Techniques to Determine the Density of DI Water and Seawater

The density of seawater plays an important role in the ecology of the marine environment. Seawater is not just water, because it contains a lot of chemicals packed into it. As water flows from land to sea it transports large amounts of conservative elements(1). Seawater density varies from place to place because it’s affected by salinity and temperature(2). The purpose of this experiment was testing the skills of accuracy and precision while using different analytical techniques to determine the density of DI water and seawater; and also determine the salinity of seawater.

There are four different analytical techniques used to determine the density of DI water and seawater; which are repeater pipette and analytical balance; B.O.D bottle and top-loading balance; density meter (or, densitometer); and conductivity meter. The repeater pipette was used for repeatedly dispensing the same volumes multiple times into multiple receptacles without having to aspire in between each dispensing step, thus increasing speed and efficiency. Positive displacement syringe tips help ensure accuracy and help prevent bubble formation and foaming in detergent containing liquids. Analytical balance is one of the most useful balances that have been found in many laboratories. 

The balance was able to detect smaller variations in mass than their higher capacity counterpart, with a readability range between 0.1mg-0.01mg. “Biological Oxygen Demand” bottle used for incubating diluted sample. The bottle was specially designed shoulder radius that sweeps all air from inside the bottle during filling. The interchangeable stoppers have a tapered bottom that also prevents air entrapment. The bottles have a flared mouth to form a water seal around the stopper that prevents air from being drawn into the bottle during incubation.

Top-loading balance was used to measure objects with a larger mass, but has less accuracy compared to analytical balance because it can’t eliminated the air error from the surrounding. Density meter was used to determine the density of the liquid. This analytical techniques was holding a fixed volume of liquid in an oscillating quartz u-tube capillary. The oscillation frequency of the u-tube depends upon the mass of the liquid within it. A conductivity meter was used to measure the amount of electrical current or conductance in a solution at a specific temperature. This instrument can determine the salinity of the seawater following calibration of the conductivity meter with a reference sample.

The combi tip then had to be refilled using the 20 mL of DI water that was measured at the start of the experiment. The DI water successfully filled the syringe with no air bubbles and the first 0.400 μL of DI water was released into a waste beaker. This syringe lasted for the next 10, 0.400 μL, increments of DI water. The same process was taken, recording the mass after every 0.400 μL of water was injected into the capped plastic bottle. The same process was repeated a second time but instead with the use of Seawater. 

By filling a 30 mL beaker with Seawater, this allowed for the repeating pipette to be rinsed twice with Seawater, then putting the Seawater that rinsed the syringe into a waste beaker. This ensured no DI water was left behind the syringe capsule, avoiding cross contamination of DI water and Seawater. About 20 mL of Seawater was put into a 30mL beaker and along with the temperature being taken, using a digital thermometer of tolerance (±0.06oC), from this the first syringe of the repeating pipette was filled. This time around there were no air bubbles and only the first 0.400 μL of Seawater was put into waste. 

This syringe of Seawater lasted for the first 11 increments being squeezed into a new capped plastic bottle, mass of 11.1334g,until it was replaced from the same 20 mL of Seawater. This syringe was the perfect amount for the next 9, 0.400 μL, increments. Like before the mass was taken after each increment and calculations were done in order to determine the mass of all 20 trials. The temperature was taken after both 20 trials were done, using a digital thermometer of tolerance (±0.06oC), for the DI and seawater. This concludes the first part of the experiment.