Question: Alkalinity is the name given to the quantitative capacity of an aqueous solution to neutralize an acid, measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from acid rain, or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions, and which ph?
Context: Alkalinity is the name given to the quantitative capacity of an aqueous solution to neutralize an acid.  Measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from rainfall or wastewater.  It is one of the best measures of the sensitivity of the stream to acid inputs.  There can be long-term changes in the alkalinity of streams and rivers in response to human disturbances. <ref name="10.1021/es401046s"> </ref> The compound hydrogen chloride has the chemical formula H Cl and as such is a hydrogen halide.  At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor.  Hydrogen chloride gas and hydrochloric acid are important in technology and industry.  Hydrochloric acid, the aqueous solution of hydrogen chloride, is also commonly given the formula HCl. Pyrophosphoric acid, also known under the name diphosphoric acid, is colorless, odorless, hygroscopic and is soluble in water, diethyl ether, and ethyl alcohol.  The anhydrous acid has two different forms, which melt at 54.3 °C and 71.5 °C.  It is best prepared by ion exchange from sodium pyrophosphate or by reacting hydrogen sulfide with lead pyrophosphate.  When phosphoric acid is dehydrated pyrophosphoric acid is produced as one of the products.  When molten, pyrophosphoric acid rapidly forms an equilibrium mixture of phosphoric acid, pyrophosphoric acid and polyphosphoric acids.  The percentage by weight of pyrophosphoric acid is around 40% and it is difficult to recrystallise from the melt.  In aqueous solution pyrophosphoric acid like all polyphosphoric acids hydrolyses and eventually an equilibrium is established between phosphoric acid pyrophosphoric acid and polyphosphoric acids.  When highly diluted, an aqueous solution of pyrophosphoric acid contains only phosphoric acid. Acid rain is rain or any other form of precipitation that is unusually acidic. Mass deacidification is a term used in Library and Information Science for one possible measure against the degradation of paper in old books (the so-called "slow fires").  The goal of the process is to increase the pH of acidic paper on a large scale.  Although acid-free paper has become more common, a large body of acidic paper still exists in books made after the 1850s because of its cheaper and simpler production methods.  Acidic paper, especially when exposed to light, air pollution, or high relative humidity, yellows and becomes brittle over time.  During mass deacidification an alkaline agent is deposited in the paper to neutralize existing acid and prevent further decay. A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually.  Hence, a pH indicator is a chemical detector for hydronium ions (HO) or hydrogen ions (H) in the Arrhenius model.  Normally, the indicator causes the color of the solution to change depending on the pH. Indicators can also show change in other physical properties; for example, olfactory indicators show change in their odor.  The pH value of a neutral solution is 7.0.  Solutions with a pH value below 7.0 are considered acidic and solutions with pH value above 7.0 are basic (alkaline).  As most naturally occurring organic compounds are weak protolytes, carboxylic acids and amines, pH indicators find many applications in biology and analytical chemistry.  Moreover, pH indicators form one of the three main types of indicator compounds used in chemical analysis.  For the quantitative analysis of metal cations, the use of complexometric indicators is preferred, whereas the third compound class, the redox indicators, are used in titrations involving a redox reaction as the basis of the analysis. In chemistry, pH ( ) (potential of hydrogen) is a numeric scale used to specify the acidity or basicity of an aqueous solution.  It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions.  More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion.  Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic.  Pure water is neutral, at pH 7 (25°C), being neither an acid nor a base.  Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively. Hydrogen iodide (H I ) is a diatomic molecule and hydrogen halide.  Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid.  Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions, whereas the other is an aqueous solution of said gas.  They are interconvertible.  HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent. A glass ionomer cement is a dental restorative material used in dentistry for dental fillings and luting cements.  It is also now commonly used as an orthodontic bracket adhesive, either as a glass ionomer, or a glass ionomer-based cement.  Glass-ionomer based cements are essentially hybrids of glass ionomers and another dental material, for example Resin-Modified Glass Ionomer Cements (RMGICs) and compomers (or modified composites).  These materials are based on the reaction of silicate glass-powder (calciumaluminofluorosilicate glass)and polyalkenoic acid, an ionomer.  Occasionally water is used instead of an acid, altering the properties of the material and its uses.  This reaction produces a powdered cement of glass particles surrounded by matrix of fluoride elements and is known chemically as Glass Polyalkenoate.  There are other forms of similar reactions which can take place, for example, when using an aqueous solution of acrylic/itaconic copolymer with Tartaric acid, this results in a glass-ionomer in liquid form.  An aqueous solution of Maleic acid polymer or maleic/acrylic copolymer with Tartaric acid can also be used to form a glass-ionomer in liquid form.  Tartaric acid plays a significant part in controlling the setting characteristics of the material. Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH).  It can have harmful effects on plants, aquatic animals and infrastructure.  Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids.  Some governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive results.  Nitrogen oxides can also be produced naturally by lightning strikes, and sulfur dioxide is produced by volcanic eruptions.  Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms, causing paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues as well as having impacts on human health.
Answer: low pH