Manual for Inorganic Nonmetallic Constituents in Water and Wastewater

Introduction to Inorganic Nonmetallic Constituents

Part 4000 of the Standard Methods for the Examination of Water and Wastewater covers the analysis of inorganic nonmetallic constituents. The methods presented include classical wet chemical techniques, automated variations, and modern instrumental techniques such as ion chromatography. These procedures are utilized for measuring various forms of chlorine, nitrogen, and phosphorus. Their primary applications include the assessment and control of receiving water quality, the treatment and supply of potable water, the measurement of process efficiency in wastewater treatment, and the evaluation of general environmental water-quality concerns. Each procedure includes references to field sampling conditions, appropriate containers, storage requirements, and the method's applicability.

Quality Assurance and Quality Control (4020)

Analytical results lack confidence without supporting quality control data. Essential quality control measurements described in Section 4020 include method calibration, standardization of reagents, assessment of individual performance capability, performance of blind check samples, and determination of test sensitivity through the Method Detection Level ($MDL$). Additionally, laboratories must conduct daily evaluations of bias, precision, and the presence of contamination or analytical interference. These procedures, including frequency and expected ranges, should be formalized in a written Quality Assurance Manual and Standard Operating Procedures.

For certain analytes such as $pH$, dissolved oxygen, residual chlorine, and carbon dioxide ($CO_2$), traditional bias determination using known additions to samples or blanks is impossible. In these cases, analysts are encouraged to purchase certified ready-made solutions of known levels to measure bias. Precision should always be evaluated through the analysis of sample duplicates. Laboratories should participate in proficiency testing ($PT$) or performance evaluation ($PE$) studies at least annually, and preferably semi-annually. Unacceptable results in these studies often indicate a failure to follow test protocols and require full investigation. Participation in $PT$ studies is often a requirement for laboratory certification in many jurisdictions.

Determination of Anions by Ion Chromatography (4110)

Ion chromatography provides a rapid, sequential measurement of common anions including bromide ($Br^-$), chloride ($Cl^-$), fluoride ($F^-$), nitrate ($NO_3^-$), nitrite ($NO_2^-$), phosphate ($PO_4^{3-}$), and sulfate ($SO_4^{2-}$). It is considered a single instrumental technique that eliminates the need for hazardous reagents and effectively distinguishes between halides and oxy-ions. This method is applicable to surface water, groundwater, wastewater, drinking water, and some industrial process waters like boiler and cooling water, following filtration to remove particles larger than 0.2
μm.

In Method 4110 B (Chemical Suppression), a water sample is injected into a carbonate-bicarbonate eluent and passed through ion exchangers. Anions are separated based on their relative affinities for a low-capacity, strongly basic anion exchanger. The separated anions pass through a cation exchanger membrane suppressor where they are converted to highly conductive acid forms, and the eluent is converted to weakly conductive carbonic acid. Detection is by conductivity, and identification is based on retention time compared to standards. Quantitation utilizes peak area or peak height.

Interferences in ion chromatography include any substance with a retention time coinciding with the target anion. High concentrations of low-molecular-weight organic acids can interfere with chloride and fluoride. High concentrations of any single ion can interfere with the resolution of others, though sample dilution or gradient elution can overcome this. Contamination must be avoided by scrupulously cleaning glassware and using high-purity water. The minimum detectable concentration is typically near 0.1
mg/L for most anions with a 100
αL sample loop.

Fluoride ($F^-$) determination is noted as difficult in unknown matrices due to two specific effects: first, the "water dip" (the elution of water) can cause negative bias at low concentrations; second, simple organic acids like formic and carbonic acid elute near fluoride and interfere. Accuracy can be improved using dilute eluent, gradient elution with $NaOH$, or alternative columns.

Apparatus and Reagents for Ion Chromatography

The required apparatus for Method 4110 B includes an ion chromatograph with an injection valve, sample loop, guard column, and separator column. The separator column must use a styrene divinylbenzene-based low-capacity pellicular anion-exchange resin. The system also requires a fiber or membrane suppressor and a temperature-compensated conductivity detector (6
αL volume or less). Reagents include deionized or distilled water with conductance < 0.1
ʱS/cm. The eluent focuses on a mixture of 0.0017M
NaHCO_3 and 0.0018M
Na_2CO_3. The regenerant solution for the suppressor is 0.025N
H_2SO_4.

Standard anion solutions (1000
mg/L) are prepared by dissolving specific salts dried to a constant weight at $105^◦C$. For example, Chloride uses 1.6485
g $NaCl/L$, Bromide uses 1.2876
g $NaBr/L$, and Nitrate uses 1.3707
g $NaNO_3/L$ (226
mg
NO_3^--N/L). Combined working standards are prepared in high and low ranges and must be replaced daily if they contain $NO_2^-$ and $PO_4^{3-}$.

Method 4110 C involves single-column ion chromatography with electronic suppression. Anions are separated based on affinity for column packing materials. Interferences include coelution of short-chain acids with fluoride and chloride. Borate/gluconate concentrate is used as part of the eluent (16.00
g sodium gluconate, 18.00
g boric acid, and 25.00
g sodium tetraborate decahydrate per liter). Samples are filtered through a 0.45
μm filter. System equilibration is achieved when a stable baseline reflects the background conductivity of the eluent (278
ʱS
±10%
).

Continuous Flow and Flow Injection Analysis (4120, 4130)

Segmented Flow Analysis ($SFA$), described in Section 4120, automates wet chemical analyses using a "conveyor belt" system. Reagents are added in a production-line manner. Air bubbles are introduced to segment the flow, which minimizes longitudinal dispersion. SFA reduces sample and reagent consumption, improves repeatability, and minimizes operator contact with hazardous materials. It is capable of $30$ to $120$ samples per hour. SFA is suitable for complex procedures like in-line distillation (for ammonia, fluoride, cyanide, phenols) and in-line digestion (for total phosphorus, nitrogen, or cyanide).

Flow Injection Analysis ($FIA$), covered in Section 4130, involves introducing a precisely measured sample portion into a continuously flowing carrier stream. The sample forms an asymmetric Gaussian gradient. If a color reaction is used, absorbance peaks are formed whose areas are proportional to analyte concentration. FIA offers the advantage of a constantly measured reagent blank and high sample throughput. Standard parameters including flow rate, volume, and residence time must be identical for standards and unknowns.

Capillary Ion Electrophoresis (4140)

Capillary Ion Electrophoresis ($CIE$) is a proposed method (4140) for determining inorganic anions. It provides an anionic "fingerprint" and is rapid, with complete analysis in less than $5$ minutes. Operating costs are significantly lower than ion chromatography. $CIE$ separates anions based on their mobility in an electric field (15
kV) as they migrate through a 75
μm-ID silica capillary.

Detection in $CIE$ is achieved via indirect UV detection (254
nm). A chromate-based electrolyte containing an electroosmotic flow modifier ($OFM$) like tetradecyltrimethyl ammonium bromide ($TTAB$) fills the capillary. Analytes displace the UV-absorbing chromate ions, resulting in a decrease in absorbance. A common interference is formate, which elutes near fluoride. Adding 5
mg/L formate to standards helps identify fluoride accurately. The migration order is always $Cl^-$, $Br^-$, $NO_2^-$, $SO_4^{2-}$, $NO_3^-$, $F^-$, and $PO_4^{3-}$. Quantitation uses time-corrected peak area, as peak area is a function of migration time.

Boron (4500-B)

Boron naturally occurs in streams at approximately 10
μg/L and in groundwater up to 10
mg/L. While essential for plants, levels above 2.0
mg/L can be deleterious to crops. Large amounts can affect the human central nervous system (borism). The preferred analytical method is Inductively Coupled Plasma ($ICP$).

The Curcumin Method (4500-B B) is suitable for $0.10$ to 1.0
mg/L. It involves forming a red product called rosocyanine when a sample is evaporated with curcumin in acidic conditions. Hardness above 100
mg/L as $CaCO_3$ interferes, which can be mitigated by ion exchange. The Carmine Method (4500-B C) is suitable for $1$ to 10
mg/L; boron causes a carmine solution in sulfuric acid to change from bright red to bluish red or blue.

Calculation for Curcumin Method: mg
B/L = \frac{A_2 \times C}{A_1 \times S} Where: $A_1 = \text{absorbance of standard}$ $A_2 = \text{absorbance of sample}$ C = \u0001μg
B \text{ in standard} $S = \text{mL sample}$

Bromide (4500-Br-)

Bromide is found in coastal areas due to seawater intrusion and in groundwaters. Even levels < 100
μg/L can lead to the formation of bromate or brominated by-products during disinfection. The Phenol Red Colorimetric Method (4500-Br- B) utilizes chloramine-T to oxidize bromide to bromine, which kemudian brominates phenol red at pH
4.5 to $4.7$. The resulting color varies from reddish to violet. Flow Injection Analysis (4500-Br- D) also utilizes chloramine-T and measures absorbance at 590
nm. Sodium thiosulfate is added to reduce chloride interference.

Carbon Dioxide (4500-CO2)

Free carbon dioxide ($CO_2$) in surface waters is typically < 10
mg/L but can be higher in groundwater. It contributes to corrosion. Estimation methods include nomographic, titrimetric, and calculation. The Titrimetric Method (4500-CO2 C) involves reacting $CO_2$ with $NaOH$ to form sodium bicarbonate, with an end-point at pH
8.3. Potentiometric or phenolphthalein indicators are used.

Calculation for $CO_2$: mg
CO_2/L = \frac{A \times N \times 44000}{mL\u0001\text{sample}} Where: $A = \text{mL titrant}$ $N = \text{normality of } NaOH$

Forms of alkalinity and free $CO_2$ can also be calculated if $pH$, total alkalinity, and total dissolved solids (TDS < 500
mg/L) are known. These calculations utilize ionization constants for carbonic acid.

Cyanide (4500-CN-)

Cyanide compounds are classified as simple (e.g., $NaCN$) or complex ($AyM(CN)x$). Molecular $HCN$ is highly toxic and forms at neutral to low $pH$. Iron-cyanide complexes are stable and less toxic in the dark but undergo rapid photolysis in sunlight to release $HCN$. Analytical methods distinguish between total cyanide, cyanides amenable to chlorination, and weak acid dissociable ($WAD$) cyanide.

Total Cyanide after Distillation (4500-CN- C) involves liberating $HCN$ gas from an acidified sample via distillation into an $NaOH$ scrubber. Magnesium chloride ($MgCl_2$) is used to help dissociate metal-cyanide complexes. The recovered cyanide is then measured by titration (silver nitrate with p-dimethylaminobenzalrhodanine indicator), colorimetry (pyridine-barbituric acid), or an ion-selective electrode.

Cyanide Amenable to Chlorination (4500-CN- G) involves comparing a chlorinated sample (to destroy amenable forms) with an untreated sample. The difference is the amenable fraction. The "Short-Cut" method (4500-CN- H) avoids distillation but is limited by thiocyanate ($SCN^-$) interference. Weak Acid Dissociable Cyanide (4500-CN- I) uses an acetate buffer at pH
4.5 to $6.0$ with zinc salts to specifically recover less stable complexes while leaving iron cyanides intact.

Samples for cyanide analysis must be preserved by raising the $pH$ to $12$ to $12.5$ with $NaOH$. Interferences such as oxidizing agents (removed with sodium arsenite), sulfides (removed with lead carbonate), and fatty acids (removed by extraction with iso-octane or hexane) must be addressed before distillation.

Residual Chlorine (4500-Cl)

Chlorination is used to destroy microorganisms but can produce carcinogens like chloroform or toxic combined chlorine (chloramines). Free chlorine consists of aqueous molecular chlorine, hypochlorous acid ($HOCl$), and hypochlorite ion ($OCl^-$). Combined chlorine forms when free chlorine reacts with ammonia or amines (monochloramine, dichloramine, nitrogen trichloride).

Amperometric Titration (4500-Cl D) is the standard method for determining free or combined chlorine. Free chlorine is titrated at pH
6.5 to $7.5$, while combined chlorine is titrated at pH
3.5 to $4.5$ in the presence of potassium iodide ($KI$). Phenylarsine oxide ($PAO$) is the preferred reducing titrant due to its stability.

The DPD Ferrous Titrimetric Method (4500-Cl F) uses N,N-diethyl-p-phenylenediamine as an indicator. Free chlorine reacts instantly to produce a red color, which is titrated to a colorless end point with ferrous ammonium sulfate ($FAS$). Monochloramine and dichloramine are subsequently determined by tiered additions of $KI$. Oxidized manganese is a primary interference and is corrected for using sodium arsenite or thioacetamide. FACTS (4500-Cl H) is a specific method for free chlorine using syringaldazine, which is unaffected by chloramines and manganese.