If you have suggestions for additions to this list of frequently asked questions, plese send them to me at sarounds@usgs.gov.
1.1 Where can I get more information?
One good source of information about alkalinity and the procedures to measure it is chapter 6.6 of the USGS National Field Manual, which may be found on-line at http://water.usgs.gov/owq/FieldManual/Chapter6/section6.6/. Additional excellent references for alkalinity concepts include Stumm and Morgan's Aquatic Chemistry and Pankow's Aquatic Chemistry Concepts. Citations for those texts are given in the reference section of the methods page.
1.2 Why is the stand-alone version written in Perl?
Because I like Perl. Also, the Internet version of the Alkalinity Calculator was written in Perl; so by writing the stand-alone version in Perl, I didn't have to change most of the original calculation routines. If I had converted the program to Excel, I would have had to rewrite most of the calculations as Visual Basic macros. While it is true that Excel could have simplified the tasks of graphing and printing results, the disadvantages outweighed the advantages. Besides, if I had written the stand-alone version for Excel, it would have been relegated to the Windows world. By sticking with Perl, the stand-alone version is usable on a variety of operating systems (Windows, linux, unix, Mac?, etc.).
The Alkalinity Calculator uses the sample temperature and conductance at the time of sample titration. The specific conductance is used to estimate the solution's ionic strength. The ionic strength and the sample temperature are used to calculate the acid dissociation constants of water and carbonic acid (Kw, K1, and K2), which are needed for some of the Calculator's methods and diagnostics. If sample temperature and conductance are unknown, values of 20 oC and 50 µS/cm will be assumed.
The Alkalinity Calculator uses two consistency tests to determine whether the sample titration data are affected by a significant amount of non-carbonate alkalinity. The first test compares the location of the measured carbonate equivalence point to its theoretical location. The second test compares the shape of the measured titration curve to a theoretical curve. If either of those tests fails, a warning is issued.
When this occurs, it is likely that something other than hydroxide, carbonate, and bicarbonate were neutralized during the titration. In these cases, it is possible that some significant amount of ammonia, boric acid, silicic acid, or humic acids also were neutralized. If that is true, then the calculated concentrations of carbonate and bicarbonate are not accurate. Their estimated concentrations also account for these other acids that also were neutralized in the titration. You may report these concentrations, but they are best reported as estimates.
2.3 How do I calculate the carbonate speciation if I had to dilute my sample before titration?
If the sample must be diluted before it is titrated, special steps must be taken to calculate the original sample's alkalinity and the concentrations of carbonate and bicarbonate. Other than accounting for the volume change, the calculation of the sample alkalinity is not affected. The alkalinity of the original sample should be equal to:
Alkoriginal = Alkdiluted x Volumediluted / Volumeoriginal
Dilution of the original sample may or may not cause its pH to decrease due to dilution of the sample's alkalinity. The Alkalinity Calculator doesn't know the pH of the undiluted sample; it will report only the speciation of the diluted sample, which could be different from that of the undiluted sample. Therefore, you must calculate the speciation on your own, using the equations of the advanced speciation method. Those equations require the acid dissociation constants (K' values) for water and for bicarbonate. Fortunately, those values are estimated from the sample's temperature and conductance and reported by the Alkalinity Calculator.
2.4 How does the inflection point method handle "ties"?
If more than one pair of data points produces the same maximum change in pH per unit of acid added in the region of an expected endpoint, the endpoint is calculated as the average titrant volume of the first and last such points, and the endpoint pH at that titrant volume is interpolated.
Note that the change in pH is associated with the mean titrant volume between the two data points that produces the pH change. For example, the change in pH between titrant volumes of 40 and 44 counts (or mL) is associated with a titrant volume of 42 counts (or mL). So, a "tie" between two pH changes produced between (a) counts 40 and 42 and (b) counts 42 and 46 is determined by averaging the average of 40 and 42 with the average of 42 and 46, resulting in an assignment of the endpoint to a titrant volume of 42.5 counts (the average of 41 and 44).
2.5 When and why might the simple speciation method fail?
The simple speciation method only works for samples with an initial pH of 9.2 or less. This method is a simplification and assumes that:
Above a pH of 9.2, these assumptions begin to fail. At a high enough pH, this method will start to report negative bicarbonate concentrations, which is an obvious problem. Before getting to that point, however, the reported carbonate and bicarbonate concentrations still can have significant errors. If this method is used for samples with an initial pH of 9.2 or less, testing has shown that the errors should be less than 10 percent, or less than 1 mg/L. See USGS Office of Water Quality Technical Memorandum 2012.05 for more information and a description of the error analysis.
For samples with pHs greater than 8.3, the simple speciation method requires that both the carbonate and bicarbonate equivalence points be determined (both A and B must be known). If the carbonate equivalence point is not detected (A is undefined), then the simple speciation method fails and the carbonate and bicarbonate concentrations are not calculated.
Because of the problems mentioned above, the simple speciation method is now deprecated and no longer used by the Alkalinity Calculator. The advanced speciation method is used instead.
This is a common occurrence for samples with pHs between 8.0 and 8.3. The simple speciation method does not take the chemistry of carbonic acid into account. It simply assumes that no carbonate is present if the titration does not detect a carbonate equivalence point. The chemistry of carbonic acid, however, tells us that a small amount of carbonate will be present even at pH values a bit less than 8.3. The advanced speciation method takes this into account and reports a more accurate carbonate concentration.
Bear in mind, however, that the amount of alkalinity assigned to carbonate in these sorts of cases typically is a small fraction of the total sample alkalinity. So, whether the carbonate is reported as zero or some small concentration probably makes little difference.
2.7 Should I always use the advanced speciation method?
Testing has shown that the simple speciation method should work relatively well for most samples with a pH of less than 9.2. Many samples, though, will exhibit some small amount of disagreement between the results of the simple and advanced speciation methods, even when the sample pH is less than 9.2. So, given that the advanced speciation method has fewer shortcomings than the simple speciation method, should the advanced speciation method be used exclusively? Generally, the answer is "yes." See USGS Office of Water Quality Technical Memorandum 2012.05 for more information.
The "pH < 9.2" guidance criteria for use of the simple speciation method was based on an analysis of theoretical titration curves from fictitious samples, comparing the results of the two speciation methods. The pH 9.2 guidance criteria does not take into account any obfuscating factors in real titrations. For example, a real-world sample may have some small amount of ammonia or silicic acid or humic acid which gets titrated in the upper pH range of the titration. This acid neutralizing capacity by something other than carbonate will cause the discrepancy between the simple and advanced speciation methods to increase.
This discrepancy occurs because all acid added above the carbonate endpoint is assumed by the simple speciation method to neutralize only carbonate. Shifting the carbonate endpoint due to the presence of other acid neutralizing species will introduce an artifact into the simple speciation method's estimate of the carbonate concentration. The advanced speciation method doesn't use the carbonate endpoint except as a check; instead it uses the sample pH and the calculated alkalinity to determine the bicarbonate and carbonate concentrations directly from the theoretical chemistry of carbonic acid.
The point is that real-world samples may have other acid-neutralizing materials that cause our calculated concentrations of bicarbonate and carbonate, by either speciation method, to be mere estimates rather than true values. Because the two speciation methods use different assumptions and methods to arrive at those concentrations, the presence of these other bases can cause the "pH < 9.2" criteria for use of the simple speciation method to be insufficient to keep its estimates consistent with the advanced speciation method.
A discrepancy between the two speciation methods for a sample with pH < 9.2 may be due to several things, but the most likely is that something other than hydroxide, carbonate, and bicarbonate got titrated. Remember, though, that the simple speciation method doesn't account for hydroxide, which means that it is guaranteed to fail at high pH (> 9.2). At a high enough pH, the simple speciation method will give negative bicarbonate concentrations, which, of course, is impossible.
In the end, there's no one answer that fits all situations. However, the advanced speciation method is the recommended method and now is the only speciation method used by USGS and the Alkalinity Calculator. Diagnostic tests in the Alkalinity Calculator allow the detection of problems that might indicate the presence of non-carbonate species with acid neutralizing properties. It is the best method available and therefore should be used whenever possible.
2.8 Why would I ever want to use the fixed endpoint method?
USGS policy is to avoid the use of fixed endpoint titrations in the determination of alkalinity. Actual endpoints are sufficiently variable from sample to sample that the use of a fixed endpoint introduces an unacceptable amount of error in the results.
So, why does the Alkalinity Calculator offer a fixed endpoint analysis method? The answer is simple. It is available just in case all of the other analysis methods fail, or fail to find the correct endpoint. For example, one can envision a situation where the inflection point method finds what clearly is the "wrong" endpoint due to some noise in the data. When all else fails, the user can determine where the endpoint should be, by looking at slope changes the titration data, then reanalyze the data by specifying that endpoint with the fixed endpoint analysis method.
2.9 Can alkalinity or ANC be negative?
Yes. Alkalinity and ANC are just a measure of the deficiency of H+ ions relative to a solution of carbon dioxide in water. A negative alkalinity or ANC simply reflects the presence of some amount of mineral acidity, or an abundance of H+ ions relative to a solution of carbon dioxide in water. Alkalinity and ANC may be positive, zero, or negative. It is instructive to note that concentrations of hydroxide, carbonate, and bicarbonate ions, unlike alkalinity and ANC, cannot be negative. Of the various titration analysis methods available in the Alkalinity Calculator, only the Gran function plot method is able to detect and report negative values of alkalinity or ANC.
3.1 Why are alkalinity titrations best done in the field at the time of sample collection?
The USGS recommends that alkalinity be analyzed in the field as soon as possible after sample collection. This is good practice because it minimizes or eliminates several potential sources of error:
3.2 How high must my sample pH be before I look for a carbonate equivalence point?
The USGS National Field Manual suggests that your titration may have a carbonate equivalence point if the sample pH is greater than 8.1. Previous versions of the Manual used a pH of 8.3 instead. Either one is a good recommendation. The carbonate equivalence point normally occurs in the vicinity of pH 8.3. To get the most out of your titration, titrate slowly in the vicinity of the carbonate equivalence point. The guidance of pH > 8.1 is good advice, but may not apply to all samples. Your experience is your best guide.
3.3 At what pH does the bicarbonate equivalence point normally occur?
The bicarbonate equivalence point for most environmental water samples occurs near a pH of 4.5, plus or minus about 0.5. However, the equivalence point can occur outside this range. For example, a sample with a low alkalinity and a low concentration of inorganic carbon (carbonate plus bicarbonate plus carbonic acid) from a pristine headwater stream can easily have a bicarbonate equivalence point above pH 5.0. Your best guide is experience; be careful not to overshoot the endpoint by titrating too quickly. If you do overshoot the endpoint, just keep going well beyond the endpoint and analyze your data with Gran's method.
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