Standard Methods for the Examination of Water and Wastewater Practice Flashcards

INORGANIC NONMETALLIC CONSTITUENTS INTRODUCTION

The analytical methods provided in Part 4000 utilize classical wet chemical techniques alongside their automated variations and contemporary instrumental methods such as ion chromatography. The focus of these methods is the measurement of various forms of chlorine, nitrogen, and phosphorus. These procedures are specifically designed for assessing and controlling receiving water quality, treating and supplying potable water, and measuring the efficiency of operation and process in wastewater treatment. Furthermore, these methods are applicable to broader environmental water-quality concerns. Every procedure's introduction includes essential references regarding field sampling conditions, the selection of appropriate sample containers, storage protocols, and the overall applicability of the method.

QUALITY ASSURANCE AND QUALITY CONTROL OVERVIEW

Analytical results reported from tests lack confidence unless accompanied by quality control (QC) results. Essential QC measurements include method calibration, the standardization of reagents, assessment of individual performance capability, the use of blind check samples, and the determination of procedure sensitivity via the method detection level (MDL). Daily evaluations must be conducted for bias, precision, and the presence of laboratory contamination or analytical interference. These procedures, including their expected ranges and the frequency of performance, must be formalized within a written Quality Assurance Manual and Standard Operating Procedures (SOPs).

For certain procedures in Part 4000, traditional bias determination using a known addition to a sample or blank is impossible. Examples include pH, dissolved oxygen, residual chlorine, and carbon dioxide. This technical limitation does not relieve the analyst of responsibility; the purchase of certified ready-made solutions of known levels is encouraged to measure bias. Precision should always be evaluated through the analysis of sample duplicates. Laboratories must participate in proficiency testing (PT) or performance evaluation (PE) studies at least annually, though semi-annually is preferred. Within many jurisdictions, this participation is a requirement for certification. An unacceptable PT result is often the first sign of a failure to follow test protocols and requires full investigation. Specific QC procedures mentioned within methods reflect the minimum necessary for successful performance, and additional controls should be used where applicable.

DETERMINATION OF ANIONS BY ION CHROMATOGRAPHY (PART 4110)

Ion chromatography provides a singular instrumental technique for the rapid, sequential measurement of common anions such as bromide, chloride, fluoride, nitrate, nitrite, phosphate, and sulfate. This method is advantageous as it eliminates the use of hazardous reagents and distinguishes effectively between halides ($Br^{-}$, $Cl^{-}$, and $F^{-}$) and oxy-ions ($SO_{3}^{2-}$, $SO_{4}^{2-}$, $NO_{2}^{-}$, and $NO_{3}^{-}$). It is applicable to surface, ground, and wastewaters, as well as drinking water and industrial process waters like boiler or cooling water, following filtration to remove particles over $0.2\text{ um}$.

The principle of ion chromatography with chemical suppression involves injecting a water sample into a carbonate-bicarbonate eluent stream. The sample passes through a guard column and a separator column containing a low-capacity, strongly basic anion exchanger. Anions separate based on their relative affinities for the exchanger. They then pass through a fiber or micromembrane suppressor bathed in a strongly acid regenerant solution ($0.025N \text{ } H_{2}SO_{4}$). The suppressor converts separated anions into their highly conductive acid forms while converting the eluent into weakly conductive carbonic acid. Detection is performed by conductivity, with anions identified by retention time and quantitated by peak area or height.

Interferences include any substance with a retention time coinciding with the target anions, such as low-molecular-weight organic acids. High concentrations of one ion can interfere with the resolution of others, though sample dilution or gradient elution can overcome this. Scrupulous avoidance of contamination is required due to small sample volumes. The minimum detectable concentration is roughly $0.1\text{ mg/L}$ for most anions using a $100\text{ uL}$ sample loop and a $10\text{ sS/cm}$ full-scale setting. Fluoride ($F^{-}$) is notable for potential bias and poor precision in unknown matrices due to the ‘water dip’ effect and interference from simple organic acids like formic and carbonic acid; special techniques such as dilute eluent or NaOH gradient elution are recommended for accurate $F^{-}$ determination.

SEGMENTED CONTINUOUS FLOW ANALYSIS (SFA) (PART 4120)

Segmented flow analysis (SFA) automates numerous wet chemical analyses by functioning as a ‘conveyor belt’ system where reagents are added in a production-line manner. Benefits include reduced sample and reagent consumption, improved repeatability due to precise timing, and minimal operator contact with hazardous materials. SFA systems typically analyze $30$ to $120\text{ samples/h}$. This technique can incorporate complex procedures such as mixing, dilution, distillation, digestion, dialysis, and solvent extractions. In-line distillation is used for ammonia, fluoride, and cyanide, while in-line digestion is used for total phosphorus and total nitrogen.

The SFA system components include a sampling device, a peristaltic pump, an analytical cartridge where chemistry occurs, and a detector. Analytical segments are separated by air bubbles (or other immiscible fluids) to prevent sample dispersion and cross-contamination. Detectors are usually spectrophotometers measuring color at specific wavelengths, although flame photometers or ion-selective electrodes can be used. Bubbles are often removed before detection to avoid signal distortion, though ‘bubble-gating’ techniques allow processing without removal using fast detector response times. Carryover can occur from longitudinal dispersion or axial dispersion (lag-phase), which is minimized by adding surfactants (wetting agents) and minimizing transmission tubing length. Contamination or blockages are indicated by abnormal bubble patterns.

FLOW INJECTION ANALYSIS (FIA) (PART 4130)

Flow Injection Analysis (FIA) is an automated method where a precisely measured portion of a liquid sample is injected into a continuously flowing carrier stream. As the sample disperses, it forms an asymmetric Gaussian gradient. If a color reaction is used, reagents merge with the carrier stream in proportions equal to relative flow rates, modifying the analyte in the gradient to produce a color proportional to the concentration. An absorbance peak is formed as the gradient passes a flow-through detector, with the peak area proportional to the analyte concentration. FIA parameters such as flow rate, sample volume, temperature, and residence time must remain identical for standards and unknowns to ensure accuracy.

CAPILLARY ION ELECTROPHORESIS (CIE) (PART 4140)

Capillary ion electrophoresis (CIE) offers rapid analysis (under $5\text{ min}$) and detects all anions present, providing an anionic ‘fingerprint.’ Its selectivity differs from ion chromatography, successfully resolving fluoride from monovalent organic acids. The principle involves a 75-um-ID silica capillary filled with a buffered aqueous electrolyte solution containing a UV-absorbing anion (sodium chromate) and an electroosmotic flow modifier (OFM). An electric field of $15\text{ kV}$ is applied, defining the detector end as the anode. Anions migrate based on mobility differences and are detected via indirect UV detection at $254\text{ nm}$ as they displace the chromate ions, causing a decrease in absorbance. Calculations use time-corrected peak area, as peak area in electrophoresis is a function of migration time.

Interferences in CIE include any anion with a similar migration time. Formate is a common potential interference for fluoride. High ionic strength in samples may decrease migration times, which is addressed by using chloride as a reference peak for normalized migration time. The electrolyte solution consists of $4.7\text{ mM}$ sodium chromate, $4\text{ mM}$ TTAOH (tetradecyltrimethyl ammonium hydroxide), $10\text{ mM}$ CHES buffer, and $0.1\text{ mM}$ calcium gluconate, with a target pH of $9 pm 0.1$. Standards should bracket the expected range from $0.1$ to $50\text{ mg/L}$.

BORON (4500-B)

Boron (B) appears in natural waters primarily as $H_{3}BO_{3}$. While essential for plants, concentrations above $2.0\text{ mg/L}$ in irrigation water are deleterious. The Curcumin Method (B) is applicable for $0.10$ to $1.0\text{ mg/L}$, forming a red product called rosocyanine. The Carmine Method (C) is suitable for $1$ to $10\text{ mg/L}$, where carmine in concentrated sulfuric acid changes from bright red to blue in the presence of boron. Samples should be stored in polyethylene or boron-free, alkali-resistant glassware.

BROMIDE (4500-Br)

Bromide occurs in coastal ground and surface waters due to seawater intrusion. The Phenol Red Colorimetric Method (B) involves oxidizing bromide with chloramine-T in the presence of phenol red at pH $4.5$ to $4.7$, resulting in a red to violet brominated compound. Chloramine-T concentration and reaction timing (exactly $20\text{ min}$) are critical. The Flow Injection Analysis Method (D) also uses the chloramine-T oxidation to bromine, followed by substitution on phenol red to produce bromphenol blue, with absorbance measured at $590\text{ nm}$. Sodium thiosulfate is added to reduce interference from chloride.

CARBON DIOXIDE (4500-CO2)

Free carbon dioxide ($CO_{2}$) contributes to corrosion and is measured via nomographic, titrimetric, or calculation methods. The Titrimetric Method (C) involves reacting free $CO_{2}$ with sodium carbonate or sodium hydroxide to form sodium bicarbonate, with an equivalence point at pH $8.3$ (phenolphthalein indicator). Analysis must be performed immediately at the sampling point to prevent gas loss. Total carbon dioxide can be calculated using the following equation:

$\text{mg total } CO_{2}/L = A + 0.44 \times (2B + C)$

Where $A$ is mg free $CO_{2}/L$, $B$ is bicarbonate alkalinity, and $C$ is carbonate alkalinity. Calculation of bicarbonate alkalinity ($T = \text{total alkalinity}$) is given by:

$\text{Bicarbonate alkalinity as mg } CaCO_{3}/L = \frac{T - 5.0 \times 10^{(pH-10)}}{1 + 0.94 \times 10^{(pH-10)}}$

CYANIDE (4500-CN)

Cyanide compounds are classified as simple and complex. Simple cyanides ($A(CN)_{x}$) dissociate to release the highly toxic $HCN$ ($pK_{a} ≈ 9.2$) or $CN^{-}$. Complex cyanides ($A_{y}M(CN)_{x}$) vary in stability; zinc and cadmium complexes dissociate easily, while iron-cyanide complexes are stable but subject to rapid photolysis into toxic $HCN$ when exposed to sunlight. Total cyanide is determined after distillation (C), where metal cyanides are converted to $HCN$ gas in an acidified boiling flask, purged with air, and absorbed in an $NaOH$ scrubbing solution. Distillation recovery of cobalticyanide is incomplete without ultraviolet radiation pretreatment.

Cyanide amenable to chlorination (G) is determined by the difference between total cyanide in an untreated sample and a sample chlorinated at pH $11$ to $12$. The Weak Acid Dissociable (WAD) procedure (I) uses a buffer of pH $4.5$ to $6.0$ and zinc salts to isolate cyanides that are not tightly complexed. Cyanogen chloride ($CNCl$) is a toxic gas formed during chlorination; its analysis (J) must occur immediately as it hydrolyzes to cyanate ($CNO^{-}$) at high pH. Cyanate ($CNO^{-}$) itself is measured (L) by hydrolyzing it to ammonia under acidic, heated conditions and calculating the difference in ammonia concentration before and after. Spot tests (K) allow for quick screening for concentrations above $50\text{ ug/L}$.

CHLORINE (RESIDUAL) (4500-Cl)

Chlorination destroys disease-producing microorganisms but can form carcinogenic chloroorganic compounds like chloroform. Free chlorine (molecular chlorine, hypochlorous acid, and hypochlorite ion) reacts with ammonia to form combined chlorine (monochloramine, dichloramine, and nitrogen trichloride). The Amperometric Titration Method (D) is the standard of comparison, using phenylarsine oxide and a noble-metal electrode to detect the end point. 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 ($KI$).

The DPD Ferrous Titrimetric Method (F) uses $N,N\text{-diethyl-}p\text{-phenylenediamine}$ as an indicator. Free chlorine reacts instantly to produce a red color at pH $6.2$ to $6.5$. Subsequent additions of iodide ion sequentially catalyze reactions for monochloramine and dichloramine. Oxidized manganese is a significant interferent, for which a sodium arsenite or thioacetamide correction is used. The FACTS method (H) is specific to free chlorine and is unaffected by chloramines or oxidized manganese. Due to the instability of chlorine in aqueous solutions, analysis must start immediately after sampling, avoiding light and agitation.