The presence of alkalinity in a water sample may be due to many different substances. However, for the sake of simplicity, the presence of bicarbonate, carbonate, and hydroxide ions is commonly considered as alkalinity. The points of change in color of phenolphthalein and methyl orange indicators, which occur at pH 8.3 and pH 4.3 provide standard reference points which are almost universally used to express alkalinity. Very high alkalinity values can be undesirable in an industrial water supply. For example, the presence of a high methyl orange alkalinity should be avoided in boiler feed water because of the resultant carbon dioxide content of the steam. Carbon dioxide usually is responsible for the corrosion of steam and return lines. High boiler water alkalinities are also undesirable because the presence of high hydroxide ion concentration is the primary cause of caustic metal embrittlement. A very high boiler alkalinity can also lead to an undesirable carryover condition. On the other hand, the alkalinity of boiler water must be sufficiently high to protect the boiler metal against acidic corrosion and to ensure precipitation of scale forming salts. Usual treatment approaches include setting both a minimum alkalinity level and an operating range. Hydroxide alkalinity may be determined by adding barium chloride prior to titration to precipitate the carbonate ion from solution, allowing direct titration of the hydroxide alkalinity. Measurement of hydroxide alkalinity is also used to control lime soda softeners. In cooling water systems, alkalinity is of major importance since the total alkalinity of a water is one factor that must be considered when predicting the tendency for the water to precipitate calcium carbonate scale. Depending on the choice of chemical treatments to protect against corrosion and scale formation, the operating alkalinity and pH ranges are chosen to balance these two features. 

This test is based on the determination of alkalinity by titration with standard acid to the phenolphthalein color change (or to a pH of 8.3) and to the methyl orange color change (or a pH of 4.3). For determination of hydroxide alkalinity only, barium chloride is added prior to titration to precipitate the carbonate ions. Titration is then taken only to the phenolphthalein end point. A pH meter may be used instead of the indicators to determine the endpoints of the titration.

 

Burette, 25 ml - 1 

Casserole porcelain 210 ml - 1 

Glass stirring rod - 1 

Graduate 50 ml - 1 

Pipet delivery 5 ml - 1 

 

 

Measure 50 ml of sample into a graduated cylinder and transfer to the casserole. Add four or five drops of phenolphthalein indicator. If P alkalinity is present, the sample will turn red. Add  standard 0.02N sulfuric acid from the buret, drop by drop, until the last trace of red color is removed by a single drop. Record volume of titrant added as the P alkalinity reading.

Calculation of Results

FORMULA :

ppm alkalinity (as CaCO3) =ml 0.02N sulfuric acid x1000 /ml sample

(for hydroxide alkalinity substitute ml 0.02N hydrochloric acid for ml 0.02N sulfuric acid) Using a 50 ml sample, the alkalinity becomes ml of acid titrant multiplied by 20.

Limitations of Test Phenolphthalein (P) and Methyl Orange (M) Alkalinity

The alkalinity measured from the titration to phenolphthalein end point or P alkalinity is considered to be equivalent to the sum of all the hydroxide ion present plus one half of the carbonate ion present. The total alkalinity by titration to the methyl orange endpoint or M alkalinity includes the other half of the carbonate ion plus any bicarbonate that was originally present. The presence of phosphate, silica, organic or other buffers may affect the titration and the calculation of the form of alkalinity present may be in error. The presence of bases other than hydroxide, carbon and bicarbonate will interfere directly. Common bases titratable by acids include sulfide (S=), hydrosulfide (HS-), cyanide (CN-) and ammonia (NH3). All these will add to the apparent concentration of one or more of the alkalinities measured. Care should be exercised in interpreting alkalinities found in these water strictly in terms of scaling tendency. However, under normal circumstances in plant control, expression of result only as the P and M alkalinity actually determined is satisfactory and is preferred from the standpoint of simplicity