Õèìè÷åñêèé ìåòîä Âèíêëåðà äëÿ îïðåäåëåíèÿ ðàñòâîðåííîãî êèñëîðîäà

Êàâåðèí À.Â.

Ñðåäè ìåòîäîâ îïðåäåëåíèÿ êîíöåíòðàöèè ðàñòâîðåííîãî êèñëîðîäà ñàìûì ñòàðûì, íî äî ñèõ ïîð íå ïîòåðÿâøèì ñâîåé àêòóàëüíîñòè, îñòàåòñÿ õèìè÷åñêèé ìåòîä Âèíêëåðà  [1].  ýòîì ìåòîäå ðàñòâîðåííûé êèñëîðîä êîëè÷åñòâåííî ðåàãèðóåò ñî ñâåæåîñàæäåííîé ãèäðîîêèñüþ Mn(II). Ïðè ïîäêèñëåíèè ñîåäèíåíèÿ ìàðãàíöà áîëåå âûñîêîé âàëåíòíîñòè âûñâîáîæäàåò éîä èç ðàñòâîðà èîäèäà â ýêâèâàëåíòíûõ êèñëîðîäó êîëè÷åñòâàõ.  ðàáîòå  [2] ïîêàçàíî, ÷òî íåëüçÿ óâåðåííî ãîâîðèòü îá îáðàçîâàíèè òîëüêî ñîåäèíåíèé ìàðãàíöà (III) èëè (IV). Ïî ìíåíèþ àâòîðîâ ýòîé ïóáëèêàöèè îáðàçóåòñÿ ñìåñü ãèäðîêñèäîâ. Âûñâîáîæäåííûé éîä äàëåå îïðåäåëÿòñÿ òèòðîâàíèåì òèîñóëüôàòîì íàòðèÿ ñ êðàõìàëîì, â êà÷åñòâå èíäèêàòîðà.

Ñòàäèÿ ôèêñàöèè êèñëîðîäà ( ùåëî÷íàÿ ñðåäà )

Mn²⁺ + 2OH^- = Mn(OH)₂

2Mn(OH)₂ + O₂ = 2MnO₂*H₂O

Èîäîìåòðè÷åñêîå òèòðîâàíèå ( êèñëàÿ ñðåäà )

MnO₂*H₂O + 4H⁺ + 2J^- = Mn²⁺ + J₂ +3H₂O

J₂ + J^- = J₃^-

J₃^- + 2S₂O₃^2- = 3J^- + S₄O₆^2-

Ïî ìåðå èñïîëüçîâàíèÿ ýòîãî ìåòîäà â ïðèðîäíûõ âîäàõ áûëî îòìå÷åíî ñóùåñòâåííîå âëèÿíèå ðåäîêñ-àêòèâíûõ ïðèìåñåé. Íî íåñìîòðÿ íà ýòî, ìåòîäè÷åñêàÿ ïðîñòîòà è íàäåæíîñòü ïîçâîëèëà óæå 1925 ãîäó âêëþ÷èòü ìåòîä Âèíêëåðà â ñáîðíèê ñòàíäàðòíûõ õèìè÷åñêèõ ìåòîäîâ àíàëèçà âîä  [ 3 ]. Îáíàðóæåííîå âëèÿíèå ðåäîêñ ïðèìåñåé èíèöèèðîâàëî ðàçðàáîòêó õèìè÷åñêèõ ìîäèôèêàöèè ìåòîäà Âèíêëåðà, íåêîòîðûå èç êîòîðûõ ïîçíåå áûëè òàêæå âêëþ÷åíû â Standard methods.  ýòèõ ìîäèôèêàöèÿõ àêòèâíî èñïîëüçóþòñÿ ïðîöåäóðû ïðîáîïîäãîòîâêè, ïðèìåíåíèå ìàñêèðóþùèõ àãåíòîâ, ìåòîäû õîëîñòîé ïðîáû, ìåòîä ïàðàëëåëüíîé éîäíîé ïðîáû, ðåãëàìåíòèðóþòñÿ óñëîâèÿ ïðîâåäåíèÿ àíàëèçà, ïðè êîòîðûõ äåéñòâèåì òîé èëè èíîé ïðèìåñè ìîæíî ïðåíåáðå÷ü. Êàê ïîêàçûâàåò àíàëèç íàó÷íîé ïåðèîäèêè íà÷àëî èññëåäîâàíèé ïî ðàçðàáîòêå òàêèõ õèìè÷åñêèõ ìîäèôèêàöèé îòíîñèòñÿ ê 20-30 ãîäàì [  4- 6 ]. Íèæå êðàòêî ïðåäñòàâëåíû òå òðóäíîñòè, êîòîðûå ìîãóò âîçíèêàòü ïðè ïðîâåäåíèè àíàëèçà ïî Âèíêëåðó ïðè îäíîâðåìåííîì ïðèñóòñòâèè â âîäå ÷àñòî âñòðå÷àþùèõñÿ ðåäîêñ-ïðèìåñåé.

Ìåøàþùåå äåéñòâèå ðåäîêñ-àêòèâíûõ ïðèìåñåé:

1. Fe(III, II)

Ñîåäèíåíèÿ äâóõâàëåíòíîãî æåëåçà íà ñòàäèè ôèêñàöèè êèñëîðîäà ìîãóò âûñòóïàòü êàê êîíêóðåíòû ïî îòíîøåíèþ ê ìàðãàíöó. Ïðîðåàãèðîâàâ ñ êèñëîðîäîì îáðàçóåòñÿ ãèäðîêñèä Fe(III), êèíåòèêà âçàèìîäåéñòâèÿ êîòîðîãî ñ èîäèäîì â êèñëîé ñðåäå çàìåäëåíà. Òàê ïðè êîíöåíòðöèè æåëåçà áîëåå 25 ìã/ë[  6,  41 ] èñïîëüçîâàíèå êëàññè÷åñêîãî âàðèàíòà ìåòîäà Âèíêëåðà ïðèâîäèò ê çàíèæåíèþ ðåçóëüòàòîâ îïðåäåëåíèé. Áûëî ïðåäëîæåíî ýëèìèíèðîâàòü âëèÿíèå æåëåçà(III) äîáàâêàìè ôòîðèäà [  7,  8 ] èëè èñïîëüçîâàíèåì ôîñôîðíîé êèñëîòîé [ 6] ïðè ïîäêèñëåíèè ïðîáû. Îáðàçóþùèéñÿ ôòîðèäíûé èëè ôîñôàòíûé êîìïëåêñ íå äàåò æåëåçó âçàèìîäåéñòâîâàòü ñ èîíàìè èîäèäà. Íî ýòîò ñïîñîá íå äàåò âîçìîæíîñòè ýëèìèíèðîâàòü âëèÿíèå äâóõâàëåíòíîãî æåëåçà.

2. Íèòðèòû

Îáû÷íî ïðèñóòñòâèå â âîäå íèòðèòîâ îáóñëîâëåíî ìèêðîáèîëîãè÷åñêèì ïðåîáðàçîâàíèåì àììîíèÿ â íèòðàò. È èçâåñòíî, ÷òî íèòðèòû â êèñëîé ñðåäå ñïîñîáíû îêèñëÿòü èîäèä èîíû, âûçûâàÿ òåì ñàìûì çàâûøåíèé ðåçóëüòàòîâ â ìåòîäå Âèíêëåðà. Òåì íå ìåíåå ïðè ñîäåðæàíèè â âîäå äî 0.05-0.1 ìãN/ë[  6,  7,  41 ] ìîæíî ïðèìåíÿòü ïðÿìîé ìåòîä Âèíêëåðà.  íàñòîÿùåå âðåìÿ ñàìûì ðàñïðîñòðàíåííûì ñïîñîáîì íåéòðàëèçàöèè âëèÿíèÿ íèòðèòîâ ñ÷èòàåòñÿ èñïîëüçîâàíèå äîáàâîê àçèäà íàòðèÿ[  9 ](ìåòîä Àëüñòåðáåðãà [  4 ]). Çäåñü íåëüçÿ çàáûâàòü, ÷òî èçëèøíåå óâåëè÷åíèå êîíöåíòðàöèè àçèäà ìîæåò ïðèâåñòè è ê îòðèöàòåëüíîé îøèáêå. Ýòî îáóñëîâëåíî âîçìîæíîñòüþ ïðîòåêàíèÿ ðåàêöèè[  10,  11 ]:

2N^3- + 2H⁺ + J₂ = 2HJ + 3N₂

Êðîìå ïðèìåíåèÿ àçèäà åñòü è äðóãèå ñïîñîáû ïîäàâëåíèÿ èëè ó÷åòà âëèÿíèÿ íèòðèòîâ: ïðèìåíåíèå ìî÷åâèíû èëè ñóëüôàìèíîâîé êèñëîòû [  8 ]. Âñå ýòè ðåàêòèâû ðàçðóøàþò íèòðèò äî ìîëåêóëÿðíîãî àçîòà.

3. Îðãàíè÷åñêèå âåùåñòâà.

Ïîíÿòíî, ÷òî âëèÿíèå îðã. âåùåñòâ, êàê âûðàæåííûõ âîññòàíîâèòåëåé áóäåò ïðîÿâëÿòüñÿ íà âñåõ ýòàïàõ îïðåäåëåíèÿ ðàñòâîðåííîãî êèñëîðîäà ïî Âèíêëåðó. Ìîëåêóëÿðíûé êèñëîðîä, îêèñëåííûå ôîðìû ìàðãàíöà, ìîëåêóëÿðíûé éîä - âñå ýòî äîñòàòî÷íî ñèëüíûå îêèñëèòåëè äëÿ âçàèìîäåéñòâèÿ ñ îðãàíè÷åñêèìè ïðèìåñÿìè. Åñëè âîäà áîãàòà îðã. âåùåñòâàìè (îêèñëÿåìîñòü 15-30ìãÎ₂/ë è áîëåå[  6,  8,  41 ]), òî îêàçûâàåòñÿ íåîáõîäèìûì ââîäèòü ïîïðàâêó íà èõ âçèìîäåéñòâèå[  6,7 ]. Íàïðèìåð â ðóêîâîäñòâå [  7 ] ïðåäëàãàåòñÿ ïðîâîäèòü ïàðàëëåëüíóþ éîäíóþ ïðîáó, íàõîäÿ òåì ñàìûì ñêîëüêî éîäà èçðàñõîäîâàëîñü íà èîäèðîâàíèå îðã. ïðèìåñåé. Íî åñòü ìåòîäû [  12 ], êîòîðûå îñíîâàíû íà ïðîâåäåíèè ìåòîäà Âèíêëåðà, â îòëè÷àþùèõñÿ îò êëàññè÷åñêèõ óñëîâèÿõ ( âðåìÿ àíàëèçà, êîíöåíòðàöèè ðåàãåíòîâ ). Òàêèì îáðàçîì óäàåòñÿ ïîäîáðàòü óñëîâèÿ, ïðè êîòîðûõ ìåøàþùèì äåéñòâèåì ïðèìåñè ìîæíî ïðåíåáðå÷ü.

Íåëüçÿ çäåñü íå îòìåòèòü îðèãèíàëüíûå ðàáîòû Ãîëòåðìàíà [  13 ].  ýòîé ðàáîòå åìó óäàëîñü ðàçðàáîòàòü õèìè÷åñêèé ìåòîä, ñî÷åòàþùèé â ñåáå îïðåäåëåíèå êîíöåíòðàöèè ðàñòâîðåííîãî êèñëîðîäà è îïðåäåëåíèå õèìè÷åñêîãî ïîòðåáëåíèÿ êèñëîðîäà(ÕÏÊ-Ñóììàðíàÿ íîðìàëüíîñòü âîññòàíîâèòåëåé, âûðàæåííàÿ â ìãÎ₂/ë.).  ñîîòâåòñòâèè ñ åãî ìåòîäèêîé ðàñòâîðåííûé êèñëîðîä â ùåëî÷íîé ñðåäå ôèêñèðóåòñÿ íå Mn(II), à ñîëüþ Ce(III).

Ce³⁺ + 3OH^- = Ce(OH)₃

4Ce(OH)₃ + O₂ +2H₂O = 4Ce(OH)₄

Âûäåëèâøåéñÿ Ce(IV) ïîñëå ðàñòâîðåíèÿ åãî â ðàñòâîðå êèñëîòû îïðåäåëÿåòñÿ ôîòîìåòðè÷åñêè èëè òèòðèìåòðè÷åñêè. Êðîìå òîãî, èñïîëüçîâàíèå ñîëåé öåðèÿ (III, IV) ïîçâîëÿåò ó÷åñòü ðàñõîä öåðèÿ (IV) íà îêèñëåíèå ïðèìåñè-âîññòàíîâèòåëÿ, ïðîâîäÿ "õîëîñòîé" îïûò, ò.å. ââîäÿ â ïðîáó âîäû íå Ce(III), à Ce(IV) íà ñòàäèè ôèêñàöèè êèñëîðîäà.

aCe(OH)₄ + bR = (a-b)Ce(OH)₃ + Ce(OH)₃, R-îðã. ïðèìåñü

4. Ñóëüôèäû è H₂S

Îáíàðóæåíî, ÷òî ñîäåðæàíèå â àíàëèçèðóåìîé âîäå ñóëüôèäîâ ïðèâîäèò ê çàíèæåíèþ ðåçóëüòàòîâ ìåòîäà Âèíêëåðà. Ïðè ýòîì îáíàðóæåíî, ÷òî âçàèìîäåéñòâèå ñóëüôèäà ñ îêèñëèòåëÿìè íîñèò ñòåõèîìåòðè÷åñêèé õàðàêòåð: 1 ìîëü êèñëîðîäà è 2 ìîëÿ ñóëüôèäà.  ðåçóëüòàòå ðåàêöèè âûäåëÿåòñÿ ýëåìåíòàðíàÿ ñåðà. Ïîñêîëüêó â ìåòîäå Âèíêëåðà ñèëüíûìè îêèñëèòåëÿìè ÿâëÿþòñÿ êðîìå êèñëîðîäà òàêæå éîä è ìàðãåíåö(III, IV), òî â ôîðìóëèðîâàíèè ìåõàíèçìà âçàèìîäåéñòâèÿ ñóëüôèäà ñ îêèñëèòåëåì åñòü ðàçëè÷íûå ìíåíèÿ. Òàê â ðàáîòå [  14 ] ñ÷èòàåòñÿ, ÷òî ñóëüôèä âçàèìîäåéñòâóåò ñ îêèñëåííûìè ôîðìàìè ìàðãàíöà, à â [  15 ] ñ éîäîì.  ðàáîòå [  16 ] ðàçðàáîòàí ìåòîä îäíîâðåìåííîãî îïðåäåëåíèÿ ñóëüôèäîâ è êèñëîðîäà â ïðîáå âîäû. Àâòîðû, èñïîëüçóÿ ñîëè Zn, îñàæäàþò ZnS, êîòîðûé äàëåå îòäåëÿþò è îïðåäåëÿþò ñïåêòðîôîòîìåòðè÷åñêè, à â îñòàâøåéñÿ íàä îñàäêîì âîäå ïðîâîäÿò îïðåäåëåíèå ðàñòâîðåííîãî êèñëîðîäà.  áîëåå ðàííåé ðàáîòå èñïîëüçîâàíà ñõîäíàÿ ñõåìà, íî èñïîëüçîâàëñÿ íå ñóëüôàò, à àöåòàò Zn[  17 ]. Ïðè âçàèìîäåéñòâèè êèñëîðîäà è ñóëüôèäà âîçìîæíî òàêæå îáðàçîâàíèå òèîñóëüôàòà, â êà÷åñòâå ïðîìåæóòî÷íîãî ñîåäèíåíèÿ [  18 ].  ðàáîòå [  16 ] ïðåäëîæåí ñïîñîá ó÷åòà òàêîãî òèîñóëüôàòà ïî ìåòîäó õîëîñòîé ïðîáû.

 çàêëþ÷åíèå íóæíî îòìåòèòü, ÷òî íàðÿäó ñ ìîäèôèêàöèÿìè è ìåòîäèêàìè, ðàçðàáîòàííûìè ñïåöèàëüíî ïîä êîíêðåòíûå ïðèìåñè, ñóùåñòâóþò áîëåå îáùèå ìåòîäèêè, íàïðàâëåííûå íà îïðåäåëåíèÿ îáùåãî ñîäåðæàíèÿ âîññòàíîâèòåëåé (ìåòîä Ðîññà [  19,  8,  40 ]) è îêèñëèòåëåé [  8 ].

Òî÷íîñòü ïðÿìîãî ìåòîäà Âèíêëåðà è åãî âîçìîæíûå îøèáêè.

Íà ïðîòÿæåíèè âñåé ïåðâîé ïîëîâèíû 20-ãî âåêà â õîäå ëàáîðàòîðíûõ è ïîëåâûõ ðàáîò áûë ñîáðàíà áîëüøàÿ ýêñïåðèìåíòàëüíàÿ áàçà ïî ðåçóëüòàòàì îïðåäåëåíèÿ êèñëîðîäà ìåòîäîì Âèíêëåðà. Áûëè îáíàðóæåíû ðàñõîæäåíèÿ â ðåçóëüòàòàõ îïðåäåëåíèé ðàñòâîðåííîãî êèñëîðîäà â îäíèõ è òåõ æå âîäàõ ïî ìåòîäàì, ðàçëè÷àþùèìñÿ òîëüêî äåòàëÿìè[  20 ], íàïðèìåð ñïîñîáîì ñòàíäàðòèçàöèè ðàñòâîðà òèîñóëüôàòà, êîíöåíòðàöèåé ðåàãåíòîâ, ñïîñîáîì òèòðîâàíèÿ (âñåãî ðàñòâîðà èëè àëèêâîòû) è äð.  áîëüøåé ìåðå ýòà ïðîáëåìà - ïðîáëåìà ñòàíäàðòèçàöèè ìåòîäà Âèíêëåðà, ïðîÿâëåòñÿ â ìíîãîîáðàçèè òàáëèö ðàñòâîðèìîñòè êèñëîðîäà[  21-26 ]. Ðàçëè÷èÿ â òàáëè÷íûõ çíà÷åíèÿõ ðàñòâîðèìîñòè êèñëîðîäà äî 6% ñïîñîáñòâîâàëè ïðîâåäåíèþ èññëåäîâàíèé ïî ïðèíöèïèàëüíûì âîïðîñàì ìåòîäè÷åñêîé îñíîâû è ìåòîäè÷åñêèì ïîãðåøíîñòÿì ìåòîäà Âèíêëåðà.  ðåçóëüòàòå òàêèõ ðàáîò áûë ñôîðìóëèðîâàí ðÿä ïîòåíöèàëüíûõ èñòî÷íèêîâ ïðèíöèïèàëüíûõ îøèáîê ìåòîäà â ÷èñòûõ âîäàõ [  20,  27 ]:

îêèñëåíèå èîäèäà êèñëîðîäîì âîçäóõà

óëåòó÷èâàíèå ìîëåêóëÿðíîãî èîäà[  28 ]

ñîäåðæàíèå ðàñòâîðåííîãî êèñëîðîäà â äîáàâëÿåìûõ ðåàêòèâàõ â ïðîöåäóðå ôèêñàöèè êèñëîðîäà[  20,  29 ðð. 285-297,  30 ]

ïðèìåñü ìîëåêóëÿðíîãî èîäà â èîäèäå

íåñîâïàäåíèå òî÷êè êîíöà òèòðîâàíèÿ è òî÷êè ýêâèâàëåíòíîñòè[  27 ]

ìàëàÿ óñòîé÷èâîñòü ðàñòâîðîâ òèîñóëüôàòà íàòðèÿ è ñîîòâåòñòâåííî íåîáõîäèìîñòü ÷àñòîé ñòàíäàðòèçàöèè

îøèáêè ïðè ñòàíäàðòèçàöèè òèîñóëüôàòà íàòðèÿ [  20 ]

òðóäíîñòü òèòðîâàíèÿ ìàëûõ êîëè÷åñòâ èîäà

èñïîëüçîâàíèå êðàõìàëà â êà÷åñòâå èíäèêàòîðà: åãî íåñòîéêîñòü è óìåíüøåíèå ÷óâñòâèòåëüíîñòè ñ ïîâûøåíèåì òåìïåðàòóðû[  31 ]

Îñòàíîâèìñÿ ïîäðîáíåå íà íàèáîëåå çíà÷èìûõ îøèáêàõ. Îêèñëåíèå èîäèäà êèñëîðîäîì óñêîðåòñÿ ñ ðîñòîì êèñëîòíîñòè. Óìåíüøèòü âëèíèå ýòîãî ïðîöåññà ìîæíî ðåãóëèðóÿ ðÍ ñðåäû. Ðåêîìåíäóåìîå çíà÷åíèå êèñëîòíîñòè ñîñòàâëåò ðÍ=2-2.5[  20,  27 ]. Óâåëè÷åíèå ðÍ áîëåå 2.7 îïàñíî, ò.ê. òàì óæå âîçìîæåí ïðîöåññ ãèäðàòîîáðàçîâàíèÿ ìàðãàíöà. Îäíîâðåìåííî ñ îêèñëåíèåì èîäèäà âîçìîæåí òàêæå è ïðîöåññ óëåòó÷èâàíèÿ éîäà. Îáðàçîâàíèå êîìïëåêñíîé ÷àñòèöû J₃^- â óñëîâèÿõ èçáûòêà èîäèäà(ñì. ñõåìó ìåòîäà Âèíêëåðà) ïîçâîëåò ñâÿçàòü ïðàêòè÷åñêè âåñü ìîëåêóëÿðíûé éîä â ðàñòâîðå. ïîíÿòíî, ÷òî ââîäÿ ðàñòâîð ñîëè ìàðãàíöà è ùåëî÷íîé ðåàãåíò(ùåëî÷ü+èîäèä), ìû òåì ñàìûì âíîñèì íåó÷òåííîå êîëè÷åñòâî êèñëîðîäà, ðàñòâîðåííîãî â ýòèõ ðåàêòèâàõ. Ïîñêîëüêó â ðàçëè÷íûõ âàðèàíòàõ ìåòîäà Âèíêëåðà èñïîëüçîâàëèñü ðåàêòèâû ðàçëè÷íûõ êîíöåíòðàöèé, òî èñïîëüçîâàòü â ðàñ÷åòàõ êàêóþ-ëèáî îäíó ïîïðàâêó áûëî íåëüçÿ. Ïðèõîäèëîñü äëÿ êàæäîãî ìåòîäà èñïîëüçîâàòü ñâîè ñîáñòâåííûå ðàñ÷åòíûå èëè ýêñïåðèìåíòàëüíûå çíà÷åíèÿ ïðèâíåñåííîãî ñ ðåàêòèâàìè êèñëîðîäà. Îáû÷íî ýòè çíà÷åíèÿ íàõîäèëèñü â èíòåðâàëå 0.005-0.0104 ððì[  29 ].

Ê ñåðåäèíå 60-õ ãîäîâ íàçðåëà íåîáõîäèìîñòü â åäèíîé ïðîöåäóðå îïðåäåëåíèÿ ðàñòâîðåííîãî êèñëîðîäà. Ýòî îò÷àñòè áûëî îáóñëîâëåíî áîëüøèì ðàçíîîáðàçèåì õèìè÷åñêèõ ìåòîäèê, ðàçâèòèåì èíñòðóìåíòàëüíûõ ìåòîäîâ è íåîáõîäèìîñòüþ èõ âçàèìíîãî ñðàâíåíèÿ. Íà îñíîâå îïóáëèêîâàííîé ðàáîòû[  27 ], Êàðïåíòåð ñôîðìóëèðîâàë ïðîöåäóðó îïðåäåëåíèÿ êèñëîðîäà ïî Âèíêëåðó[  32 ].  ýòîì âàðèàíòå áûëè ó÷òåíû ïðàêòè÷åñêè âñå ïîòåíöèàëüíûå îøèáêè âûÿâëåííûå ðàííåå.  ñîâìåñòíîé ðàáîòå[  20 ] Êýððèò è Êàðïåíòåð äîïîëíèëè ýòó ìåòîäèêó ïîïðàâêîé íà ó÷åò ðàñòâîðåííîãî â ðåàêòèâàõ êèñëîðîäà(0.018ìë/ë). Ýêñïåðèìåíòàëüíî èçìåðåííàÿ â ðàáîòå [  30 ] âåëè÷èíà íåñêîëüêî îòëè÷àëàñü è ñîñòàâëÿëà 0.011 ìë/ë.

Ïðè îïðåäåëåíèè òî÷íîñòíûõ õàðàêòåðèñòèê õèìè÷åñêîãî ìåòîäà Âèíêëåðà èññëåäîâàòåëè ñòîëêíóëèñü ñ ïðîáëåìîé òî÷íîãî çàäàíèÿ êîíöåíòðàöèè ðàñòâîðåííîãî êèñëîðîäà. Äëÿ ýòîãî èñïîëüçîâàëèñü íàñûùåíèå âîäû âîçäóõîì èëè êèñëîðîäîì ïðè çàäàííîé òåìïåðàòóðå, ñòàíäàðòíàÿ äîáàâêà ðàñòâîðà êèñëîðîäà â îáåçêèñëîðîæåííóþ âîäó[  27 ], ýëåêòðîõèìè÷åñêîå ãåíåðèðîâàíèå êèñëîðîäà[  34 ], èñïîëüçîâàíèå àëüòåðíàòèâíûõ èíñòðóìåíòàëüíûõ ìåòîäîâ îïðåäåëåíèÿ êèñëîðîäà[  35 ]. Íå ñìîòðÿ íà äîëãóþ èñòîðèþ ýòîé ïðîáëåìû è ìíîãî÷èñëåííûå ðàáîòû, îêîí÷àòåëüíîå ðåøåíèå ïîêà íå íàéäåíî è âîïðîñ ïî-ïðåæíåìó îñòàåòñÿ îòêðûòûì. Íàèáîëåå ïîïóëÿðíûì ñïîñîáîì çàäàíèÿ êîíöåíòðàöèè êèñëîðîäà â âîäå áûë è îñòàåòñÿ äî ñèõ ïîð - ïðîöåäóðà íàñûùåíèÿ âîäû êèñëîðîäîì âîçäóõà ïðè ôèêñèðîâàííîé òåìïåðàòóðå. Îäíàêî îòñóòñòâèå åäèíîîáðàçèÿ ïðîöåäóðû(îáúåì ðàñòâîðà, óñëîâèÿ ïåðåìåøèâàíèÿ, ñïîñîá è ñêîðîñòü ïðîäóâàíèÿ êèñëîðîäà) ïðèâîäèò ê çíà÷èòåëüíûì îøèáêàì, äîñòèãàþùèì 2%[  21 ].  áîëüøåé ìåðå ýòî ïðîÿâëÿëîñü ïðè ðàáîòå â îáëàñòè ìåíüøå 5 ìãÎ₂/ë.

Îïèðàÿñü íà âûñîêîòî÷íîå ïðèãîòîâëåíèå ðàñòâîðîâ êèñëîðîäà, âíåñåíèåì ñòàíäàðòíîé äîáàâêè â îáåçêèñëîðîæåíóþ âîäó, Êàðïåíòåðó óäàëîñü äîñòèãíóòü ïðàâèëüíîñòè 0.1% è âîñïðîèçâîäèìîñòè 0.02% íà óðîâíå 5 ìãÎ₂/ë äëÿ âàðèàíòà ìåòîäà Âèíêëåðà ñ ôîòîìåòðè÷åñêèì òèòðîâàíèåì[  21 ].  Òàáëèöå 1 ïîêàçàíà ïîãðåøíîñòü êëàññè÷åñêîãî âàðèàíòà ìåòîäà Âèíêëåðà íà ðàçëè÷íûõ óðîâíÿõ êîíöåíòðàöèè ðàñòâîðåííîãî êèñëîðîäà. Òàáëèöà 1 ñîñòàâëåíà ïî îïóáëèêîâàííûì ðåçóëüòàòàì ïîëåâûõ è ëàáîðàòîðíûõ îïðåäåëåíèé.

Òàáëèöà 1. Ïîãðåøíîñòü ìåòîäà Âèíêëåðà â ÷èñòûõ âîäàõ.[ 9, 20, 36-38 ]

ìãÎ₂/ë

ïîãðåøíîñòü

0.05

~30%

0.2-0.3

10-20%

0.8-1.7

3-5%

3-...

~1%, íî ïðè òùàòåëüíîé ðàáîòå âîçìîæíî ñíèæåíèå äî 0.1%.

Äðóãèì âàæíûì ïàðàìåòðîì, õàðàêòåðèçóþùèì âîçìîæíîñòè ìåòîäà ÿâëÿåòñÿ íèæíÿÿ ãðàíèöà îïðåäåëåíèÿ.  ëèòåðàòóðå öèòèðóåòñÿ äâà çíà÷åíèÿ íèæíåé ãðàíèöû: ~0.05[  7,  39] è ~0.2 [  8,  9 ]ìãÎ2/ë. Ïîíÿòíî, ÷òî ïðåäåë îáíàðóæåíèÿ ìîæåò îïðåäåëÿåòüñÿ ñëåäóþùèìè êðèòåðèÿìè:

íàðóøåíèå ñòåõèîìåòðèè ðåàêöèé, ëåæàùèõ â õèìè÷åñêîé îñíîâå ìåòîäà Âèíêëåðà

÷óâñòâèòåëüíîñòü éîä-êðàõìàëüíîé ðåàêöèè

êîíöåíòðàöèåé èñïîëüçóåìîãî ðàñòâîðà òèîñóëüôàòà è ðàçðåøàþùÿÿ ñïîñîáíîñòü áþðåòêè

 ðàáîòå Ïîòòåðà ïîêàçàíî, ÷òî äàæå íà óðîâíå 0.0007(!)ìãÎ₂/ë[  29 ðð. 317-328] ñòåõèîìåòðèÿ îñíîâîïîëàãàþùèõ ðåàêöèé ñîõðàíÿåòñÿ.  ýòîé æå ðàáîòå ãîâîðèòñÿ, ÷òî îñíîâíîé ïðè÷èíîé, îïðåäåëÿþùåé íèæíèé ïðåäåë ÿâëÿåòñÿ ÷óâñòâèòåëüíîñòü éîä-êðàõìàëüíîé ðåàêöèè, êîòîðàÿ îöåíèâàåòñÿ êàê ~2·10^-6Í (0.02-0.05 ìãÎ₂/ë)[  27,  29,  42, 43 ]. Òàêèì îáðàçîì ìîæíî ñêàçàòü, ÷òî óðîâåíü 0.05 ìãÎ₂/ë - ýòî íèæíèé ïðåäåë îáíàðóæåíèÿ, à óðîâåíü 0.2 ìãÎ₂/ë ìîæíî òðàêòîâàòü, êàê íèæíèé ïðåäåë ìåòîäà(èëè çíà÷èìîñòè îïðåäåëåíèÿ), ò.å. òîò óðîâåíü, íà êîòîðîì ïîãðåøíîñòü äîñòèãàåò 10-20% è áîëåå.

Õðîíîëîãè÷åñêîå ðàçâèòèå ìåòîäà Âèíêëåðà.

1888

Ïåðâàÿ ïóáëèêàöèÿ Âèíêëåðîì íîâîé ìåòîäèêè[  1 ].

1920-ûå

Âêëþ÷åíèå ìåòîäà Âèíêëåðà â ñáîðíèê Standard methods(1925) [  3 ]. Ïîÿâëåíèå ïåðâûõ õèìè÷åñêèõ ìîäèôèêàöèé[  4-6 ].

1930-50ûå

Ðàçâèòèå àëüòåðíàòèâíûõ èíñòðóìåíòàëüíûõ ìåòîäîâ(ãàçîìåòðè÷åñêèå, ôîòîìåòðè÷åñêèå).

1960-ûå

Èçó÷åíèå îñíîâîïîëàãàþùèõ ïðèíöèïîâ ìåòîäà Âèíêëåðà[  2,  27,  29,  30 ]. Ïîïûòêè ðàçðàáîòêè óíèôèöèðîâàííîé ïðîöåäóðû îïðåäåëåíèÿ ðàñòâîðåííîãî êèñëîðîäà íà îñíîâå ðàáîò Êýððèòà è Êàðïåíòåðà[  2,  32 ].

1970-ûå

Ðàçâèòèå àìïåðîìåòðè÷åñêèõ àíàëèçàòîðîâ. ÃÎÑÒ 22018-84[  33 ], ÑÒ ÑÝ 6130-87

1980-ûå

Ðàçðàáîòêà ñòàíäàðòîâ ïî îïðåäåëåíèþ ðàñòâîðåííîãî êèñëîðîäà íà îñíîâå âàðèàíòà Êàðïåíòåðà. ÈÑÎ 5813-83[  9 ], ÈÑÎ 5814-84[  26 ]

1990-ûå

Ïðîáëåìà êàëèáðîâêè è ñðàâíåíèÿ ìåòîäîâ îïðåäåëåíèÿ ðàñòâîðåííîãî êèñëîðîäà â îáëàñòè ìèêðîêîíöåíòðàöèé (ìåíüøå 1 ìãÎ2/ë) [  44, p. 29].

Ñïèñîê ëèòåðàòóðû

[1] Winkler L.W. Die Bestimmung des im Wasser geloesten Sauerstoffes. // Chem. Ber. 1888, v. 21, pp. 2843-2855.

[2] Carrit D.E., Carpenter J.H. Comparison and evalution of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater. A NASCO report. // J. Mar. Res. 1966, v. 24, _3, pp. 286-313.

[3] Standard methods of water analysis. N.Y., 1925, 6 ed.

[4] Alsterberg G. Die Winklersche Bestimmungsmetode fuer in Wasser geloesten, elementaren Sauerstoff sowie ihre Anwendung bei Anwesenheit oxyderbaran Substansenn. // Biochem. Z. 1926, v. 30, p.130

[5] Ridel S., Steward C.G. Determination of dissolved oxygen in seawater in the presence of nitrites and organic matter. // Analyst 1901, v. 26, pp. 141-148

[6] Á.À. Ñêîïèíöåâ, Þ.Ñ. Îâ÷èííèêîâà Îïðåäåëåíèå ðàñòâîðåííîãî êèñëîðîäà â âîäàõ, ñîäåðæàùèõ ðàçëè÷íûå îêèñëèòåëè è âîññòàíîâèòåëè. // ÆÏÕ 1933, ò. 6, _6, ñòð. 1173-1179.

[7] Î.À. Àëåêèí, À.Ä. Ñåìåíîâ, Á.À. Ñêîïèíöåâ Ðóêîâîäñòâî ïî õèìè÷åñêîìó àíàëèçó âîä ñóøè. Ë. Ãèäðîìåòåîèçäàò, 1973ã., ñòð. 36-44.

[8] Þ.Þ. Ëóðüå, À.È. Ðûáíèêîâà Õèìè÷åñêèé àíàëèç ïðîèçâîäñòâåííûõ ñòî÷íûõ âîä. Ì. Õèìèÿ, 1974ã. 4-å èçä., ñòð. 45-54.

[9] ÈÑÎ 5813-83. Îïðåäåëåíèå ðàñòâîðåííîãî êèñëîðîäà. Èîäîìåòðè÷åñêèé ìåòîä.

[10] M.J. Barcelona, E.E. Garske Nitric oxide interference in the determination of dissolved oxygen by the azide-modified Winkler method. // Anal. Chem. 1983, v. 55, _6, pp. 965-967.

[11] Jones K. In "Comprehensive inorganic chemistry" Pergamon Press, Oxford, 1973, chapter 19, pp. 147-388.

[12] Standard methods for the examination of water, sewage and industrial wastes. APHA, N.Y., 1955, 10 ed., pp. 250-260.

[13] Graaf Bierbrauwer I.M., Golterman H.L. The determination of oxygen in fresh water with trivalent cerium salts. // Proceedings IBP-symposium; held at Amsterdam and Nieuwersluis October 1966, pp. 158-165.

Golterman H.L., Wisselo A.G. Ceriometry, a combined method for chemical oxygen demand and dissolved oxygen (with discussion on precision of the Winkler technique). // Hydrobiologia 1981, v. 77, _ 1, pp. 37-42.

[14] Riley J.P. Analytical chemistry of seawater. In Riley J., Skirrow G.(eds), Chemical oceanography, v. 3, 1975, 2 ed., pp. 258-259.

[15] Mor E., Beccaria A.M. Determination of dissolved oxygen in seawater in the presence of sulfide. // Ann. Chim. (Rome), 1971, v. 61, pp. 363-371.

[16] K. Ingvorsen, B.B. Jorgensen Combined measurement of oxygen and sulfide in water sample. // Limnol. Oceanogr. 1979, v. 24, _2, pp. 309-393.

[17] Pomeroy R. Auxillary pretreatment by zink acetate in sulfide analyses. // Anal. Chem. 1954, v. 26, pp. 571-572.

[18] Chen K.Y., Morris J.C. Kinetics of oxidation of aqueous sulfide by O2. // Environ. Sci. Technol., 1972, v. 6, pp. 529-537.

[19] Ross F.F. The determination of oxygen in polluted waters. // Water Waste Treat. J., 1964, v. 9, pp. 528-531.

[20] Carrit D.E., Carpenter J.H. Comparison and evalution of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater. A NASCO report. // J. Mar. Res. 1966, v. 24, _3, pp. 286-313.

[21] Carpenter J.H. New measurements of oxygen solubility in pure and natural water. // Limnol. Oceanogr. 1966, v. 11, _2, pp. 264-277.

[22] Winkler L.W. Die Loeslichkeit des Sauerstoffs in Wasser. // Ber. Deut. Chem. Ges. 1889, v. 22, pp. 1764-1774.

[23] Winkler L.W. // Ber. Deut. Chem. Ges. 1891, v. 24, p. 3602.

[24] Elmore H.L., Hayes T.W. Solubility of atmospheric oxygen in water. // Proc. Am. Soc. Civil Engrs., 1960, v. 86(SA4), pp. 41-53.

[25] Montgomery H.A.C., Thom N.S., Cockburn A. Determination of dissolved oxygen by the Winkler method and the solubility of oxygen in pure water and sea water. //J. Appl. Chem., 1964, v. 14, _ 7, pp. 280-296.

[26] ÈÑÎ 5814-84. Îïðåäåëåíèå ðàñòâîðåííîãî êèñëîðîäà. Ìåòîä ýëåêòðîõèìè÷åñêîãî äàò÷èêà.

[27] Carpenter J.H The accuracy of the Winkler method for dissolved oxygen analysis. // Limnol. Oceanogr., 1965, v. 10, _1, pp. 135-140.

[28] Knapp G., Stalcup M.C., Stanley R.J. Iodine losses during Winkler titrations. // Deep-Sea res., 1991, v. 38, _ 1, pp. 121-128.

[29] Potter E.C. The microdetermination of dissolved oxygen in water. // J. Appl. Chem. 1957, v. 7, _ 6, pp. 285-328.

[30] Murray C.N., Riley J.P., Wilson T.R.S. The solubility of oxygen in Winkler reagents used for the determination of dissolved oxygen. // Deep-Sea res. 1968, v. 15, pp. 237-238.

[31] Êîëüòãîô È.Ì., Ñåíäýë Å.Á. Êîëè÷åñòâåííûé àíàëèç. ïîä ðåä. Ëóðüå Þ.Þ., Ãîñõèìèçäàò, 3-å èçä., 1948ã., ñòð. 639.

[32] Carpenter J.H. The Chesapeake Bay Institute technique for the Winkler dissolved oxygen method. // Limnol. Oceanogr. 1965, v. 10, _1, pp. 141-143.

[33] ÃÎÑÒ 22018-84 Àíàëèçàòîðû ðàñòâîðåííîãî â âîäå êèñëîðîäà. Àìïåðîìåòðè÷åñêèå ÃÑÏ. Îáùèå òåõíè÷åñêèå òðåáîâàíèÿ (ÑÒ ÑÝ 6130-87).

[34] P. Jeroschewski, D. zur Linden A flow system for calibration of dissolved oxygen sensors. // Fresenius J. Anal. Chem., 1997, v. 358, _ 6, pp. 677-82.

[35] Wheatland A.B., Smith L.J. Gasometric determination of dissolved oxygen in pure and saline water as a check of titrimetric methods. // J. Appl. Chem., 1955, v. 5, pp. 144-48.

[36] ×åðíÿêîâà À.Ì., Ñàëëèâàí Ä.Ï., Ñòóíæàñ Ï.À., Íàëáàíäîâ Þ.Ð., Ïîÿðêîâ Ñ.Ã., Êàëâàéòèñ À.Í., Ñîëîìîí Ä.Ë. Î ñîïîñòàâëåíèè îïðåäåëåíèé ðàñòâîðåííîãî â âîäå êèñëîðîäà ïî ìåòîäó Âèíêëåðà. // Îêåàíîëîãèÿ, 1983, ò. 23, _4, ñòð. 681-687.

[37] Culberson C.H. Dissolved oxygen. In WOCE Operational Manual. Volume. 3: The observation programm. Section 3.1: WOCE Hydrographic Programm. part 3.1.3: WHP Operations and methods. WHP Office Report WHPO 91-1/WOCE Report No 68/91., July 1991.

[38] G.E. Friederich, L.A. Codispoti, C.M. Sakamoto An easy-to-construct automated Winkler titration system. Appendix 1: On the determination of dissolved oxygen in sea water(by L. Codispoti). In MBARI Technical report No 91-6, August 1991.

[39] À.Ì. ×åðíÿêîâà, Ì.Ï. Ìàêñèìîâà, Ý.È. Êîííèê Ìåòîäû îïðåäåëåíèÿ ñîäåðæàíèÿ êèñëîðîäà è åãî ðàñòâîðèìîñòè â ìîðñêîé âîäå. // Îáçîðíàÿ èíôîðìàöèÿ Ðûáíîãî õîçÿéñòâà. Ì. 1970ã., ñåð. 1 (Ïðîìûñëîâàÿ èõòèîëîãèÿ è îêåàíîëîãèÿ), âûï. 3, ñòð. 18-35.

[40] Ñêîïèíöåâ Á.À., Ìèòÿãèíà Î.Â. Îá îïðåäåëåíèè ðàñòâîðåííîãî â âîäå êèñëîðîäà â ïðèñóòñòâèè âîññòàíîâèòåëåé. // ÆÏÕ 1939, ò. 12, _8,

[41] Ðóêîâîäñòâî ïî õèìè÷åñêîìó àíàëèçó âîä ñóøè. Ãèäðîìåòåîèçäàò, 1941ã., ñòð. 40-49.

[42] È.Ì. Êîðåíìàí Àíàëèòè÷åñêàÿ õèìèÿ ìàëûõ êîíöåíòðàöèé. Ì. Õèìèÿ, 1966ã., ñòð. 58.

[43] G. Knowles, G.F. Lowden Methods for detecting the end-point in the titration of iodine with thiosulphate. // Analyst, 1953, v. 78, _ 924, pp. 159-164.

[44] C.H. Culberson, G. Knapp, R.T. Williams, F. Zemlyak A comparison of methods for the determination of dissolved oxygen in seawater. WHP Office report, WHPO-91-2, February 1991.

[Updated 8/99]