| Introduction | | | | This is especially likely with a closed vessel such as |
| A sample taken for chemical analysis is supposed to | | | | the Lachat Microdist. |
| be a snapshot of the quantity of analyte present at | | | | Samples that contain sulfide at concentrations above |
| the time the sample was taken. Reactions of the | | | | 50 ppm lose significant amounts of cyanide within 24 |
| analyte with other constituents in the sample matrix | | | | hours. Once Sulfide is reduced below 50 ppm the |
| cannot be allowed to proceed, nor can loss of the | | | | holding time can be extended. Even so, samples |
| analyte by evaporation, precipitation, or oxidation. | | | | should be analyzed as soon as possible and |
| The act of adding a chemical and refrigeration is | | | | preferably with a method that uses on-line sulfide |
| intended to preserve the analyte concentration. | | | | abatement such as ASTM D6888-04 or OIA 1678. In |
| Unfortunately, it is possible that the very act of | | | | fact, with slight reagent modification, ASTM |
| attempting to preserve an analyte concentration in | | | | D6888-04 and OIA 1678 can handle sulfide |
| the sample bottle actually makes matters worse. | | | | concentrations up to 200 ppm. However, remember |
| Depending on conditions and the sample matrix itself | | | | that cyanide concentrations are rapidly depleting as |
| our attempts at stabilizing what we are looking for | | | | cyanide remains in contact with sulfide. Basically, all |
| generates more of it. Sometimes we cause it to | | | | methods listed in the CFR for sulfide removal just |
| disappear. But even worse, sometimes we are killing | | | | don%u2019t work. Headspace expelling and dynamic |
| it and creating it all at the same time within that one | | | | stripping leave residual sulfide behind, which interferes |
| little bottle. It would not be so bad except for | | | | with distillation and analysis. The headspace and |
| decisions are made on the measurements finally | | | | stripping methods are difficult to use and essentially |
| made. These decisions could result in fines for analyte | | | | require a mobile laboratory. pH adjustments, as well |
| concentrations that weren't really there, or a false | | | | as flow rates, must be precise. Since the methods |
| assurance that analyte was absent when it really | | | | are volatilizing high levels of sulfide these procedures |
| wasn't. Other problems arise on questions of | | | | must be done under a hood, or with plenty of |
| compliance. In the regulated community not following | | | | ventilation. Recall that these procedures are removing |
| sampling and preservation protocol means sample | | | | sulfide by generating hydrogen sulfide gas. |
| collection was not valid. Changing the sample | | | | Precipitation with cadmium in the presence of iron |
| collection procedure is only allowed if the laboratory | | | | cyanide complexes forms a very stable and insoluble |
| has data to support the change, however, in most | | | | cadmium iron cyanide complex. When the cadmium |
| cases laboratories are unable to fully characterize the | | | | sulfide is filtered off so is the iron cyanide. Mercury |
| matrix of a sample prior to sample collection. We will | | | | cyanide (a WAD, CATC, or Available cyanide species) |
| look into the potential interferences that impact | | | | is also lost by precipitation with cadmium. The |
| cyanide analysis and what can be done to minimize | | | | precipitation with cadmium was put in place to |
| these impacts. | | | | replace precipitation with lead. This was because lead |
| Purpose of Sample Preservation | | | | sulfide rapidly reacts with cyanide to form |
| Cyanide methods are developed as an attempt to | | | | thiocyanate lowering the results. Other precipitants, |
| measure various cyanide species. These species | | | | such as Bismuth, also result in lower recoveries. |
| range from the most toxic free cyanide to the ultra | | | | The only sulfide removal procedure that recovers CN |
| conservative estimate of cyanide toxicity we know | | | | quantitatively is dilution of the sample till sulfide is no |
| as total cyanide. Total cyanide measurements include | | | | longer detected by the lead acetate test strips. An |
| free cyanide, available cyanide, and non toxic strong | | | | argument against dilution is the increase in detection |
| metal complexes. Also included in the definition of | | | | limit, however, most automated methods are |
| total cyanide are insoluble particulate or colloidal | | | | sufficiently sensitive so that dilutions of up to 10 X |
| cyanide complexes. The method that the samples | | | | still allow detection at about 5 ppb. Also, besides |
| are being collected for needs to be known at the | | | | diluting sulfide other interferences are being diluted as |
| time of sample collection. In most instances total | | | | well. Again, the only way to remove sulfide is to |
| cyanide will be analyzed. We will be discussing | | | | dilute the sample till sulfide is no longer detected on |
| protocols for sampling and preservation of cyanide | | | | the lead acetate test strips. Then analyze the sample |
| defined in Part 136, however, since these potential | | | | ASAP by a method that utilizes on-line sulfide |
| interferences apply to all samples this discussion | | | | abatement. If samples must be distilled, ASTM D7284 |
| should be applicable to all intended uses of data and | | | | was developed specifically to handle samples |
| to all cyanide methods. | | | | containing sulfide. This method utilizes gas-diffusion |
| Sample Pretreatment | | | | amperometry as the measurement step after |
| Oxidizers must be removed immediately since they | | | | samples are distilled. |
| rapidly react with CN decreasing its concentration. | | | | 40 CFR Part 136 specifically states that if sulfite, |
| The presence of oxidizers is determined by starch | | | | thiosulfate, or thiocyanate are thought to be present |
| iodide test strips, or using field portable DPD kits. | | | | to use a UV digestion method, or a non distillation |
| Oxidizers must be removed prior to any pH | | | | gas-diffusion method. Many people have problems |
| adjustment or they will rapidly oxidize any free and | | | | with this statement because at present there are no |
| most available cyanide present. Do not add a reducing | | | | commercial suppliers of the Kelada, and OIA 1677 is |
| agent unless oxidizers are detected, or known to be | | | | not a total method. The intent of this statement was |
| present. A literature search on the web of laboratory | | | | that since distilled colorimetrically determined cyanide |
| SOP%u2019s reveals that ascorbic acid and pH | | | | results from samples that contain these substances |
| adjustment to 12 - 13 is the most commonly | | | | cannot be trusted, an available cyanide result by |
| practiced preservation technique for cyanide samples. | | | | method 1677 is a more accurate estimate of toxic |
| This is unfortunate because: | | | | cyanide than what is possible by distillation |
| Ascorbic acid is a carbon source that can actually be | | | | colorimetry. The problem with naming the Kelada |
| a precursor for CN generation during storage. | | | | method is that it is a distillation/colorimetric method, |
| Multiple holding time studies have demonstrated that | | | | and does not eliminate, or even significantly minimize, |
| samples containing CN and ascorbic acid rapidly lose | | | | the interferences experienced because of these |
| CN upon storage at high pH. | | | | compounds. More so, since the Kelada is an |
| Use of ascorbic acid combined with hydroxide both | | | | automated colorimetric method any sulfur dioxide |
| destroys cyanide and creates it. | | | | that distills into the absorber solution can react with |
| If ascorbic acid is used for dechlorination the holding | | | | cyanide forming cyanate and react with chloramine T |
| time is reduced to about 24 hours. For example, a | | | | increasing the chlorine demand. In effect, any samples |
| synthetic sample prepared at 200 ppb CN with | | | | that contain sulfite cannot be determined by the |
| ascorbic acid added and the pH adjusted to 12 | | | | Kelada method. This is evidenced by a similar |
| recovered less than 25 % of the original cyanide | | | | methods that state that sulfite concentrations higher |
| present after storage for 3 days. Again, not only | | | | than 1 mg/l interfere. Remember again that |
| does ascorbic acid cause cyanide to be lost, but it | | | | thiosulfate reacts under heated acid conditions |
| can cause it to be generated as well. Basically, the | | | | (distillation) to elemental sulfur and sulfur dioxide. Since |
| use of ascorbic acid to dechlorinate samples unless | | | | the Kelada method is a distillation method, this means |
| analysis is possible within 24 hours. | | | | that samples that contain thiosulfate cannot be |
| Sodium thiosulfate may be used to dechlorinate | | | | analyzed. In fact the Kelada method says that |
| samples, however, it must not be added in excess. | | | | thiosulfate was evaluated for oxidant removal and |
| There are no spot tests available to estimate amount | | | | caused an interference with the method. |
| of thiosulfate present in a sample. Boiling hot sulfuric | | | | Thiocyanate in the presence of nitrate or nitrite |
| acid solution (cyanide distillation) containing thiosulfate | | | | reacts to generate cyanide. Sulfamic acid has been |
| generates colloidal sulfur and sulfur dioxide. Sulfur | | | | used to minimize this effect. Thiocyanate alone |
| dioxide distills into the absorber solution. If the | | | | reacts with the small amounts of oxidant that form |
| absorber solution is analyzed immediately, and | | | | due to irradiation and generate cyanides as well. The |
| chloramine T is doubled 80% recovery is possible. | | | | Kelada method suggests an alkaline digest be used in |
| However, as solutions sit the SO2, now Sulfite, | | | | the presence of thiocyanate to minimize degradation |
| reacts in the basic solution with the NaCN oxidizing it | | | | of thiocyanate to cyanide. However, if sulfite is also |
| to cyanate and lowering recovery. Therefore, if | | | | present in the sample, contact of sulfite and cyanide |
| thiosulfate is suspected to be present, samples need | | | | in alkaline solution rapidly oxidizes cyanide to cyanate. |
| to be analyzed as soon as possible after distillation, | | | | Solutions and Conclusions |
| and the analyst needs to verify that the amount of | | | | ASTM D 7284 analyzes total cyanide after distillation |
| chloramine T added is enough to guarantee a chlorine | | | | by gas-diffusion amperometry. The method has been |
| residual. This means that automated methods that | | | | validated by extensive single laboratory studies and |
| use colorimetry (335.3 and Kelada 01) should not be | | | | has been evaluated for performance in the presence |
| used because there is no way the analyst can verify | | | | of multiple interferences. The method was developed |
| that enough chloramine T was added. | | | | specifically to overcome sulfide interferences with |
| Sodium arsenite has been demonstrated as an | | | | colorimetry, but in the process of evaluating |
| effective preservative in most cases, however, a | | | | interferences it was found to overcome sulfite and |
| few studies have found slight false positives when | | | | thiosulfate interferences as well. A modification of the |
| combining sodium arsenite with distillation methods. | | | | currently published procedure eliminates, or at least |
| Since sodium arsenite is an arsenic compound no one | | | | significantly minimizes thiocyanate plus nitrate |
| really wants to carry it around in the field, or be | | | | interferences. OIA 1678 is a UV irradiation |
| adding it to sample bottles. | | | | gas-diffusion amperometry method. It differs from |
| Sodium borohydride is mentioned in the Kelada 01 | | | | the Kelada and EPA 335.3 methods because no heat |
| method. There are legitimate concerns with its use | | | | is necessary to separate CN from the acidified |
| since it generates hydrogen gas upon acidification. | | | | matrix. Earlier literature documents that automated |
| Anyone familiar with analyzing arsenic and selenium | | | | distillation alone only liberates free cyanide and that in |
| by hydride generation are familiar with this. Since | | | | automated methods UV irradiation is needed to |
| distillations are taking place near a heat source the | | | | analyze total cyanide. OIA 1678 relies on gas diffusion |
| hydrogen generation could result in an explosion | | | | instead of distillation to separate cyanide from the |
| hazard. Also, rapid generation of hydrogen gas within | | | | acidified matrix. Since OIA 1678 does not need heat |
| a digestion vessel could result in exploding vessels. | | | | the interferences are minimized. |