correction.

Solvent error: 

The optical rotation of optically active substances is heavily based on the structure of the substance. Additionally, it may also be associated with the thickness of the material, the wavelength, and the temperature of the light. For solutions, the influencing factor is concentration, followed by the solvent. Therefore, the optical rotation of optically active substances is usually different under different conditions. 

Things to Know Before Using a Polarimeter:

  1. The sample and polarimeter must be placed in a constant temperature chamber with a temperature of 20℃±0.5℃ before starting the measurement. A constant temperature bath can be used as an alternative. 
  2. Closely inspect the room for any foreign objects or the position of the sodium light source before turning on the machine. Check if the indicator switch is in the off position and whether the instrument is placed in a suitable position. 
  3. The normal starting time for a sodium lamp is 20 min to reach stability. A DC power supply should be used for the sodium lamp to ensure stability. If there is a polarity switch, the polarity should be changed frequently after shutdown to prolong the service life of the sodium lamp.
  4. To move the analyzer away from the optical zero position to the left or right, repeatedly press the remeasure switch when the polarimeter is set to zero. The repeatability and stability of the instrument can be checked by observing the stopping points of the left and right retests. 
  5.  The measuring tube filled with distilled water or blank solvent is to be placed in the sample chamber. Any bubbles in the measuring tube should float on the convex neck first. Dry the glass at both ends of the clear surface with a soft cloth. Additionally, the position and direction of the measuring tube should be fixed. 
  6.  The specific and optical rotation directions are bound to change with different solvents employed at different pH values or under multiple conditions. To avoid this, a specified solvent is recommended. 
  7. The solution should undergo centrifugation or filtration if it’s cloudy or contains small particles. Some material solutions whose optical rotation changes significantly after exposure to light should be protected from light. 
  8.  When measuring the test solution’s blank zero point or stop point, the readings should be taken three times, with the average result employed for calculation. The zero point should be corrected with a blank solvent before each determination for strict determination. The reagent should determine whether the zero point has changed following the determination.
  9. The measurement tube must be cleaned and washed properly after the measurement process is complete. It should be placed in a dry, covered space to avoid dust buildup. 

How to use a polarimeter?

  1. Connect the polarimeter to the 220V AC power supply and turn on the power switch. The sodium lamp can work after 5 minutes.
  2. Ensure the accuracy of the polarimeter’s zero position when the polarimeter is not placed in a test tube or filled with distilled water. Observe whether the brightness of the field of view is consistent at zero degrees. Inconsistency indicates zero error therefore, the deviation value should be subtracted or added to the measurement reading. Choose a test tube of appropriate length and fill it with the tester liquid. Install a rubber ring and screw on the nut until no water leaks. The protective glass will stress if the nut is tightened too much, which will impact the accuracy of the reading. Wipe off any residual solution to minimize accuracy deviation. 
  3. For evaluation of optical rotation reading, the dial requires turning the analyzer and finding a spot with the same brightness in the field of vision. A right-handed substance is a positive reading, whereas a left-handed chemical is a negative reading.
  4. Using the double Vernier reading method, the result can be obtained according to the following formula: Q=(A+B)/2
  5. Optical rotation and temperature are correlated. For most substances measured with λ=5893A° (sodium light), when the temperature increases by one °C, the optical rotation decreases by about 0.3%


What is the difference between sodium lamps and LED light sources for an automatic polarimeter?

Generally, the basic principle involves all-optical rotational measurement at a constant wavelength. The wavelength of the sodium lamp in the air is 589.3nm. The LED lamp can reach the wavelength of the sodium lamp with the help of a special filter. The life of sodium lamps generally ranges from 50 to 200 hours, while the life of LED lamps is generally at least 5,000 hours. Additionally, LED lights are cheaper than sodium lamps which is why LEDs have replaced conventional sodium lamps. 

Why is temperature control necessary for automatic polarimeter testing?

As discussed previously, temperature and optical rotation share a direct relationship. Therefore, all sample testings have a particular temperature requirement. However, Users who don’t have high standards for results and are willing to overlook the data divergence caused by temperature can purchase a polarimeter without temperature control. Specific substances, like sugar, include a relatively full temperature coefficient adjustment table. Other tests will have strict requirements on temperature. 

What metrics should I consider when buying a polarimeter?

The two critical metrics for the instrument are accuracy and repeatability. The instrument’s sensitivity may be reflected in the sample transmittance. Samples having a transmittance of 1% are often considered to be relatively sensitive. The temperature control accuracy is particularly crucial when purchasing a temperature-controlled polarimeter. 

What are the benefits of automatic calibration of polarimeters?

The instrument contains an automatic calibration function that enables users to continuously monitor its performance and timely change the test accuracy of the instrument to maintain optimal performance throughout.

What measurement modes does the polarimeter have?

There are mainly four modes: 

  • Optical rotation, 
  • Specific rotation, 
  • Concentration 
  • Sugar content. 

Several instruments launched by our company have the function of switching these four modes at will, which saves the user the tedious calculation again.

How often does a polarimeter with its calibration function need to be calibrated?

You can decide whether to calibrate once a week or once a month, depending on the instrument’s use and accuracy. Users can also select a quartz standard polarimeter close to the test angle to calibrate the instrument daily if they are focused on measuring a certain angle.