TROLL 9500 Operator's Manual - Geotech Environmental Equipment

Multi-ParameterWater Quality TROLL ®11 pHWhat Is pH?The term pH is derived from “p” meaning power and “H” for the elementhydrogen and literally means “power of hydrogen.” pH is definedas the negative logarithm of the hydrogen ion activity (or concentrationin moles/liter):pH = - log [ H + ] or [H + ] = 10 -pHWater (H 2O) dissociates into hydrogen ions (H + ) and hydroxide ions(OH – ) in aqueous solution. At 25°C there are 1.0 x 10 –7 moles/L ofhydrogen ions and 1.0 x 10 –7 moles/L of hydroxide ions in pure water.Thus the water is neutral (pH = 7) because there are equal amountsof each ion. Addition of a substance with hydrogen or hydroxide ionswill shift the balance and cause the water to become either acidic orbasic.The pH scale ranges from 0 (most acidic) to 14 (most basic or leastacidic). A change of 1 pH unit corresponds to a tenfold change inhydrogen ion concentration.Why Measure pH?A pH value indicates the amount of hydrogen ion that is present in anaqueous environment. The hydrogen ion concentration gives an indicationof the acidity of a substance. pH is an important measurementin natural waters because most chemical and biochemical processesare pH dependent. The physiological chemistry of most living organismscan tolerate only small changes in pH and still provide the chemicalreactions that sustain life. The solubility of many chemicals is pHdependent. Thus, pH determines their availability to living organisms.Typical pH valuesFluidpH unitsAcid rain < 5Distilled water 5.6Most natural waters 8Safe for freshwater fish 6-9Properly chlorinatedswimming pool 7.2 - 7.6Natural waters usually have pH values in the range of 4 to 9. Mostnatural waters are slightly basic (~ pH 8) because of the presence ofcarbonates (CO 32–) and bicarbonates (HCO 3–). Extremely fresh watercan even be slightly acidic (~ pH 6), depending on the concentrationof dissolved carbon dioxide (CO 2). The carbon dioxide combines withwater to form a small amount of carbonic acid (H 2CO 3) and this processlowers the pH. Nitrogen oxides (NO x) and sulfur dioxides (SO 2)from automobile exhaust and the burning of coal combine with waterin the atmosphere to form nitric (HNO 3) and sulfuric acid (H 2SO 4).This falls to the ground as acid rain and accumulates in surface water.Runoff from mining spoils and the decomposition of plant materialscan also cause acidic surface water.pH values below 5 in natural waters are generally considered to betoo acidic. Freshwater fish seem to do well in the pH range of 6 to 9.Acidic drinking water is a concern because of its corrosive characteristicsto plumbing and appliances. pH also affects the ammonia/ammonium(NH 3/NH 4+) equilibrium in water. Even a small amount ofammonia is detrimental to fish while a moderate amount of ammoniumis tolerated. At a pH of 6.5 almost all ammonia is in the form ofammonium. However, as the pH becomes slightly basic, ammonium ischanged into harmful ammonia. The lethal dose of ammonia for troutis only 0.2 mg/L.The pH SensorpH electrodes use a potentiometric method to measure the pH ofa solution. The pH sensor consists of a pH-sensitive glass whosevoltage is proportional to the hydrogen ion concentration. A secondsensor (electrode) serves as a reference, which supplies a constantstable output. Electrical contact is made with the solution using asaturated potassium chloride (KCl) solution. The electrode behavior isdescribed by the Nernst equation:E m= E o+ (2.3 RT/nF) log [H + ]whereE mis the potential from the pH electrode,TROLL 9500 Operator’s Manual660095110 rev. 007 01/09