Determination of Acidity of Water
Acidity of a water is its quantitative capacity to react with a strong base to a designated pH. The measured value may vary significantly with the end-point pH used in the determination. Acidity is a measure of an aggregate property of water and can be interpreted in terms of specific substances only when the chemical composition of the sample is known. Strong mineral acids, weak acids such as carbonic and acetic, and hydrolysing salts such as iron or aluminium sulphates may contribute to the measured acidity according to the method of determination.
Acids contribute to corrosiveness and influence chemical reaction rates, chemical speciation, and biological processes. The measurement also reflects a change in the quality of the source water.
Titration Method to determine acidity of water
Hydrogen ions present in a sample as a result of dissociation or hydrolysis of solutes react with additions of standard alkali. Acidity thus depends on the end-point pH or indicator used. The construction of a titration curve by recording sample pH after successive small measured additions of titrant permits identification of inflection points and buffering capacity, if any, and allows the acidity to be determined with respect to any pH of interest.
In the titration of a single acidic species, as in the standardization of reagents, the most accurate end point is obtained from the inflection point of a titration curve. The inflection point is the pH at which curvature changes from convex to concave or vice versa.
Because accurate identification of inflection points may be difficult or impossible in buffered or complex mixtures, the titration in such cases is carried to an arbitrary end-point pH based on practical considerations. For routine control titrations or rapid preliminary estimates of acidity, the colour change of an indicator may be used for the end point. Samples of industrial wastes, acid mine drainage, or other solutions that contain appreciable amounts of hydrolysable metal ions such as iron, aluminium, or manganese are treated with hydrogen peroxide to ensure oxidation of any reduced forms of polyvalent cations, and boiled to hasten hydrolysis. Acidity results may be highly variable if this procedure is not followed exactly.
Ideally the end point of the acidity titration should correspond to the stoichiometric equivalence point for neutralization of acids present. The pH at the equivalence point will depend on the sample, the choice among multiple inflection points, and the intended use of the data.
Dissolved carbon dioxide (CO2) usually is the major acidic component of unpolluted surface waters; handle samples from such sources carefully to minimize the loss of dissolved gases. In a sample containing only carbon dioxide-bicarbonates-carbonates, titration to pH 8.3 at 25°C corresponds to stoichiometric neutralization of carbonic acid to bicarbonate. Because the colour change of phenolphthalein indicator is close to pH 8.3, this value generally is accepted as a standard end point for titration of total acidity, including CO2 and most weak acids. Metacresol purple also has an end point at pH 8.3 and gives a sharper colour change.
For more complex mixtures or buffered solutions selection of an inflection point may be subjective. Consequently, use fixed end points of pH 3.7 and pH 8.3 for standard acidity determinations via a potentiometric titration in wastewaters and natural waters where the simple carbonate equilibria discussed above cannot be assumed. Bromophenol blue has a sharp colour change at its end point of 3.7. The resulting titrations are identified, traditionally, as methyl orange acidity (pH 3.7) and phenolphthalein or total acidity (pH 8.3) regardless of the actual method of measurement.
Dissolved gases contributing to acidity or alkalinity, such as CO2, hydrogen sulphide, or ammonia, may be lost or gained during sampling, storage, or titration. Minimize such effects by titrating to the end point promptly after opening sample container, avoiding vigorous shaking or mixing, protecting sample from the atmosphere during titration, and letting sample become no warmer than it was at collection.
In the potentiometric titration, oily matter, suspended solids, precipitates, or other waste matter may coat the glass electrode and cause a sluggish response. Difficulty from this source is likely to be revealed in an erratic titration curve. Do not remove interferences from sample because they may contribute to its acidity. Briefly pause between titrant additions to let electrode come to equilibrium or clean the electrodes occasionally.
In samples containing oxidizable or hydrolysable ions such as ferrous or ferric iron, aluminium, and manganese, the reaction rates at room temperature may be slow enough to cause drifting end points.
Do not use indicator titrations with coloured or turbid samples that may obscure the colour change at the end point. Residual free available chlorine in the sample may bleach the indicator. Eliminate this source of interference by adding 1 drop of 0.1M sodium thiosulfate (Na2S2O3).
Selection of procedure:
Determine sample acidity from the volume of standard alkali required to titrate a portion to a pH of 8.3 (phenolphthalein acidity) or pH 3.7 (methyl orange acidity of wastewaters and grossly polluted waters). Titrate at room temperature using a properly calibrated pH meter, electrically operated titrator, or colour indicators.
Use the hot peroxide procedure (4a) to pre-treat samples known or suspected to contain hydrolysable metal ions or reduced forms of polyvalent cation, such as iron pickle liquors, acid mine drainage, and other industrial wastes.
Colour indicators may be used for routine and control titrations in the absence of interfering colour and turbidity and for preliminary titrations to select sample size and strength of titrant (4b).
The range of acidities found in wastewaters is so large that a single sample size and normality of base used as titrant cannot be specified. Use a sufficiently large volume of titrant (20 mL or more from a 50-mL burette) to obtain relatively good volumetric precision while keeping sample volume sufficiently small to permit sharp end points. For samples having acidities less than about 1000 mg as calcium carbonate (CaCO3)/L, select a volume with less than 50 mg CaCO3 equivalent acidity and titrate with 0.02N sodium hydroxide (NaOH). For acidities greater than about 1000 mg as CaCO3/L, use a portion containing acidity equivalent to less than 250 mg CaCO3 and titrate with 0.1N NaOH. If necessary, make a preliminary titration to determine optimum sample size and/or normality of titrant.
Sampling and storage:
Collect samples in polyethylene or borosilicate glass bottles and store at a low temperature. Fill bottles completely and cap tightly. Because waste samples may be subject to microbial action and to loss or gain of CO2 or other gases when exposed to air, analyse samples without delay, preferably within 1 d. If biological activity is suspected analyse within 6 h. Avoid sample agitation and prolonged exposure to air.
Electrometric titrator :
Use any commercial pH meter or electrically operated titrator that uses a glass electrode and can be read to 0.05 pH unit. Standardize and calibrate according to the manufacturer’s instructions. Pay special attention to temperature compensation and electrode care. If automatic temperature compensation is not provided, titrate at 25 ± 5°C.
The size and form will depend on the electrodes and the sample size. Keep the free space above the sample as small as practicable, but allow room for titrant and full immersion of the indicating portions of electrodes. For conventional-sized electrodes, use a 200-mL, tall-form Berzelius beaker without a spout. Fit beaker with a stopper having three holes, to accommodate the two electrodes and the burette. With a miniature combination glass-reference electrode use a 125-mL or 250-mL Erlenmeyer flask with a two-hole stopper.
e. Flasks, volumetric, 1000-, 200-, 100-mL.
f. Burets, borosilicate glass, 50-, 25-, 10-mL.
g. Polyolefin bottle, 1-L.
a. Carbon dioxide-free water: Prepare all stock and standard solutions and dilution water for the standardization procedure with distilled or deionized water that has been freshly boiled for 15 min and cooled to room temperature. The final pH of the water should be ≥ 6.0 and its conductivity should be <2 μmhos/cm.
b. Potassium hydrogen phthalate solution, approximately 0.05N: Crush 15 to 20 g primary standard KHC8H4O4 to about 100 mesh and dry at 120°C for 2 h. Cool in a desiccator. Weigh 10.0 ± 0.5 g (to the nearest mg), transfer to a 1-L volumetric flask, and dilute to 1000 mL.
c. Standard sodium hydroxide titrant, 0.1N: Prepare solution approximately 0.1N as indicated under Preparation of Desk Reagents (see inside front cover). Standardize by titrating 40.00 mL KHC8H4O4 solution (3b), using a 25-mL burette. Titrate to the inflection point (1a), which should be close to pH 8.7. Calculate normality of NaOH:
A = g KHC8H4O4 weighed into 1-L flask,
B = mL KHC8H4O4 solution taken for titration, and
C = mL NaOH solution used.
Use the measured normality in further calculations or adjust to 0.1000N; 1 mL = 5.00 mg CaCO3.
d. Standard sodium hydroxide titrant, 0.02N: Dilute 200 mL 0.1N NaOH to 1000 mL and store in a polyolefin bottle protected from atmospheric CO2 by a soda lime tube or tight cap. Standardize against KHC8H4O4 as directed in 3c, using 15.00 mL KHC8H4O4 solution and a 50-mL burette. Calculate normality as above (3c); 1 mL = 1.00 mg CaCO3.
e. Hydrogen peroxide, H2O2, 30%.
f. Bromophenol blue indicator solution, pH 3.7 indicator: Dissolve 100 mg bromophenol blue, sodium salt, in 100 mL water.
g. Metacresol purple indicator solution, pH 8.3 indicator: Dissolve 100 mg metacresol purple in 100 mL water.
h. Phenolphthalein indicator solution, alcoholic, pH 8.3 indicator.
i. Sodium thiosulfate, 0.1M: Dissolve 25 g Na2S2O3.5H2O and dilute to 1000 mL with distilled water.
If sample is free from hydrolysable metal ions and reduced forms of polyvalent cations, proceed with analysis according to b, c, or d. If sample is known or suspected to contain such substances, pre-treat according to a.
a. Hot peroxide treatment: Pipet a suitable sample (see 1e) into titration flasks. Measure pH. If pH is above 4.0 add 5-mL increments of 0.02N sulfuric acid (H2SO4) to reduce pH to 4 or less. Remove electrodes. Add 5 drops 30% H2O2 and boil for 2 to 5 min. Cool to room temperature and titrate with standard alkali to pH 8.3 according to the procedure of 4d.
b. Colour change: Select sample size and normality of titrant according to criteria of 1e. Adjust sample to room temperature, if necessary, and with a pipet discharge sample into an Erlenmeyer flask, while keeping pipet tip near flask bottom. If free residual chlorine is present add 0.05 mL (1 drop) 0.1M Na2S2O3 solution, or destroy with ultraviolet radiation. Add 0.2 mL (5 drops) indicator solution and titrate over a white surface to a persistent colour change characteristic of the equivalence point. Commercial indicator solutions or solids designated for the appropriate pH range (3.7 or 8.3) may be used. Check colour at end point by adding the same concentration of indicator used with sample to a buffer solution at the designated pH.
c. Potentiometric titration curve:
1) Rinse electrodes and titration vessel with distilled water and drain. Select sample size and normality of titrant according to the criteria of 1e. Adjust sample to room temperature, if necessary, and with a pipet discharge sample while keeping pipet tip near the titration vessel bottom.
2) Measure sample pH. Add standard alkali in increments of 0.5 mL or less, such that a change of less than 0.2 pH units occurs per increment. After each addition, mix thoroughly but gently with a magnetic stirrer. Avoid splashing. Record pH when a constant reading is obtained. Continue adding titrant and measure pH until pH 9 is reached. Construct the titration curve by plotting observed pH values versus cumulative millilitres titrant added. A smooth curve showing one or more inflections should be obtained. A ragged or erratic curve may indicate that equilibrium was not reached between successive alkali additions. Determine acidity relative to a particular pH from the curve.
d. Potentiometric titration to pH 3.7 or 8.3: Prepare sample and titration assembly as specified in 4c1. Titrate to preselected end-point pH (1d) without recording intermediate pH values. As the end point is approached make smaller additions of alkali and be sure that pH equilibrium is reached before making the next addition.
A = mL NaOH titrant used,
B = normality of NaOH,
C = mL H2SO4 used (4a), and
D = normality of H2SO4.
Report pH of the end point used, as follows: ‘‘The acidity to pH _______ = _______ mg CaCO3/L.’’ If a negative value is obtained, report the value as negative. The absolute value of this negative value should be equivalent to the net alkalinity.