Lab 3:

Alkalinity


Reading Assignment

Read Chapter 2 in Simplified Procedures for Water Examination.


Introduction

Alkalinity is a measure of the capacity of a solution to neutralize a strong acid.  We can calculate the amount of alkalinity in water by titrating the sample water with an acid, using an indicator to determine when the sample water's pH has dropped to a certain level - the endpoint

Titration does not tell us the actual types of alkalinity present in the water (though we can make an educated guess, as discussed below.)  So we cannot report our results as ppm of carbonate, bicarbonate, etc.  Instead, we report the amount of alkalinity as calcium carbonate equivalent, which is the amount of alkalinity in terms of its equivalent value of calcium carbonate.  This is the same unit of measurement we will use to measure hardness, as you will learn in a later lab. 

The pH of water determines what type(s) of alkalinity can be found in the water.  By performing two different alkalinity tests with different endpoint pH values, we are able to estimate the proportions of carbonate, bicarbonate, and hydroxide alkalinity in the water.  The two tests also help us determine whether the water is corrosive and whether it is caustic.  This page will present the lab procedures for testing both total and phenolphthalein alkalinity and for using these test results to calculate all of the factors mentioned above. 




Equipment



Reagents

The reagents needed include the indicator and the acid used for the titration.  The acid is N/50 sulfuric acid, which is the same as 0.02 N sulfuric acid.  The indicator for the total alkalinity test will be:

And for the phenolphthalein alkalinity test, you will need the following indicator:
In either test, a pH meter can be used instead of an indicator, if so desired.  If  using a pH meter, do not add the indicator in step 3 of the procedure below.  In step 4, the acid is added not until a color change occurs but until the pH reaches the desired endpoint - 4.5 for the total alkalinity test or 8.3 for the phenolphthalein alkalinity test. 




Total Alkalinity Procedure
  1. Clean the burette and fill almost to the top with N/50 sulfuric acid.  Then run some acid to waste until the "zero" mark is reached.  This should leave the stopcock and tip of the burette full of the solution. 

  2. Measure out 100 mL of the water to be tested and pour into a clean white porcelain evaporating dish.

  3. With a dropping bottle, add 2 or 3 drops of methyl orange or methyl purple indicator to the sample and stir.  When alkalinity is present, the solution becomes yellow when methyl orange is added or becomes green when methyl purple is added.  (Note:  When a high chlorine residual is present, the chlorine bleaches the color and makes the determination of endpoint difficult.  The chlorine may be removed using sodium thiosulfate, or an additional indicator may be added.)

  4. Slowly and carefully add N/50 sulfuric acid from the burette to the contents of the dish until the faintest pink coloration appears - that is, until the color of the solution is no longer yellow.  While adding the acid, the solution should be gently stirred with the stirring rod.  It is often advantageous to set up two 100 mL samples, adding methyl orange to each and acid to only one while the other is held alongside so that the colors may be compared and the color change to pink can be better recognized. (Note: When using methyl purple, the color is changed to purple.  A gray tint precedes the end point and warns the operator of its approach.)

  5. Record the volume of sulfuric acid used to reach the endpoint.

  6. Calculate the total alkalinity, as follows:
T. alk. = (mL of acid) × (10 ppm/mL)

For example, if 1.5 mL of acid were used in the titration, then the total alkalinity would be:

T. alk. = (1.5) × (10) = 15 ppm CaCO3

(Note that this simple formula requires that you use the exact acid concentration and sample volume listed in this procedure.  If you use different values, you must calculate the alkalinity as follows:

calculating alkalinity

This more complicated equation can also be used to calculate phenolphthalein alkalinity.)


Phenolphthalein Alkalinity Procedure

  1. Clean the burette and fill almost to the top with N/50 sulfuric acid.  Then run some acid to waste until the "zero" mark is reached.  This should leave the stopcock and tip of the burette full of the solution. 

  2. Measure out 100 mL of the water to be tested and pour into a clean white porcelain evaporating dish.  Stir the sample. 

  3. With a dropping bottle, add 5 drops of phenolphthalein indicator to the sample and stir.  When phenolphthalein alkalinity is present, the solution becomes pink.  No color indicates the phenolphthalein alkalinity is zero and that the test is complete.  No color also indicates that free carbon dioxide is present, and the same sample may be used to test for carbon dioxide.

  4. If pink color results after the indicator is added, slowly and carefully add N/50 sulfuric acid from the burette to the contents of the dish until the coloration disappears.  While adding the acid, the solution should be gently stirred with the stirring rod. 

  5. Record the volume of sulfuric acid used to reach the endpoint.

  6. Calculate the phenolphthalein alkalinity, in ppm, as follows:
P. alk. = (mL of acid) × (10 ppm/mL)

For example, if 2.4 mL of acid were used, the phenolphthalein alkalinity would be:

P. alk. = (2.4) × (10) = 24 ppm CaCO3





Corrosive Water

lkalinity is important in the water treatment plant because it promotes coagulation and inhibits corrosion.  Here, we will be concerned with the amount of alkalinity which must be present in the finished water to prevent corrosion in the pipes of the distribution system. 

Corrosive water results from low alkalinity, which tends to promote acidic water.  Since coagulation uses up alkalinity, operators must sometimes add lime or soda ash to water to prevent corrosive finished water.  However, there is no set amount of alkalinity which must be present in water to prevent corrosion.  Instead, the relation between alkalinity and pH is usually the determining factor as to whether or not the finished water will be corrosive.  The following three requirements must be met in order for treated water to be non-corrosive:

1. The phenolphthalein alkalinity, calculated as CaCO3, should not be greater than:

15 ppm + (0.4 × T. alk.)

This requirement limits the permissible pH to about 10.6 at 25°C (77°F).

2. The normal carbonate alkalinity should not exceed 120 ppm.  This requirement may be met by keeping the total alkalinity within the limits suggested in the table below.  These values apply to water at 25°C (77°F).  
 

Limits of alkalinity for various pH ranges

pH Range
Limit for Total Alkalinity 
(ppm as CaCO3)
8.0 to 9.6 
400 
9.7 
340 
9.8 
300 
9.9 
260 
10.0 
230 
10.1 
210 
10.2 
160 
10.3 
180 
10.4 
170 
10.5 to 10.6 
160 


3. The total alkalinity should not exceed the hardness by more than 35 ppm (calculated at CaCO3).  (You will learn to measure hardness in the next lab.)

 

Does your water meet the first two criteria for being non-corrosive?  You will need to test the pH of the water to determine whether your water meets the second criteria.  Record your answer in the Data section.




Caustic Water

Another problem related to alkalinity is caustic water, which causes a burning sensation when it is tasted.  Caustic water is caused hydroxide alkalinity.  Perform the following calculations to determine whether your water is caustic, then record the result in the Data section.

1. Calculate:

C = (0.5) × (T. alk.)


2. Is phenolphthalein alkalinity greater than C?  If so, the water is caustic. 



Proportions of Alkalinity Sources

The primary causes of alkalinity in water are carbonate and bicarbonate, with hydroxide present in smaller amounts.  Once you measure both total and phenolphthalein alkalinity, you can calculate the approximate amount of these three types of alkalinity using the table below:

When...
Hydroxide
Carbonate
Bicarbonate
P = O
0
0
T
2P <  T
0
2P
T - 2P
2P = T
0
2P
0
2P > T
2P - T
2(T - P) 
0
P  = T
T
0
We

We'll consider a couple of examples to help you use the table above.  In the first example, our phenolphthalein test showed no alkalinity (P=0) and the total alkalinity is 80 ppm as CaCO3 (T=80.)  As we can see from the table, when phenolphthalein alkalinity is 0, all alkalinity is bicarbonate.  As a result, our water contains 80 ppm bicarbonate alkalinity and 0 ppm hydroxide and carbonate alkalinity. 

Let's assume that P=50 ppm and T=80 ppm.  In this case, 2P is more than T, so we read the fourth row of the table.  We can calculate the components of the alkalinity as follows:

Hydroxide:
2P - T
2(50) - 80
20 ppm
Carbonate:
2(T - P)
2(80 - 50)
60 ppm

Bicarbonate:

0

So we can see that, in this second example, 20 ppm of the alkalinity results from hydroxide and 60 ppm of the alkalinity results from carbonate.  There is no bicarbonate alkalinity present in the sample. 

It can sometimes be important to understand the various alkalinity components of water because they can act differently.  For example, hydroxide is the cause of caustic water.  You should calculate the amount of each type of alkalinity in your sample water and record the results of your calculations in the Data section. 


 

Data


Water Source
mL of acid used
Alkalinity (ppm as CaCO3)
Total alkalinity test



Phenolphthalein alkalinity test




1. What is the pH of the water? ______________________

2. Does the water meet the first two criteria for being non-corrosive? _________________________

3. Is the water caustic? ____________________

4. How much of the alkalinity is hydroxide? ________________

5. How much of the alkalinity is carbonate? __________________

6. How much of the alkalinity is bicarbonate? ___________________




Assignment

The procedures outlined on this page are manual titration procedures.  You should also view the virtual lab which shows how to test alkalinity using a titrator.  There is an assignment that needs to be completed concerning the virtual lab, so please print the assignment first and then answer the questions as you perform the lab. Once you have the assignment completed, log in and complete the assignment online to be entered directly into the database.



Source

American Public Health Association, American Water Works Association, and Water Environment Federation.  1998.  Standard Methods for the Examination of Water and Wastewater.  American Public Health Association, Washington, D.C.