Digital Refractometer for Seawater Analysis
The HI96822 Digital Refractometer eliminates the uncertainty associated with mechanical refractometers by measuring the refractive index to determine the salinity of natural and artificial seawater including brackish, ocean water or saltwater aquarium. The instrument converts the refractive index of a sample to PSU, ppt and specific gravity. These conversions are based on respected scientific publications using the physical properties of seawater (not sodium chloride). The HI96822 is an easy to use tool for measuring salinity in natural and artificial seawater whether in the field, lab or at home.
- Designed for seawater salinity analysis (not NaCl)
- Automatic Temperature Compensation
- High accuracy measurements displayed as PSU, ppt and specific gravity
Making a Standard Solution
Sodium Chloride solutions can be used to check the accuracy of the meter. The table below lists two Sodium Chloride solutions and their expected ppt Seawater value.
To make a Standard NaCl Solution (g/100 g), follow the procedure below
- Place container (such as a glass vial or dropper bottle that has a cover) on an analytical balance.
- Tare the balance.
- To make an X NaCl solution weigh out X grams of high purity dried Sodium Chloride (CAS #: 7647-14-5: MW 58.44) directly into the container.
- Add distilled or deionized water to the beaker so the total weight of the solution is 100g.
Example Standard NaCl Solution
| NaCl (g) | Water (g) | Total | Expected Seawater value (ppt) | |
| 3.5% NaCl | 3.50 | 96.50 | 100.000 | 34 |
| 10% NaCl | 10.00 | 90.00 | 100.000 | 96 |
Refractive Index
The HI96822 takes measurements based on a sample's refractive index. Refractive index is a measurement of how light behaves as it passes through the sample. Depending on the sample's composition, the light will refract and reflect differently. By measuring this activity with a linear image sensor, the sample's refractive index can be assessed and used to determine its physical properties such as concentration and density. In addition to the linear image sensor, the HI96822 uses an LED light, prism and lens to make measurement possible.
Variations in temperature will affect the accuracy of refractometry readings, so the use of temperature compensation is highly recommended for reliable results. The HI96822 contains a built-in temperature sensor and is programmed specialized algorithms then apply temperature compensation to the measurement and convert the refractive index to PSU (Practical Salinity Units), ppt (part per thousand) or S.G. (Specific Gravity) (20/20). PSU is defined as the conductivity ratio of seawater to a standard KCl solution. It is based upon the work of the UNESCO, ICES, SCOR, and IAPSO. This information is published in The Joint Panel of Oceanographic Tables and Standards. An older salinity scale is ppt (10-3), where salinity is defined by "the salt content is the weight of the inorganic salts contained in 1 kg of seawater if all bromide and iodide are replaced by an equivalent amount of oxidesin. (Knudsen, 1901). Specific Gravity (20/20) is based upon the published relationship between density at 20 °C and the mass of dissolved salts in the seawater sample (CRC Handbook of Chemistry and Physics, 87th Edition).
Features-at-a-glance
- Calibrate with distilled or deionized water
- Sample size can be as small as 2 metric drops (100 ?l)
- Readings are displayed in approximately 1.5 seconds
- The dual-level LCD displays measurement and temperature readings simultaneously
- Battery percent level remaining at startup and low battery indicator
- To conserve battery life the meter shuts off automatically after three minutes of non-use
- Water resistant ABS plastic casing designed to perform under laboratory and field conditions.
- Easy to clean and corrosion-resistant
- Samples automatically compensated for temperature variations
