Химический метод Винклера для определения растворенного кислорода

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]. Â 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. Â 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- + 2S2O32- = 3J- + S4O62-

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. Â 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. Â 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 ]:

2N3- + 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 ]. Â 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.). Â 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. Â 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. Â 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. Â 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 ]. Â ðàáîòå [  16 ] ïðåäëîæåí ñïîñîá ó÷åòà òàêîãî òèîñóëüôàòà ïî ìåòîäó õîëîñòîé ïðîáû.

Â 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. Â 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. Â 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 ]. Â ýòîì âàðèàíòå áûëè ó÷òåíû ïðàêòè÷åñêè âñå ïîòåíöèàëüíûå îøèáêè âûÿâëåííûå ðàííåå. Â 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 ]. Â 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 ]. Â 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. Â 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 

Â ðàáîòå Ïîòòåðà ïîêàçàíî, ÷òî äàæå íà óðîâíå 0.0007(!)ìãÎ2/ë[  29 ðð. 317-328] ñòåõèîìåòðèÿ îñíîâîïîëàãàþùèõ ðåàêöèé ñîõðàíÿåòñÿ. Â 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 ÑÝÂ 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 ÑÝÂ 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]