pH and Alkalinity for Lakes

**For the Mass. Department of Environmental Protection-approved SOP, get the pdf file for lakes or for rivers**

Background Information

pH is a measure of the hydrogen ion concentration of the water as ranked on a scale of 1.0 to 14.0. The lower the pH of water, the more acidic it is. The higher the pH of water, the more basic, or alkaline, it is. pH affects many chemical and biological processes in the water and different organisms have different ranges of pH within which they flourish. The largest variety of aquatic animals prefer a pH range of 6.5 - 8.0. pH outside of this range reduces the diversity in the stream because it stresses the physiological systems of most organisms and can reduce reproduction. Low pH can also allow toxic elements and compounds such as heavy metals to become mobile and "available" for uptake by aquatic plants and animals. Again, this can produce conditions that are toxic to aquatic life, particularly to sensitive species like trout.

Changes in acidity can be caused by atmospheric deposition (acid rain or acid shock from snowmelt), surrounding rock, and wastewater discharges. Technically, the pH scale measures the logarithmic concentration of hydrogen (H+) and hydroxide (OH-) ions, which make up water (H+ + OH- = H20). When both types of ions are in equal concentration, the pH is 7.0 or neutral. Below 7.0, the water is acidic (there are more hydrogen ions than hydroxide ions). When the pH is above 7.0, the water is alkaline, or basic (there are more hydroxide ions than hydrogen ions). Since the scale is logarithmic, a drop in the pH by 1.0 unit is a 10-fold increase in acidity. So, a water sample with a pH of 5.0 is ten times as acidic as one with a pH of 6.0. pH 4.0 is 100 times as acidic as pH 6.0.

Alkalinity is a measure of a river's "buffering capacity," or its ability to neutralize acids. Alkaline compounds in the water such as bicarbonates (baking soda is one type), carbonates, and hydroxides remove H+ ions and lower the acidity of the water (which means increased pH). They do this usually by combining with the H+ ions to make new compounds. Without this acid neutralizing capacity, any acid added to a river would cause an immediate change in the pH. Measuring alkalinity is important to determining a river's ability to neutralize acidic pollution (as measured by pH) from rainfall or snowmelt. It's one of the best measures of the sensitivity of the river to acid inputs. Alkalinity comes from rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges. Total alkalinity is measured by collecting a water sample, and measuring the amount of acid needed to bring the sample to a pH of 4.2. At this pH all the alkaline compounds in the sample are "used up." The result is reported as milligrams per liter (mg/l) of calcium carbonate.

The Massachusetts Acid Rain Monitoring Project ranks waters according to their alkalinity as follows:

0* mg/l: Acidified 5-10 mg/l: Highly Sensitive
0-2 mg/l: Critical 10-20 mg/l: Sensitive
2-5 mg/l: Endangered 20 mg/l: Not Sensitive

*and pH less than 5.0.

Sampling Method for Lakes

Field Equipment

  • High Density Polyethylene sample bottle,500 ml
  • Field data sheet
  • Cooler and Koolits or ice
  • Zip-loc bag (1 gallon size)

Method

  • Rinse a 500 ml bottle (including cap) three times with surface water at the sampling site. Be sure to empty your rinse water away from your sampling location.
  • Lower open sample bottle upside down to arm's length.
  • Underwater, turn bottle upside right and let it fill up.
  • When no more bubbles come up, cap the bottle underwater and bring back up to the surface.
  • Put sample in cooler for delivery to lab
  • On your lake field sheet , record sample ID and write 'pH/ANC' in the Chemistry column
    Holding time is 8 hours for pH and 14 days for alkalinity.

Note: Put enough ice or equivalent in cooler to reduce sample temperature to 4° C as soon as possible.

Analysis Method

pH Measurement

Equipment required:

  • pH-Meter
  • Buffers (4.01 and 7.00)
  • Deionized or distilled water
  • 150ml Glass Beaker
  • Magnetic Stirrer
  • Stir Bar
  • 100ml Graduated Cylinder (optional for pH measurement)

After calibrating your meter with the buffers, rinse the electrode(s) and glassware with distilled or deionized water. Carefully measure 100 ml of your sample and place in a 150 ml beaker for the pH and alkalinity part. Place the rinsed electrode in the test sample. We strongly encourage letting all samples come to room temperature in the tightly capped bottle before analyzing. If you are conducting other analyses with the sample water, keep in mind that pH should be analyzed within 5 minutes of uncapping the sample bottle. The sample should be stirred very gently, preferably with a magnetic stirrer. It may take up to 3 minutes for the reading to become stable. When stable, but not in excess of 5 minutes, record the sample pH to the nearest 0.01 pH unit

Alkalinity Measurement

Equipment required:

  • pH-Meter
  • Refillable Electrode
  • Buffers (4.01 and 7.00)
  • Deionized or distilled water
  • 150ml Glass Beaker
  • Magnetic Stirrer
  • Stir Bar
  • 100ml Graduated Cylinder
  • Digital Titrator
  • 0.16N Sulfuric Acid Cartridge

After placing the sulfuric acid cartridge in position in the Hach Digital Titrator, be sure to advance the plunger manually until titrant is forced out of the delivery tip. Do this as you would a hypodermic syringe, with the delivery tip up to remove bubbles. Get all the bubbles out! Then advance the plunger using the delivery knob on the end of the titrator until you are sure that the delivery tip is filled with solution. Check for leaks where the tip connects to the cartridge. Rinse the tip WELL with distilled water or sample; this is important because the titrant is concentrated and a little bit goes a long way. Reset the counter to zero and you are ready to titrate.

After completing a titration and recording the digits of titrant used, rinse the delivery tip with distilled water or the next sample, reset the counter (THIS IS EASILY FORGOTTEN WHEN BUSY), and you are immediately ready for the next sample.

Titrations go better if the delivery tip is positioned under the surface of the solution being titrated. For one or two samples, the titrator can be held in the hand, however, it is easier to mount the titrator on a ring stand using a clamp. Try to keep the titrator vertical through all titrations; putting the titrator horizontally on the bench between titrations may introduce bubbles in the tip.

The acid cartridges provided are 0.16N sulfuric acid. Our waters are typically quite low in alkalinity, so we use a special double end-point alkalinity procedure to accurately measure alkalinity below 20 mg L-1.

After reading and recording the pH as described above, titrate with the digital titrator and sulfuric acid cartridge to pH 4.5; record titrant used to this point as A. Continue the titration to pH 4.2. Record the titrant used to this point as B. If the initial pH is less than 4.5, record the initial pH value. Titrate until the pH is 0.3 units below the starting point. Enter the digits of titrant used as B; A = 0. Write down the pH reading where you stopped (as an accuracy check). We will use computers to calculate the alkalinity, but you may do your own calculations using the formulas below. The examples will help to clarify what can be somewhat confusing formulas.

A = digits used to pH 4.5
B = digits used to pH 4.2 or 0.3 pH units below initial value (total titrant including A)
Double end-point alkalinity= (2A - B) x 0.1

EXAMPLE: A sample required 120 digits to reach pH 4.5. An additional 15 digits were required to reach pH 4.2, for a total of 135 digits. Therefore, A = 120 and B = 135.
Double end-point alkalinity = (240 - 135) x 0.1 = 10.5 mg/l

EXAMPLE: A sample had an initial pH of 4.3. The sample required 22 digits to lower the pH to 4.0. Therefore, A = 0; B = 22.
Double end-point alkalinity = (0 - 22) x 0.1 = -2.2 mg/l

Although the negative alkalinity value may seem not to make much sense, it is an extremely important measurement for assessment of acidification.

pH Electrode Care and Maintenance:

General electrode care and handling procedures are very important in your lab because pH measurements will only be as good as the condition of your electrode(s). For greater accuracy in your measurements and longer electrode life, there are a few areas of electrode care with which you should be familiar.

Storage

Glass combination or separate pH and reference electrodes should be kept wet. The reference electrode requires a free-flowing junction, so be sure to maintain the reference filling solution at a level significantly above the storage or sample solution level at all times This will provide a positive head pressure, which forces the filling solution out through the junction rather than the storage solution into the probe.

For dry storage, the sleeve or plug should cover the filling hole to reduce the flow of filling solution. During the measurement or storage in pH 4 buffer, however, this sleeve or plug must be slid away or removed to allow flow of the reference solution into the sample.

To obtain a faster electrode response, the glass electrode should be stored in a slightly acidic solution. In the protective cap for the glass electrode, put a drop or two of pH 4 buffer and put the cap on the electrode, carefully. Distilled water extracts ions from the bulb causing a slower response; pH 7 buffer over a long time period ages the electrode slightly.

If using a separate reference electrode, the best solution would be to place the reference electrode in its own filling solution but this can be messy. Providing KCl to both sides of the junction keeps it flowing freer. To reduce the salt crust of saturated solution, an approximately 0.1 M KCl solution may be used, but for storage only. Experience indicates that simply covering the filling hole with the protective sleeve and storing dry suffices in most instances as long as the soaking procedure is followed.

For combination electrodes, store the electrode in a combined solution of approximately 0.1 M KCl in pH 4 buffer.

One day or more prior to analysis, soak both electrodes in pH 4 buffer and, during analysis, place the electrodes in the same buffer when not in use.

Reference Electrode Filling Solution

Read the instructions that came with your electrodes carefully. Saturated calomel reference electrodes such as those used by the Acid Rain Monitoring Project must not be filled with filling solutions containing silver chloride (AgCl). We use 4M KCl solutions only. However, the most common filling solution for combination electrodes is 4 M KCl saturated with AgCl. Be sure to ascertain which filling solution is correct for your electrode(s) and double check that your filling solution matches these requirements.

Permanently filled or Gel electrodes Due to their unique micropore junction, it is recommended that they be stored hanging dry.

Preliminary Electrode Response Testing

If your electrode exhibits slow response, poor span between two buffer values or undue sensitivity to movement of the electrode, rejuvenation may be necessary to improve performance.

Response varies with the electrode and the solution it is in. Generally working electrodes reach 0.05 pH units of the final reading in buffer within 10 seconds. A stable reading (less than 0.01 pH units per minute change) should be reached in fresh water samples within a minute or two. If you have to wait too long (5 minutes or more) then the pH itself may change due to the contact of the water sample with air.

Electrodes may also require adjusting the slope to values significantly different from 100% for two point calibration. Perform the following test if in doubt:

Set your meter to 100% slope and room temperature, then standardize as usual with pH 7 buffer. Without moving the slope dial, read a pH 4 buffer. It should read between 3.85 and 4.15; set the slope to read pH 4, the slope should be 95% to 105%.

If your electrode exhibits either of the above problems or is sensitive to movement, rejuvenation is in order.

Glass Electrode Rejuvenation

To treat the bulb of the pH electrode:

Use 1 bottle each of acid and base (0.1N).

BE CAREFUL WHEN HANDLING THESE SOLUTIONS. IF YOU GET ANY ON YOU RINSE OFF WITH LOTS OF WATER.

  • Dip the electrode bulb into the acid and wait until meter reads pH ~ 1
  • Rinse electrode, then dip into the base. Wait until meter reads pH ~ 13
  • Repeat this rinsing and dipping cycle several times (at least 3 times, 6 times are better)
  • For the last cycle, you can leave electrode in base 5 min, then in acid briefly until it reaches pH ~ 1
  • Then rinse the electrode under tap water and let sit in pH 4 buffer for 2 hours
  • Rinse electrodes and re-calibrate meter as you normally do with pH 7 and pH 4 buffers.

To treat the reference electrode:

Replace the 4M KCl solution in the reference electrode and get rid of crystals that may have formed. If there are lots of crystals, then shake out the solution and put deionized pure water into the filling hole and soak the electrode tip in hot tap water for 15 minutes or so until the crystals have dissolved. Then shake all the liquid out of the filling hole in the reference electrode and refill with fresh 4 M KCl. Let the electrode sit at room temperature for ½ hour before use. Frequently add more 4M KCl solution to the reference electrode since it will continually leak out and evaporate. The solution in the electrode should be within ½ inch of the filling hole. The hole should be open when reading pH but close it when you are through for the day or else the solution will evaporate and new crystals will form (but do not close the hole if you will be storing the electrode soaking in pH 4 solution). If you still have problems with slow response, try rubbing the tip on your blue jeans or on very fine (600 grit) sandpaper.

For combination electrodes, do both treatments described above.

Final Test For Linearity

Standardize the meter as described below. Rinse the electrodes and your sample cup with pure deionized water. Then titrate 100.0 ml of deionized water with your 0.16N acid as follows: Make sure your digital titrator is working and reset to zero. Add 10 digits of acid, record digits and pH, increase acid to 20 digits, record pH; repeat until you have added 100 digits of acid and stop. Send the results to us and we will send you a report. If you want to see the results yourself, try plotting the hydrogen ion concentration (H = 10(-pH)) vs. digits and see if the line is straight.

Movement Sensitivity

If your meter gives wild readings and is sensitive to your touch, it may not be properly grounded. Try using a three prong power plug or attach a wire from the meter to a cold water pipe. Sometimes a problem of fluctuating readings or consistency wrong readings can be solved by disconnecting and reconnecting the electrode connectors several times. Apparently an oxide layer can sometimes cause these symptoms.

Calibration

The pH meter should be standardized (calibrated) prior to sample analyses and after every 25 sample analyses. Buffers should be at room temperature (68°F). Remove the electrodes from the pH 4 buffer solution where they have been soaking for at least one day. Rinse with deionized water. Insert the electrodes in pH 7.00 buffer and adjust the calibration dial until exactly pH 7.00 shows on the meter. Remove the electrodes and rinse with deionized water. Place the electrodes in pH 4.01 buffer and adjust the slope until the meter shows pH 4.01. Rinse with deionized water.

A note on buffers. The accuracy of your pH measurement is in direct relation to the accuracy of the standard buffer solution used to calibrate your pH meter. In order to maintain a reasonable degree of accuracy when making a pH measurement, a number of precautions concerning the care and use of buffers should be observed. These include:

  1. Do not use buffers after their expiration date. Mold growth, CO2 absorption and contamination cause changes in the buffer pH.
  2. Do not use buffers which have mold growth floating in the buffer.
  3. Always cap the buffer container when storing to prevent contamination and reduce CO2 pickup.
  4. pH buffer values change with temperature. Be sure to measure the temperature of the buffer and look up its value at that temperature before standardizing the meter (see below).
  5. Do not pour used buffer back into the bottle.

Buffer Values at Various Temperatures

Temperature Buffers
°C °F pH 4 pH 7
0 32 4.003 7.119
5 41 3.998 7.086
10 50 3.996 7.058
15 59 3.996 7.035
20 68 3.999 7.015
25 77 4.004 7.000
30 86 4.011 6.998

pH Alkalinity QA/QC

Quality control for pH and alkalinity consists of normal pH measurement and titration of a sample prepared by the WRRC and sent to you prior to field collection. There will be three of these samples. Several days prior to sampling, you will receive the first QA/QC sample from us, along with a postcard for reporting your results. This is a diagnostic sample. Follow the procedures described for pH and alkalinity measurement. Analyze two separate aliquots of this sample and report your results to us on the postcard. You will be called if we find a significant discrepancy between what we expect and what you measured. We will work with you to troubleshoot the problem so that you are confident of quality analysis for the field samples. Two other QA/QC samples will arrive just before field sampling. Unlike the first QA/QC sample, these are used to document data quality by helping us to statistically define the accuracy and precision of your analyses. Analyze two separate aliquots of one of these immediately prior to measuring pH and alkalinity on field samples; analyze two separate aliquots of the second QA/QC sample immediately after analyzing the field samples. In other words, the first two samples analyzed should be from one of the QA/QC bottles, then analyze the field samples, and finally analyze two samples from the other QA/QC bottle. Results should be reported on the pH alkalinity lab data sheet.