Temperature Dependence of the pH of pure Water - Chemistry LibreTexts
Acid-Base. Relationships at Higher Temperatures. L. F. AUDRIETH and THERALD MOELLER. Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois. Pure water has a pH level of 7, but this changes with fluctuations in temperature. However, pure water is always considered a neutral. So whatever affects the [H+] of a given solution, pH of that soultion will also experience an inverse change. Now talking of corelation of pH and temperature, .
In addition, pH levels outside of 6. An minor increase in pH levels can cause a oligotrophic rich in dissolved oxygen lake to become eutrophic lacking dissolved oxygen. Even minor pH changes can have long-term effects.
Acid-temperature relationship | Physics Forums
In an oligotrophic lake, or a lake low in plant nutrients and high in dissolved oxygen levels, this can cause a chain reaction.
With more accessible nutrients, aquatic plants and algae thrive, increasing the demand for dissolved oxygen. This creates a eutrophic lake, rich in nutrients and plant life but low in dissolved oxygen concentrations. Factors that Influence the pH of Water There are many factors that can affect pH in water, both natural and man-made.
Most natural changes occur due to interactions with surrounding rock particularly carbonate forms and other materials. In addition, CO2 concentrations can influence pH levels. Carbon Dioxide and pH pH levels can fluctuate daily due to photosynthesis and respiration in the water. The degree of change depends on the alkalinity of the water.
Photosynthesis, respiration and decomposition all contribute to pH fluctuations due to their influences on CO2 levels. This influence is more measurable in bodies of water with high rates of respiration and decomposition. While carbon dioxide exists in water in a dissolved state like oxygenit can also react with water to form carbonic acid: However, this equation can operate in both directions depending on the current pH level, working as its own buffering system.
However, as CO2 levels increase around the world, the amount of dissolved CO2 also increases, and the equation will be carried out from left to right. This increases H2CO3, which decreases pH. The effect is becoming more evident in oceanic pH studies over time. Total change in annual oceanic pH levels from s to s. World Ocean Atlas ; photo credit: Plumbago; Wikipedia Commons Carbon dioxide in the atmosphere decreases the pH of precipitation.
The above equations also explain why rain has a pH of approximately 5. As raindrops fall through the air, they interact with carbon dioxide molecules in the atmosphere. A pH level of 5. Natural, unpolluted rain or snow is expected to have pH levels near 5. Acid rain requires a pH below 5. Natural pH Influences Carbonate materials and limestone are two elements that can buffer pH changes in water. When carbonate minerals are present in the soil, the buffering capacity alkalinity of water is increased, keeping the pH of water close to neutral even when acids or bases are added.
Additional carbonate materials beyond this can make neutral water slightly basic. Limestone quarries have higher pH levels due to the carbonate materials in the stone. Lightning can lower the pH of rain. As mentioned earlier, unpolluted rain is slightly acidic pH of 5.
pH of Water
If rain falls on a poorly buffered water source, it can decrease the pH of nearby water through runoff. Decomposing pine needles can decrease pH. Anthropogenic causes of pH fluctuations are usually related to pollution. Acid rain is one of the best known examples of human influence on the pH of water. Any form of precipitation with a pH level less than 5. This precipitation comes from the reaction of water with nitrogen oxides, sulfur oxides and other acidic compounds, lowering its already slightly acidic pH.
These chemicals can come from agricultural runoff, wastewater discharge or industrial runoff. Wastewater discharge that contains detergents and soap-based products can cause a water source to become too basic.
Typical pH Levels Recommended minimum pH levels for aquatic life. Typical pH levels vary due to environmental influences, particularly alkalinity.
The alkalinity of water varies due to the presence of dissolved salts and carbonates, as well as the mineral composition of the surrounding soil. The recommended pH range for most fish is between 6. Oceanic organisms like clownfish and coral require higher pH levels.
Sensitive freshwater species such as salmon prefer pH levels between 7.
Environmental Considerations Natural precipitation, both rain and snow, has a pH near 5. Most grasses and legumes prefer soils with a pH of 4.
The acidity of the surrounding environment can also affect the pH of water. This is most obvious near mining areas, but the effect can also occur naturally. This may be tolerable for some aquatic species such as frogs but not for most fish. Some frogs and other amphibians can often tolerate pH levels as low as 4.
That is why angel fish and discus from the Amazon River Basin can thrive quite happily in waters with a pH as low as 5. Seawater has a pH around 8. In deeper lakes where stratification layering occurs, the pH of water is generally higher 7.
Some states, such as Alaska, are attempting to maintain a pH standard for water quality. Stratification can cause pH levels within a body of water to differ above and below the cline. These layers are separated by clines, known as thermoclines temperature divides or chemoclines chemistry gradients.
Chemoclines can be based on oxygen, salinity, or other chemical factors that do not cross the cline, such as carbon dioxide. Differences in pH levels between water strata are due to increased CO2 from respiration and decomposition below the thermocline. In crater lakes such as Lake Nyos or Lake Monoun, the pH rapidly drops from a surface level around 7 to 5.
This significant drop comes from the saturated CO2 that is stored up in the lower strata of the lake. Adaptability While ideal pH levels for fish are fish blood has a pH of 7. A dramatic fluctuation is considered a shift in pH of 1. For saltwater fish, the pH of water should remain between 7. In both cases, hydrostatic pressures allowed water to remain liquid at such cold temperatures Salt water, however, has a lower freezing point.
That is why salt is used in winter to de-ice roads and sidewalks. Average seawater has a salinity level of 35 PPT parts per thousandwhich shifts the freezing point to Ice floats on top of the denser water.
That does not sound like a large difference, but it is enough to keep ice floating on top of water and allows aquatic organisms to survive the winter. This drop in density occurs because the hydrogen bonds in water create an open hexagonal lattice, leaving space between the molecules Ice formed in seawater is even less dense than freshwater ice When saltwater begins to freeze, the water molecules begin to form a crystal lattice just like they do in freshwater.
These crystals only include water molecules, not salt ions, and the formation is known as brine exclusion As the ice structure grows, pockets of concentrated saltwater can be trapped inside the ice, but are not incorporated into its structure.
The trapped water can eventually drain, leaving a small air bubble in the ice. The air bubbles left behind reduce ice density significantly — down to 0. Multiyear ice in Antarctica is fresher than new sea ice. Older ice structures, called multiyear ice, have no brine left and are fresh enough to drink once melted The lake mixes every spring and fall, realigning the temperature throughout the lake. The thermocline exists at different depths depending on the season.
Stratification is the division of a water column into strata, or layers, of water with different properties. These divisions are usually defined by temperature and density, though other parameters such as salinity and chemical distinctions can also be used Stratification occurs because work force and displacement is required to mix liquids of different densities As seasons progress, sunlight, wind, ambient temperature and ice in winter cause the lake to restratify Lake stratification — the different layers are separated by thermoclines, or temperature gradients.
When referring to temperature and density strata within a lake, the layers are usually called the epilimnion, metalimnion and hypolimnion from top to bottom The upper layer, the epilimnion, is exposed to solar radiation and thermal contact with the atmosphere, keeping it warmer.
The epilimnion will extend as far as sunlight and wind will allow, and is usually deeper in lakes with greater surface areas Below the epilimnion is a layer of water with a rapidly changing temperature range known as the metalimnion The metalimnion serves as the boundary between the upper and lower layers of water. The temperature in this strata can vary greatly between its top and bottom depths In addition, the metalimnion can fluctuate in thickness and depth due to weather conditions and seasonal changes The metalimnion is bordered on both top and bottom by an edge called the thermocline.
The thermocline is defined as the plane of maximum temperature decrease In other words, when the water temperature begins to significantly drop, the thermocline has been crossed.
As temperature and density are related, a second cline, known as a pycnocline exists at the same depths. The pycnocline divides water column strata by density Below the second thermocline and pycnocline is the hypolimnion. This strata is usually too deep to be affected by wind, solar radiation and atmospheric heat exchanges The temperature of the hypolimnion is usually determined by the spring turnover. This temperature may only change minimally, if at all, while stratified Lakes that completely mix at least once per year are known as holomictic lakes There are six types of holomictic lakes, with definitions based on average temperature and how frequently temperatures align These lakes and their dividing factors can be seen in this flow diagram: Lakes that do not mix completely are called meromictic lakes These lakes have a lower strata that remains isolated throughout the year.
This bottom layer is known as the monimolimnion, and is usually divided from the collective layers above it mixolimnion by a halocline salinity-based cline Meromictic conditions can occur in a holomictic lake when unusual weather conditions cause the lake to stratify before it has time to completely mix Pressure and Water Temperature Points Pressure does not directly alter water temperature.
Instead, it shifts the freezing, boiling and maximum density points. The temperature at which boiling and freezing occur will only hold true at sea level 3. Pressure can change the boiling point of water. This is due to the effect of atmospheric pressure.
At a lower pressure higher altitudewater will boil at a lower temperature. On the other side of the scale, at higher pressures such as in a pressure cookerwater will boil at a higher temperature Atmospheric pressure does not affect the temperature of the water itself, but only its ability to become vapor, thus shifting the boiling to the left or right. As the hydrostatic pressure increases, the freezing point lowers At high elevations lower pressurethere is a slight increase in the freezing point, but the change in pressure is not enough to significantly affect the point What Factors Influence Water Temperature?
Water temperature can be affected by many ambient conditions. Shallow and surface waters are more easily influenced by these factors than deep water Sunlight The greatest source of heat transfer to water temperature is from sunlight Sunlight, or solar radiation, is a form of thermal energy The result is a daily fluctuation in water temperature based on the amount of sunlight received by the water.
Solar radiation is the greatest influence on water temperature. If a body of water is deep enough to stratify, sunlight will only transfer heat through the photic zone light-reaching. Most of this energy greater than half is absorbed in the first 2 m of the water This energy will continue to be absorbed exponentially until the light is gone.
Water dissociation and pH
The photic zone varies in depth but can be up to m deep in the oceans The depth of the photic zone is based on the amount of solids and other light-scattering elements present in the water. The temperature of water below the photic zone is generally only altered when the water is mixed Thus shallower bodies of water tend to warm quicker and reach higher temperatures than deeper water bodies 1.
Atmosphere Rivers can appear to steam in winter when colder air flows over the warmer water. As heat always flows from a higher temperature to a lower temperature, this transfer can go both ways 6. When the air is cold, warm water will transfer energy to the air and cool off.
If the air is hot, cold water will receive the energy and warm up. The extent of this transfer is based on the thermal inertia and specific heat of water Water temperature fluctuations are more gradual than air temperature fluctuations Turbidity Turbidity monitoring during the Passaic river dredge project.
Turbidity can increase water temperatures. Increased turbidity will also increase water temperature. Turbidity is the amount of suspended solids in water. These suspended particles absorb heat from solar radiation more efficiently than water The heat is then transferred from the particles to water molecules, increasing the temperature of the surrounding water Confluence As the river flows into the lake, it can affect the temperature of the water.
Roberto Araya Barckhahn via Wikimedia Commons Groundwater, streams and rivers can alter the temperature of the body of water into which they flow.
If a spring or groundwater source is colder than the river it flows into, the river will become cooler. Recalling the rules of heat transfer energy flows from hot to coldthe river loses energy to the cooler water as it warms it up 6.
If the inflow is large or fast enough, the equilibrium temperature of the water will be close to the temperature of the inflow 1. Glacial fed streams will keep conjoining rivers cooler near the source of the flow than further downstream 1. Man-made Influences Thermal pollution from municipal and industrial effluents can negatively affect water quality.
Vmenkov via Wikimedia Commons Man-made influences on water temperature include thermal pollution, runoff, deforestation and impoundments. Thermal Pollution Thermal pollution is any discharge that will dramatically alter the temperature of a natural water source This pollution commonly comes from municipal or industrial effluents 1.
If the temperature of discharge is significantly warmer than the natural water, it can negatively affect water quality. There are several significant consequences of thermal pollution, including diminished dissolved oxygen levels, fish kills and influxes of invasive species Runoff from parking lots and other impervious surfaces are another form of thermal pollution.
Water that flows off of these surfaces absorb much of their heat and transfer it to a nearby stream or river, elevating the temperature 9.
Deforestation It is not only manmade additions that can affect water temperature. Water that is shaded by vegetation and other objects will not absorb as much heat as sunlit water When trees or riparian canopies are removed, a body of water can become unusually warm, altering its natural cycle and habitats Impoundments The McKenzie dam altered the water temperature pattern downstream, affecting fish behaviors, particularly reproduction.
Impoundments such as dams can drastically affect water temperature cycles. While a dam does not directly contribute heat to the water, it can affect the natural patterns of water temperature warming and cooling 9. An operational dam without a sliding gate assembly can alter the water temperatures downstream of the dam, which can affect local fish population behaviors.
Shifting the temperature pattern can affect the migration, spawning and hatching of local fish species 9. The temperature pattern will shift if the reservoir stratifies and the dam release is too high or too low, releasing unusually cool or unusually warm water into the stream 9.
Typical Temperatures Seasonal temperature fluctuations across the US. Typical temperatures are dependent on 1 type of water body 2 depth 3 season 4 latitude 5 surrounding environment. Even a specific body of water can vary due to any of these sources; a lake might freeze over one winter, but it might not freeze the following year due to a warm winter.
It follows the same warming cooling pattern both years, but it does not reach the same temperatures. Rivers and streams tend to experience greater, more rapid and temperature fluctuations than lakes and oceans Likewise broad, shallow lakes will be warmer than their deeper counterparts.
Due to the shifting angle of solar radiation and the effects of atmospheric heat transfer, water temperatures will vary seasonally As solar radiation is more intense near the equator, water at lower latitudes wills to be warmer than water at higher latitudes Shaded streams will not be as influenced by solar radiation as their exposed counterparts and can remain cooler.
Bodies of water that are influenced by groundwater flow or a glacial fed stream will also be cooler 1. Oceanic temperatures also vary by season, latitude, depth, ocean currents and convection Surface water will vary more with season and latitude than deeper waters, and show diurnal daily fluctuations due to solar radiation and wind This diurnal variation can be as great as 6 degrees Celsius Due to its massive size and the high specific heat of water, the ocean has an equally large heat capacity This means that fluctuations between seasons or due to unusual events will only have a slight impact Studies have shown the ocean has warmed approximately 0.
While this number seems small, it is quite considerable in regards to the size of the ocean. Sea-Surface Temperatures in December, Hurricanes, cyclones, thunderstorms and other weather events can form depending on the temperature of the ocean Monsoons can occur when there is a large temperature differential between land and sea, causing cyclical precipitation and storms Hurricanes and cyclones develop over warm water, where the heat can be rapidly transferred to the air via convection In a similar vein, lake-effect snow and other heavy precipitation conditions can form when cold air flows over a large, warmer body of water This warming in turn affects weather and temperature patterns across the globe