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Several college athletes died from these symptoms medicine 2016 buy secnidazole 500mg visa, Today we can often add a cellular or molecular explanation medications ranitidine buy 500mg secnidazole with visa. Case Studies and Research Results 2 disorders are typically recessive because the half normal amount at the chapter’s end discusses this problem treatment borderline personality disorder generic secnidazole 500mg amex. The one normal allele symptoms concussion buy secnidazole 500mg low price, therefore, compensates for the mutant one, to which it is dominant. A recessive trait is said to arise from a “loss-of-function” Key Concepts Questions 4. State two factors that can influence single gene inherited disorders are said to be due to a “gain-of-function,” inheritance patterns. Distinguish between autosomal dominant traits and disease results from a gain-of-function in which the dominant autosomal recessive traits. What is the effect of relatives having children with the normal protein from functioning in certain brain cells. How are Mendel’s first law and logic used to solve who are missing one copy of the gene do not have the illness. Explain how recessive traits or illnesses can result abnormal, not absent, to cause the disease. Recessive disorders tend to be more severe, and produce symptoms earlier than dominant disorders. In contrast, if a dominant mutation arises that harms health early in life, peo of More Than One Gene ple who have the allele are either too ill or do not survive long enough to reproduce. The allele eventually becomes rare in In a second set of experiments, Mendel examined the inheritance the population unless it arises anew by mutation in the gam of two traits at a time. Today, the ability of exome and genome etes of a person who does not have the disease. Dominant dis sequencing to reveal gene interactions makes Mendel’s experi orders whose symptoms do not appear until adulthood, or that ments on the inheritance of more than one trait more relevant than ever because many genes are considered simultaneously. The two genes are said to “independently assort” because they are packaged into gametes at random (figure 4. A loss-of-function mutation is typically recessive and Mendel looked at seed shape, which was either round or results in a deficit or absence of the gene’s protein product. A wrinkled (determined by the R gene), and seed color, which gain-of-function mutation is typically dominant and alters the was either yellow or green (determined by the Y gene). Each of these combinations has one gene for each From their appearance, Mendel deduced that round is domi trait. A Punnett square for this cross shows that the four types nant to wrinkled, and yellow to green. These classes appeared in a ratio are present in the ratio 9:3:3:1, just as Mendel found. Mendel then crossed each plant from the third gen Solving a Problem in Following eration to plants with wrinkled, green seeds (genotype rryy). Probability predicts 9:3:3:1 proportion of progeny classes only if one gene does not the likelihood of an event. The independent assortment of genes carried on different chromosomes results from the random alignment of chromosome pairs during metaphase of meiosis I. The allele combination depends upon which chromosomes are packaged together in a gamete—and this happens at random. A Punnett square can represent the random combinations of gametes produced by dihybrid individuals. An underline in a genotype (in the F2 generation) indicates that either a dominant or a recessive allele is possible. Test crosses with rryy plants revealed the genotypes of the F2 generation, depicted in the 16-box Punnett square. Consider the probability of obtaining a plant with wrinkled, green peas (genotype rryy) from dihybrid (RrYy) par 3. Do the reasoning for one gene at a time, then multiply the different chromosomes independently assort. How are Punnett squares and probability used to A Punnett square depicting a cross of two Rr plants indi follow inheritance of more than one trait Therefore, the chance of dihybrid parents (RrYy) producing homozygous recessive (rryy) offspring is 1/4 multi 4. Now consult the 16-box Punnett square for Mendel’s dihybrid cross again (see figure 4. Only one of the For genetics researchers and genetic counselors, families are 16 boxes is rryy, just as the product rule predicts. Bb yellow (normal) urine bb beeturia (red urine after eating beets) Hh colored eyelids hh normal eyelids Ee brachydactyly (short fingers) and depict which relatives have specific phenotypes and, ee normal fingers sometimes, genotypes. A human pedigree serves the same purpose as one for purebred dogs or cats or thoroughbred Figure 4. His partner has red urine after she eats differs from a family tree in genealogy, and from a genogram beets, and has normal eyelids and short fingers. The in social work, in that it indicates disorders or traits as well chance that their child will have red urine after eating as relationships and ancestry. Verti cal lines represent generations; horizontal lines that connect disorder hemophilia (see figure 6. The mutant gene proba two shapes at their centers depict partners; shapes con bly originated in Queen Victoria of England in the nineteenth nected by vertical lines that are joined horizontally repre century. Squares indicate males; circles, females; and a pedigree of a family with colorblindness using musical diamonds, individuals of unspecified sex. Roman numerals notation—half notes for unaffected females, quarter notes designate generations. Arabic numerals or names indicate for colorblind females, and filled-in and squared-off notes to individuals. Colored or shaded shapes indicate century, eugenicists tried to use pedigrees to show that traits individuals who express a trait, and half-filled shapes are such as criminality, feeblemindedness, and promiscuity were known carriers. Today, pedigrees are important both for helping fami lies identify the risk of transmitting an inherited illness and Pedigrees Then and Now as starting points for identifying and describing, or anno the earliest pedigrees were strictly genealogical, not indicat tating, a gene from the human genome sequence. The mons and the Amish, are invaluable in helping research term pedigree arose in the fifteenth century, from the French ers follow the inheritance of particular genes. Within this section of a chro ness was an extensive family tree of several European mosome lies the causative mutation. Discovery of a mutation royal families, indicating which members had the clotting that causes an early-onset form of Alzheimer disease, for 80 Part 2 Transmission Genetics Symbols Pedigrees Display Mendel’s Laws, = Normal female, male Visual learners can easily “see” a mode of inheritance in a pedigree. Consider a pedigree for an autosomal recessive, = Female, male who expresses trait trait, albinism. Homozygous recessive individuals in the third, = Female, male who carries an (F2) generation lack an enzyme necessary to manufacture the allele for the trait but does not pigment melanin and, as a result, hair and skin are very pale express it (carrier) (figure 4. For some disorders, carriers have half the = Sex unspecified wild type amount of a key biochemical in a body fluid, such as blood or urine. A typical pedigree for an autoso = Spontaneous abortion mal dominant trait has some squares and circles filled in to (miscarriage) indicate affected individuals in each generation. A pedigree may be inconclusive, which means that either autosomal recessive or autosomal dominant inheritance Lines can explain the pattern of filled-in symbols. According to the pedigree, this trait can be passed in an autosomal dominant mode because it = Partners affects both males and females and is present in every genera = Adoption tion. However, the pedigree can also depict autosomal reces sive inheritance if the individuals represented by unfilled = Siblings symbols are carriers. Inconclusive pedigrees tend to arise when families are small and the trait is not severe enough to = Identical twins impair fertility. People may hesitate to supply information = Fraternal twins because the symptoms embarrass them. Families with adop tion, children born out of wedlock, serial relationships, blended = Parents closely related families, and assisted reproductive technologies (see chapter 21) (by blood) may not fit easily into the rules of pedigree construction. Many = Former relationship people cannot trace their families back far enough to reveal a mode of inheritance. Symbols representing individuals connect to form pedigree trait or condition, his or her parents are inferred to be carriers charts, which display the inheritance patterns of traits. His unaf fected parents, Kizzy and Ike, must each be heterozygotes example, took researchers to a remote village in Colombia, (carriers). Deshawn’s sister, Taneesha, is also healthy, and she is where the original mutation present today in a 1,000-plus expecting her first child. Taneesha’s husband, Antoine, has no member family came from a Spanish settler who had arrived family history of sickle cell disease.

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After securing the tube medications 5 rs discount secnidazole 500 mg, maintain the patient’s head in a neutral position; neck flexion may push the tube farther into the airway medicine reminder app generic secnidazole 500mg fast delivery, and extension may pull the tube out of the airway medicine journey order 500mg secnidazole visa. Note that transtracheal ventilation primarily supports oxygenation as tidal volumes are usually too small to symptoms 2 days before period generic secnidazole 500mg effectively remove carbon dioxide. This technique is intended for temporary use while a more effective airway is obtained. Do not insert the suction catheter beyond the end of the endotracheal tube to avoid injuring the mucosa. Use a maximum suction force of 80 to 120 mm Hg for suctioning the airway via an endotracheal tube. Higher suction pressures applied through Part 12: Pediatric Advanced Life Support 23 large-bore noncollapsible suction tubing and semirigid pharyngeal tips are used to suction the mouth and pharynx. The compression-to ventilation ratio differs (newly born and newborns – 3:1; infant two rescuer – 15:2) and how to provide ventilations in the presence of an advanced airway differs (newly born and newborns – pause after 3 compressions; infants – no pauses for ventilations). It may be necessary to temporarily interrupt chest compressions to determine the child’s rhythm. Less frequently there is a sudden impairment of cardiac output with an initially normal rhythm but without pulses and with poor perfusion. The second rescuer delivers ventilations at a rate of 1 breath every 6 seconds (10 breaths per minute). Rotate the compressor role approximately every 2 minutes to prevent compressor fatigue and deterioration in quality and rate of chest compressions. If at any time the rhythm becomes “shockable,” give a shock (Step 7) and immediately resume chest compressions for 2 minutes before rechecking the rhythm. Minimize time between chest compressions and shock delivery (ie, check rhythm and deliver shocks immediately after compressions rather than after rescue breaths, if possible) and between shock delivery and resumption of chest compressions. Outcome of shock delivery is best if rescuers minimize the time between last compression and shock delivery, so rescuers should be prepared to coordinate (brief) interruptions in chest compressions to deliver shocks, and should resume compressions immediately after shock delivery. Continuous monitoring is helpful in tracking responses to treatment and changes in clinical condition. When appropriately trained personnel are available, echocardiography may be considered to identify patients with potentially treatable causes of the arrest, particularly pericardial tamponade and inadequate ventricular filling. If a patient has an indwelling arterial catheter, the waveform can be used as feedback to evaluate chest compressions. Although a central venous catheter can provide more secure long-term access, its placement requires training and experience, and the procedure can be time-consuming. Therefore central venous access is not recommended as the initial route of vascular access during an emergency. If both central and peripheral accesses are available, administer medications into the central circulation since some medications (eg, adenosine) are more effective when administered closer to the heart, and others (eg, calcium, amiodarone, procainamide, sympathomimetics) may be irritating when infused into a peripheral vein. The length of a central catheter can contribute to increased resistance, making it more difficult to push boluses of fluid rapidly through a multilumen central than a peripheral catheter. Table 3: 2010 Medications for Pediatric Resuscitation Open table in a new window Medications for Pediatric Resuscitation Medication Dose Remarks Adenosine 0. The effectiveness of endotracheal epinephrine during cardiac arrest is controversial. Some studies showed it to be as effective as vascular administration164,169,170 while other studies have not found it to be as effective. Non-lipid-soluble drugs (eg, sodium bicarbonate and calcium) may injure the airway; they should not be administered via the endotracheal route. However, they also cause intense vasoconstriction and increase myocardial oxygen consumption, which might be detrimental. Two pediatric observational out-of-hospital studies196,197 had too many confounders to determine if vasopressors were beneficial. This recommendation was based predominantly on pediatric case series or extrapolation from adult studies that used short-term outcomes. The same study did not show an association between lidocaine or amiodarone use and survival to hospital discharge. Manual defibrillators can also be used with hands-free pads that are self adhesive. Use the largest paddles or self-adhering electrodes200-202 that will fit on the child’s chest without touching (when possible, leave about 3 cm between the paddles or electrodes). An appropriate paddle or self-adhesive pad size is “Adult” size (8 to 10 cm) for children >10 kg (> approximately 1 year) “Infant” size for infants <10 kg 4. Place manual paddles over the right side of the upper chest and the apex of the heart (to the left of the nipple over the left lower ribs) so the heart is between the two paddles. Neither the energy dose specifically related to automated external defibrillators, nor the energy dose for cardioversion was evaluated in this evidence review. In adults with prolonged arrest215,216 and in animal models,217defibrillation is more likely to be successful after a period of effective chest compressions. Ideally chest compressions should be interrupted only for ventilations (until an advanced airway is in place), Part 12: Pediatric Advanced Life Support 33 rhythm check, and shock delivery. If a “shockable” rhythm is still present, continue chest compressions after a rhythm check (when possible) while the defibrillator is charging (so chest compressions are delivered until shock delivery). It is important to minimize the time between chest compressions and shock delivery and between shock delivery and resumption of postshock compressions. In in-hospital settings with continuous invasive monitoring, this sequence may be modified at the expert provider’s discretion. Epinephrine should be administered during chest compressions, but the timing of drug administration is less important than the need to minimize interruptions in chest compressions. Just prior to the rhythm check, the rescuer operating the defibrillator should prepare to recharge the defibrillator (4 J/kg or more with a maximum dose not to exceed 10 J/kg or the adult dose, whichever is lower). If at any time the rhythm check shows a “nonshockable” rhythm, proceed to the “Pulseless Arrest” sequence (Steps 10 or 11). Instead, the compressing rescuer gives continuous chest compressions at a rate of at least 100 per minute without pause for ventilation. The rescuer delivering ventilation provides about 1 breath every 6 seconds (10 breaths per minute). Two or more rescuers should rotate the compressor role approximately every 2 minutes to prevent compressor fatigue and deterioration in quality and rate of chest compressions. This algorithm applies to the care of the infant or child with bradycardia and cardiorespiratory compromise, but a palpable pulse. Part 12: Pediatric Advanced Life Support 35 Figure 2: Pediatric Bradycardia With a Pulse and Poor Perfusion Algorithm Emergency treatment of bradycardia is indicated when the rhythm results in hemodynamic compromise. Part 12: Pediatric Advanced Life Support 36 (Box 4) After 2 minutes reevaluate the patient to determine if bradycardia and signs of hemodynamic compromise persist. Verify that the support is adequate (eg, check airway, oxygen source, and effectiveness of ventilation). The choice of therapy is determined by the patient’s degree of hemodynamic instability. Attempt vagal stimulation (Box 7) first, unless the patient is hemodynamically unstable or the procedure will unduly delay chemical or electric cardioversion. Pharmacologic cardioversion with adenosine (Box 8) is very effective with minimal and transient side effects. If there is no effect and there are no signs of toxicity, give additional doses (Table 3). Avoid the simultaneous use of amiodarone and procainamide without expert consultation. Do not use adenosine in patients with known Wolff-Parkinson-White syndrome and wide-complex tachycardia. In hemodynamically unstable patients: Electric cardioversion is recommended using a starting energy dose of 0. Involve a qualified surgeon early and, if possible, transport a child with multisystem trauma to a trauma center with pediatric expertise. The following are special aspects of trauma resuscitation: When the mechanism of injury is compatible with cervical spinal injury, restrict motion of the cervical spine and avoid traction or movement of the head and neck. If the airway cannot be opened with a jaw thrust, use a head tilt–chin lift because you must establish a patent airway. Because of the disproportionately large head of infants and young children, optimal positioning may require recessing the occiput234 or elevating the torso to avoid undesirable backboard-induced cervical flexion. Suspect thoracic injury in all thoraco-abdominal trauma, even in the absence of external injuries.

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When different examiners were asked to medicine for yeast infection discount 500 mg secnidazole with mastercard report the extent to symptoms diagnosis discount secnidazole 500 mg visa which the spleen tip extended below a Hooking Maneuver of Middleton With Patient Supine speci c bony landmark (eg symptoms bladder cancer buy secnidazole 500mg low price, the xiphisternal-sternal junc the patient is asked to treatment esophageal cancer generic 500mg secnidazole visa lie at with his or her left st under tion), their estimates varied on average by 6 cm. If a spleen is greatly of obesity (more false-negative results), and its speci city was enlarged, it may be possible to feel a hilar notch along its decreased by recent food intake (more false-positive results). The rst 2 studies17,18 assessed the accuracy of the routine Among the same patients, a second study14 showed that the examination for splenomegaly by abstracting the clinical interexaminer agreement for palpation ranged from 0. Most When tested among 50 patients with alcoholism, agree enlarged spleens were missed (a high rate of false-negative ment among different examiners (using 2-handed palpation results, leading to low sensitivity), but few examiners reported with the patient in the right lateral decubitus and 1-handed palpating spleens that were not there (a low rate of false palpation with the patient supine) demonstrated an inter positive results, leading to high speci city). In the other 5 palpation studies4,5,9,14,19 (Table 46-2), the examination for splenomegaly was performed as part of the Table 46-1 Studies of the Accuracy of Percussion study. Because the examiners knew that they were under scrutiny, it is not surprising that both their true-positive No. Nonobese patients 78 (29/37) 82 (54/66) One study9 compared percussion methods and palpation who have not and demonstrated that the Castell method of percussion may eaten recently12 be somewhat more sensitive than palpation (82% vs 71%) 65 Scintigra Nixon method9 59 (10/17) 94 (45/48) (Tables 46-1 and 46-2). Finally, if splenomegaly was declared phy when any of the 4 signs (2 for percussion and 2 for palpation) Castell method9 82 (14/17) 83 (40/48) were positive, true-positive and false-positive declarations of aEach patient was examined by 1 to 3 examiners, for a total of 245 examinations. The discriminating ability refers to the lege were found to have unexplained palpable spleens1 and, probability of correctly selecting the patient with splenomeg on incomplete follow-up, appeared to fare none the worse20; aly between 2 patients: one with an enlarged spleen and one similarly, 12% of otherwise healthy postpartum women at a with a normal spleen. The discriminating abil Guidelines for examining for splenic enlargement are sum ity of palpation and percussion was similar, although the test marized in Table 46-3. Splenomegaly is uncommon but occurs in a wide variety of the most important nding of this study was that palpa conditions. Given the low sensitivity of the clinical exami tion was a better discriminator among patients in whom per nation, it can be argued that the routine examination for cussion result was positive. Rather, the examination for splenomegaly is most poor discriminator (55%) or only slightly better than chance. The bedside examination of the spleen should start with patients in whom palpation is a useful test. If percussion is not dull, there is no need to pal dullness is absent, there is no need to palpate, because palpa pate because the results of palpation will not effectively rule tion is a poor test among such patients. If the possibility of miss Finally, this study also demonstrated that, given a clinical ing splenic enlargement remains an important clinical con suspicion (the prior probability or disease prevalence) of sple cern, then ultrasonography or scintigraphy is indicated. In nomegaly before examining the patient of 10% to 90%, it is the presence of percussion dullness, palpation should fol dif cult to substantially decrease the likelihood of an enlarged low. Maneuvers are not sufficiently sensitive to rule out splenomegaly Dr Grover is a research scholar and Dr Barkun is a clinical Given the low pretest probability of splenomegaly, test specificity scholar supported by the Fonds de la recherche en sante du of clinical examinations is not sufficiently high to rule in splenic Quebec. Weight of the spleen, I: range of normal in a nonhospi high, order ultrasonography because palpation in the presence of tal population. Dimensions of the normal If percussion result is positive but palpation result is negative, then adult spleen scan and prediction of spleen weight. Spleen size in polycythemia: To con dently rule out splenomegaly, a radiologic procedure is necessary a clinical and scintigraphic study. Sonographic measurements of the normal liver, spleen, pan of splenomegaly was at least 10% before examination). Ultrasonographic determination of the splenic size required to con dently rule in or rule out splenomegaly. Reliability of clinical techniques for detecting Returning to the 3 patients originally described at the splenic enlargement. A Biographical History of Medicine: Excerpts and Essays on the plaining of fatigue if your preexamination clinical suspicion Men and Their Work. The bedside assessment of splenic you may choose to examine for splenic enlargement in the enlargement. The relative accuracy of estimation of enlargement of the liver and spleen by radiologic and clinical methods. Correlation of liver and spleen size: determinations by nuclear medicine studies and physical examination. Prediction of spleen size by routine tistics, McGill University, Montreal, Quebec, Canada (Drs radioisotope scintigraphy. The incidence of palpable liver and spleen in the postpartum Clinical Epidemiology and Biostatistics of McMaster Univer period. The medical history is also that of Details of the Update intermittent sweats and some weight loss. Your examination A study from Brazil suggested that combining the results of 2 reveals diffuse adenopathy. The inference is that having a colleague con rm your ndings for Original Review splenomegaly might be useful. The rational clinical Two studies from India suggested that palpation maneu examination: does this patient have splenomegaly Furthermore, in one of the studies, false-negative results for Traube space percussion were signi cantly higher Our literature search used the parent search strategy for the 3 in smaller patients. Therefore, although palpation may per Rational Clinical Examination series, combined with the sub form better than percussion in lean patients, we do not know ject “exp splenomegaly,” published in English from 1991 to whether the same test characteristics apply to patients with 2004, and articles that referred to the original review. The clinical utility appears enhanced when results yielded 136 articles, for which we reviewed the titles the results of both percussion and palpation are considered and abstracts. We found 5 articles suitable for review, but should be con rmed in other studies in which the order although 1 was a duplicate publication. One of the studies had sion maneuvers were relatively insensitive but had better information on the interobserver variability of examination 5 speci city, supporting the ndings of the original Rational techniques. However, they also noted • Palpation might have greater accuracy than percussion, signi cant interobserver variability that did not depend on especially in lean patients. The poor reliability was evident tion performance may be biased because in a number of in the broad range of individual assessors’ sensitivity and studies, palpation followed percussion maneuvers. Because the evaluation of Table 46-4 Likelihood Ratios of Percussion and Palpation Maneuvers for Splenomegaly such a large number of individual ndings may have lacked independence, and because the total number of patients was Maneuver (No. In addition, we used the diagnostic odds ratios to assess whether the overall accuracy for some maneu vers might be better than others. No federal guidelines discuss the assessment of splenomegaly by using physical examination. The per cussion ndings have a lower accuracy than the palpation signs (as suggested by the diagnostic odds ratios). Middle the prevalence of palpable splenomegaly in an otherwise ton maneuver, in which the physician stands to the left of the healthy student population is low, approximating 3%8; 12% patient and hooks the examining hand under the ribs, may of normal postpartum women had palpable spleens. When it remains important not to miss splenomeg 66%10), or in areas in which schistosomiasis is prevalent. Prevalence and clinical signi cance of splenomegaly in asymp Traube’s space—a useful index of splenic enlargement. J Assoc Physicians tomatic human immunode ciency virus type 1-infected adults: Swiss India. Hepatosplenomegaly aFor the Evidence to Support the Update for this topic, in infectious mononucleosis, assessed by ultrasonic scanning. The assess maneuver ments were carried out by a physician blinded to the patient’s Percussion Maneuvers clinical history and laboratory results. Furthermore, the generalizability may Abdominal palpation was performed with patients in the be questioned, considering the patient population characteristics decubitus position during deep inspiration, by 2 indepen (the mean Quetelet index of the studied patients was low by 2 dent physicians in a blinded fashion. The overall prevalence of between the splenic border and the costal margin was also splenomegaly suggests that this population may differ consider independently measured by the examiners. We do not know whether “leanness” as evi denced by a low body mass index creates a bias that favors the 2 examination maneuvers were considered positive for palpation or percussion. How useful is Traube’s space per Splenomegaly was de ned as a splenic length greater than cussion in assessing splenic enlargement A spleen was palpated by both physicians in 37 cases (discordance between examiners occurred in 5 cases). This suggests that clinicians communities with an independent assessment by ultra sonography. Table 46-8 Likelihood Ratios for Percussion, Palpation, and a Combination of the 2 Findings for Splenomegaly Reviewed by Alan N. However, the maximal clinical utility appeared to be tympanitic (resonant) or dull. Dullness to percussion is an achieved when both percussion and palpation were considered.

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  • They can live in clothing for up to 1 month.
  • Mitral stenosis
  • Overnight oxygen level measurements (oximetry)
  • Dark urine -- comes and goes
  • Feeling weak or tired more often than usual, or with exercise
  • All adults should have their blood pressure checked every 1 to 2 years if their blood pressure was less than 120/80 mmHg at their most recent reading.

The conventional chlorination facility for use in potable water treatment consists of three elements medicine ball workouts cheap 500mg secnidazole, namely chlorine source symptoms 8 days after ovulation order secnidazole 500 mg overnight delivery, metering system and injection system symptoms schizophrenia buy cheap secnidazole 500 mg on-line. Some aspects are common to symptoms 3 months pregnant purchase secnidazole 500mg without prescription all three, such as the requirement for proper mixing and contact with the water. Selection of the chlorine compound to be used the liquid or solid hypochlorite products are normally applied at relatively small water purification plants and their use at larger installations is usually only for very specific purposes. If the capacity of a treatment plant warrants the installation and use of liquid chlorine, then the remoteness and locality of the purification plant should not be a deterring factor. Not only is the cost of liquid chlorine (expressed per mass of active compound) less than that of the hypochlorite products, the former also has advantages such as: Some systems where chlorine gas is dosed do not require electrical power to operate as the chlorine is drawn off directly from a 68 kg container in the gaseous form. If the cylinder is placed such that the outlets are vertically above each other position gaseous chlorine can be withdrawn from the topmost valve and liquid from the bottom outlet. Sodium hypochlorite is normally supplied at a concentration of 15% mass by volume, but depending on the storage temperature may lose about 6,5 % of the original concentration within four weeks at 15 C and as much as 37 % of the original concentration within four weeks at 30 C. In these systems sodium chloride is used as the feedstock to produce a sodium hypochlorite solution of about 5 % mass by volume. By using this chlorine solution it is possible to manage the persistent loss in chlorine concentration because a lower initial chlorine concentration is used and in general the storage period is short. Calcium hypochlorite in solid form is available as granules, tablets and briquettes. The presence of these metals can greatly reduce the stability of the hypochlorite compounds, especially the solutions, as these metals act as powerful catalysts in the decomposition of hypochlorites. Typical problems include the blockage of small-bore sodium hypochlorite dosing lines and corrosion of chlorinators or equipment that comes into contact with chlorine compounds or vapours. However, once chlorine gas has reacted with water, including moisture in the atmosphere, very corrosive hydrochloric acid is formed. Therefore the correct materials of construction must be selected at all sites where chlorine is handled or stored. Chlorine compounds must never be stored together with organic material such as oil or grease, nor with flammables such diesel, paraffin or petrol. To avoid rapid decomposition, hypochlorite solutions must be kept at the lowest possible temperatures and exposure to direct sunlight must be avoided. Liquid chlorine containers must also be stored at the lowest practical temperature and away from any external heat source, including direct sunlight. If the chlorine containers are exposed to high temperatures the chlorine will expand and eventually cause the container to rupture should the temperature rise too high. However, if chlorine containers are kept at temperatures of 10 C or less the flow of chlorine will be sluggish and erratic. Ideally they should be stored and used at between 18 C and 25 C with this temperature being lower than the chlorinator to avoid condensation in the chlorinator. The maximum chlorine abstraction rates of chlorine gas or liquid from containers must be observed at all times. The maximum respective withdrawal rates are 1,3 kg/hour for 68kg cylinders and 9,0 kg/hour from one ton drums. Safety precautions when handling chlorine compounds All the chlorine compounds used in water purification are highly toxic. These include pure chlorine in the liquid or gaseous form as well as liquid sodium hypochlorite and solid calcium hypochlorite. When handling chlorine all necessary precautions must be taken and the required personal protective equipment must be worn at all times when working with connections to chlorine containers. Accidental spills or leaks must be contained and cleaned up using recommended methods in accordance with legal and safety requirements. The suppliers of chlorine products and chlorine dosing equipment can provide information on the safe handling of chlorine products and equipment. At the one end are waters having virtually no dissolved salts, with low alkalinity and low pH; examples occur to a lesser or greater extent in virtually all countries worldwide, but in South Africa principally along eastern seaboard regions. At the other end of the spectrum, are waters with high concentrations of dissolved salts, high alkalinity and variable pH and in addition, for underground waters, high concentrations of dissolved iron, manganese and possibly carbon dioxide. Between these extremes, waters may be found with virtually any combination of chemical quality reflecting the history of the water and the region from which it is derived. Most waters require some form of treatment before discharge to a distribution system. The chemical quality of the water entering the distribution system always has some impact on the system: Waters with appropriate qualities have minimal impact and the systems have long life expectation; waters with inappropriate qualities can have a major impact and reduce expectation of the lives of systems substantially. Impact of such waters on distribution systems divides into three types: the water may cause extensive precipitation of calcium and iron minerals on the walls of the conduits, so extensive that the carrying capacity of the conduit may be reduced severely, to zero on occasion. Economic costs in maintaining distribution systems have not been estimated countrywide in South Africa, but must be substantial this is probably the case in most countries worldwide. Studies in this regard are long overdue, but qualitative discussions with municipal officials in charge of water supplies and with consumers, have forced the conclusion that problems with corrosion and aggression in water distribution systems are indeed widespread. In many instances, but not in all, these effects can be nullified or minimised by relatively simple corrective chemical treatment of the water. Appropriate stabilisation, however, demands an understanding of the underlying chemistry of the water and the interactions between the water and the material of the retaining structure. Extensive research has been conducted into the chemistry of water in the treatment of water supplies for municipal use. Perhaps the most influential contribution has been that of Langelier (1936) in both recognising the importance of the carbonate weak acid system in establishing pH in water, and the solubility of calcium carbonate 145 mineral as an important consideration in stabilisation. He proposed that water be treated to a state of oversaturation with respect to calcium carbonate, so that a protective film formed on pipe walls. Although more than 60 years have past since the pioneering work of Langelier, his criteria are still applied, in many instances indiscriminately. Since Langelier’s work, extensive research studies have been affected on the chemistry of water and the chemical responses of water in contact with iron metals and cement materials. The influence of other factors, such as chloride and sulphate ions, dissolved oxygen concentration, velocity of flow, etc. In this chapter, the currently accepted criteria used in South Africa for preventing corrosion and aggression in drinking water distribution systems are set out. The problem(s) associated with attaining these criteria via chemical treatment and judicial pipe material use are highlighted. Two distinct aggressive processes have been identified: (1) the water has chemical characteristics that cause dissolution of some of the minerals in the cement matrix. This exposes “fresh” free lime to reaction, and, in this fashion the aggression 146 progressively penetrates deeper into the concrete mass, weakening the matrix strength and exposing any steel reinforcing to the elements. With regard to (2) sulphate attack, when the dissolved sulphate concentration is high, the hydrated calcium oxide, alluminates and allumino ferrite react with sulphate species to form sulphate minerals that occupy greater volumes than the original minerals, causing expansion and consequent physical disintegration of the cement matrix. Furthermore, hydrated calcium silicate also reacts with sulphate ions forming a product which has much lower strength than the original mineral. Criteria for prevention of corrosion of iron piping Corrosion of metal components of pipes carrying water is a result of redox reactions at sites on the metal-water interface, by the formation of electrochemical cells each with an anodic and cathodic area. At the anode, metal molecules lose electrons to form metal ions that pass into solution; at the cathode, the electrons pass to some electron acceptor (usually oxygen) in the water adjacent to the cathode. These reactions induce significant changes in the chemical composition of the water adjacent to the anodic and cathodic areas. Depending on the circumstances, the reactions may cause continuous ionisation of the metal at the anode (corrosion) or may give rise to precipitation of minerals over the anode and cathode thereby reducing the areas of active electrochemical sites and hence reducing the rates of overall reactions, eventually stopping the corrosion completely (passivation of the surface). For waters “higher” in chlorides and sulphates, the formation of a protective film is adversely affected by these ions and requires special consideration. The overall corrosion rate always is governed by the slower of the anodic and cathodic reactions, because the rates of these two half reactions must be equal. Interactively, this causes changes in the chemical condition of the water layer next to the metal at both the anode and cathode. This precipitate, which is porous to some degree, reduces the rate of ionic and molecular diffusion to the surface. While the overall reaction rate is still high, the rate is controlled by the diffusion rate at the cathode. At this stage the film that forms at the anode has no affect on the reaction rate, because the reaction products that form are readily permeable and do not isolate the metal.

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