Emphasis on Acids and Bases prepared by
J. Chem. Educ. Staff
Acidity or Basicity in Soils and Natural Waters Most soils, natural waters and many components of naturally occurring materials, including plants and animals, are found to be either slightly acidic or slightly basic. T h e p H of ocean water, for example, is 7.89, that of rain water is about 6.2. Normal arterial blood has a p H between 7.35 and 7.45. About 98.3% of any sample of soil consists of compounds of the eight elements oxygen, silicon, aluminum, iron, calcium, magnesium, potassium, and sodium. The first four elements tend to make soils acidic or only slightly basic; the last four tend to make it distinctly basic. I n general, soils in humid regions are acidic (pH approaching 4.8); those in arid regions are basic (pH 8.3 or higher). Crops like cranberries require acid soils @H -4); others like alfalfa and many clovers require neutral or only slightly acid soils ( p H -4.5). Many rocks, especially the carbonates, sulfates and certain oxides are basic; gases like carbon dioxide and sulfur dioxide are acidic. As a first approximation, sea water might be thought of as .. the product of continuous, massive acid-base reactions. Arid3 from vulvanoes aud f n m gases like CO:,in the air, and bases from components of weathered rocks make contact in the sea and react. Among the reactions involved are: acid-forming reactions1 (from dissolving gases) base-forming reaction (weatheringof limestone)
COz + H20
HzC03 Hf, + HCO3SO2 H20 e H2S03 = Ht, + HS03CaZ+ CaC03+ H+., + HCOa- (a base in water)
+
-
The acidity of rainwater is the result of dissolved gases like COz reacting with water as shown in the first reaction above.
0 Acid-Base Balance in Body Chemistry Normal operation of the human hody requires that hody fluids be maintained a t a constant pH, just on the alkaline side of neutrality in the p H range 7.3-7.45. However, cellular oxidation of foodstuffs produces acids such as carhonic, H2C03, phosphoric, H3P04, sulfuric, H2S04, and lactic, HC3Hs03, acids. For example, a normal individual produces between 10 and 20 moles of carbonic acid per day from the carbon dioxide resulting from oxidation of foodstuffs. These acids pass into the blood. To maintain normal operation, the hlood must receive these acids without a sign~ficantlowering of its pH, and transport rhrm UI points irom which thc.vcan beeliminated by the body. The blood p H is held approximatc~lyconstant, even as acid IS added, by chemical sysienis known as buffers. Key blood buffrrs are the cnrhmic arill-l~icarbonarrion system and hemoglt,hin. Acidanre eliminated by the body at the lungsand H>O, and the whtw carbonic arid denmposes into (:0, ('0,is exuelled-and at the kidnevs where the acids are in ?ffectneutrai~zedand pass out as sodium or potassium salts. I'he sodium and notassium ions lost in this wav must he redaced from the diet. 26 1 Journal of Chemical Education
0 Acid Foods and Basic Foods Components of food containing sodium, potassium, calcium or magnesium ions are said to be base forming and those containing phosphorus, sulfur and chlorine are said to be acid forming. Which of these predominates in the food determines whether the food itself leaves an acidic or a basic residue. Acid residues are left by meat, fish, eggs, cheese and cereals; alkaline residues are left by milk, vegetables and some fruits. Fats and sugars are neutral. Acid-tasting citrus fruits are actually alkali formers. Although they contain both citric acid and sodium citrate, oxidation converts the citric acid and citrate ion to carbon dioxide and water, leaving the alkaline sodium oxide or sodium carbonate. Bender, A.E.,"DietionaryofNutrifionand Food Chsmistry,"Ch~mical PublishingcomPmy, NOWYork, 1976
0 Approximate pH Values of Common Fluids Fluid Gastric juice Lemon juice Vinegar Soft Drinks Blood serum Pancreatic juice Bile
pH
Fluid
PH
1.3 2.3 3.0 3.0 7.4 8.0 7.8-8.6
Grapefruit juice Black coffee Urine Saliva
3.2 5.0 4.8-7.5 6.8-7.2
0 Examples of Acidic, Basic and Neutral Drugs Acidic Drugs
Neutral Drugs
Basic Drugs
Barbiturates Diphenylhydantoin
Caffeine Meprobamate (tranquilizer) Phenacetin Thebromine (cocoa)
Amphetamines Cocaine
Salicylates (aspirin) Sulfanamides
Codeine Morphine
Commercial Preparation of Nitric Acid Nitric acid is prepared commercially by the oxidation of ammonia using a platinum-rhodium catalyst. Equations for the reactions involved are
In this process, ammonia, mixed with air preheated to about 300°C, is passed through a reactor which contains a platinum-rhodium wire-gauze catalyst. The temperature of the Chem I Supplement is a bimonthly feature planned to give teachers of first courses in ehemistrv , additional chemical information that mtght be used in rhsa. pusted un hulletin b u d s , or used rosr~mthw ndditional student invulvrmenr in artivirirs relaled tumem~stry. ~~~~~~
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reactor is about 750°C, the pressure inside is about 6.8 atm. The hot nitric oxide, NO, produced in the reactor and the unreacted air are cooled by passing them through a heat exchanger. Then they are passed, along with more cool air, into a stainless steel absorption tower where conversion of NO to NO2 occurs. Because the equilibrium for this reaction favors the production of NO2 a t lower temperatures, the reactor is
added to the reactor. The NO2 reacts with the water and a solution containing 6145% nitric acid in water is formed. This can he drawn from the bottom of the reactor and further concentrated if desired. Nearly 70% of the nitric acid manufactured is used to make explosives, mostly for the military; about 10%is used in the manufacture of synthetic polymers and about 5% in fertiliz-
Volume 54, Number 10, October 1977 1 627
