Sunday, May 31, 2020

Pickled Lime Brine; or, how salty was the ocean in 1868?

In the midst of preparing to brine my 3 tests of pickled limes that are soaked in saltwater, I decided I wanted the salinity of my brine to roughly resemble seawater, since that's what my research suggests limes would have been preserved in on their way to Boston. Which is about when the rabbit hole opened up next to me and I jumped on in.


One of the first articles I saw when I went looking for information on seawater was that seawater composition shifts over time, and a lot of those shifts are due to environmental changes (hello again, global warming). So then I wondered: how much has seawater changed since the mid-19th century?

It turns out, there's actually a wealth of information on ocean temperature, salinity, etc. at multiple depths. Scientists wrote and published about both the makeup of the earth and the ocean throughout the middle of the century, but the records of ocean information that continue to today began in roughly 1870 when the H.M.S. Challenger set sail.

image of the ship, via the Smithsonian Library

Considered the start of modern oceanography, the Challenger voyage from 1872-1876 was specifically intended to collect data on features of the ocean--the first mission of its kind. During the five year voyage, the crew of the Challenger collected data on ocean currents, sea floor topography, marine life, and components of the water (e.g. temperature, chemical composition) at various depths. Over its 4-year voyage, the 6 scientists aboard discovered over 4,000 new species and many new elements of the ocean floor--including the Marianas Trench.

The chemical laboratory on board HMS Challenger.
the laboratory aboard the H.M.S. Challenger, via Dive and Discover
Most helpful to me, the team eventually published their findings: the 50-volume, 29,500-page report was finally finished 23 years after the expedition. Part of what took so long was that the scientific members of the crew brought many samples back to England, and over the time after their return worked with experts on a variety of additional research to better understand what they had collected.

As you might imagine, there's a lot of data in the Challenger report. And much of it has been digitized, which is really cool! In fact, NOAA (the National Oceanic and Atmospheric Administration) has documentation of ocean data from 1870 through today, including for the Challenger expedition. But as you also might imagine, not being a chemist it was kind of difficult to just find the one number I wanted.

Sir C. Wyville Thomson | Scottish naturalist | Britannica
Sir C. Wyville Thompson, one of the zoologists aboard the Challenger and authors of the Challenger Report on ocean chemistry
After a lot of searching, I landed on two helpful sources: The EarthIts Physical Condition and Most Remarkable Phenomena by W. Mullinger Higgins (published 1858); and Report on the Scientific Results of the Voyage of  H.M.S. Challenger during the years 1873-1876, Physics and Chemistry - vol 1 (published 1884). The latter had a lot of discussed on how the amount of salt in the water was determined (letting the water evaporate and weighing the leftover salt was the main method while at sea!), many inscrutable charts of chemical breakdowns of various samples, and some description of average salt content per kilo of water. With that last part, I was able to determine that a ration of 34.751 grams of salt to 1 kilo of water was generally what I wanted. From the former, I learned that northern oceans were generally considered to have a higher salinity than southern oceans (within a narrow range). Since the pickled limes being imported to Boston during the 1860s were coming from the West Indies, a slightly lower salinity would make sense. According to Higgins, northern ocean salinity ranged from 3.27%-3.91%. Since my 34 grams per kilo falls within that range (and vaguely on the lower side), I felt pretty confident going with it.

One last internet search to figure out how many cups of water and salt I would need for that ratio (and a little rounding later), I settled on 4.5 cups of water to 2 tablespoons of salt. And I was off!

Little Women - PART ONE: CHAPTER SEVEN - Amy's Valley of Humiliation
illustration for Little Women chapter 7: Amy's Valley of Humiliation

Higgins, 1858
The Challenger Report in full
Physics and Chemisty, vol 1
Salinity at Ocean Sciences
Early Determination of Salinity
NOAA Historical Data Index
The Challenger Expedition, Dive and Discover
H.M.S. Challenger: Humanity's First Real Glimpse of the Deep Oceans, Discover Magazine
Then and Now: Oceanic Expeditions, NOAA

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